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Influenza Overview


Quick Facts

Influenza


Click the image above to learn how to stay healthy this flu season and about the influenza vaccine.
  • Influenza, often referred to as “flu,” is a viral respiratory disease caused by type A or type B influenza viruses, which constantly mutate, and are infectious in humans and animals.1  Different influenza strains cause outbreaks and epidemics and, infrequently, cause pandemics that spread globally and are usually associated with more severe disease and increased mortality. Historically, influenza pandemics have involved type A influenza strains like the one that caused the 1918-19 influenza pandemic.2  
  • Symptoms of influenza include fever, chills, headache, sore throat, runny or stuffy nose, coughing, sneezing, and sometimes vomiting and diarrhea. Serious complications of influenza infection include dehydration, bronchitis, bacterial or viral pneumonia, otitis media (ear infection) and, in very severe cases, death. The majority of people recover from type A or B influenza without serious complications. The elderly, very young children, pregnant women and persons with certain chronic diseases, like asthma and heart or lung disease, are at higher risk for influenza complications. 3  
  • About 80 percent of all respiratory infections that occur during the “flu season” are not type A or type B influenza because there are many other viruses and bacteria that can cause respiratory “influenza-like illness” (ILI). ILI infection symptoms are similar to influenza symptoms and only lab tests can confirm whether an individual has been infected by influenza or an ILI. 4 5
  • Influenza viruses are primarily spread through coughing and sneezing. Public health officials say that, for a limited period of time, influenza can also be transmitted if an uninfected person touches or uses items that have been recently handled by an infected person. 6  
  • Frequent hand washing; covering the mouth while coughing; staying home when sick and avoiding contact with infected individuals; staying hydrated and eating nutritious food; lowering stress and getting plenty of exercise; sleep and vitamin D are helpful in the preventing influenza and ILI infections. 7 8 9 10 (View NVIC's 3 minute flu prevention video.

Influenza (Flu) Vaccines 

  • There are several different influenza vaccines licensed by the U.S. Food and Drug Administration (FDA) and distributed by manufacturers for use in the U.S. that are recommended by the US Centers for Disease Control (CDC) for different age groups.11  Most seasonal influenza vaccines in the U.S. contain either two type A influenza viruses and one type B influenza virus (Trivalent) or two type A influenza viruses and two type B influenza viruses (Quadrivalent) that are selected every year by the World Health Organization (WHO) and U.S. Centers for Disease Control (CDC) for inclusion in influenza vaccines given during the current flu season.12
  • Most of the influenza vaccines in use in the U.S. are injectable, inactivated vaccines that are made using chicken embryos, insect cells, or dog kidney cells. Depending upon the vaccine manufacturer, some influenza vaccines contain an oil in water squalene adjuvant that hyper-stimulates the immune system to produce a stronger antibody response. 13 Injectable influenza vaccines packaged in multi-dose vials contain the mercury preservative thimerosal, and inactivated influenza vaccines packaged in single dose vials are either thimerosal-free or contain trace amounts of the mercury preservative, while the live attenuated nasal vaccine contains no thimerosal. 14 In 201615 and in 201716, the CDC’s Advisory Committee on Immunization Practices (ACIP) withdrew its recommendation for use of the nasal spray influenza vaccine after analyzing data demonstrating it failed to prevent influenza infection most of the time. However, in February 2018, the CDC’s Advisory Committee on Immunization Practices (ACIP) reversed its decision and voted to approve a new formulation of the nasal spray influenza vaccine for the 2018-2019 influenza season. This decision was made despite the lack of vaccine efficacy studies to support whether it will offer any improved protection from influenza. 17  
  • The CDC recommends that all Americans six months of age or older get a flu shot every year and that babies between six and eight months old should receive two doses of influenza vaccine one month apart in the first year of life. 18 The CDC reports that between 2004/2005 and 2017/2018, overall influenza vaccine effectiveness ranged from 10 percent (2004/2005) and 60 percent (2010/2011) and less the 50 percent effective in ten out of 14 flu seasons.
  • Using the MedAlerts search engine, as of June 30, 2018, there have been more than 153,034 reports of influenza vaccine reactions, hospitalizations, injuries and deaths following influenza vaccinations made to the federal Vaccine Adverse Events Reporting System (VAERS), including 1,476 related deaths, 11,739 hospitalizations, and 3,084 related disabilities. Moderate reactions reported include fever, local reactions (pain, redness, swelling at the site of the injection), headache, fatigue, sore throat, nasal congestion, cough, joint and muscle pain, and nausea. Serious vaccine complications include brain inflammation and neurological damage, convulsions, Bell’s palsy, limb paralysis, neuropathy, shock, wheezing/asthma and other breathing problems, and death. Influenza vaccinations can cause Guillain Barre Syndrome (GBS), a painful and disabling immune and neurological disorder of the peripheral nervous system that can cause temporary or permanent paralysis and death.
  • In 2013 the Federal Advisory Commission on Childhood Vaccines (ACCV) voted to add GBS to the Vaccine Injury Table (VIT) within the federal Vaccine Injury Compensation Program (VICP) and was officially added in 2017. As of August 8, 2018, there had been 4,527 claims filed in the federal Vaccine Injury Compensation Program (VICP) for injuries and deaths following Influenza vaccination, including 144 deaths and 4,383 serious injuries.
  • IMPORTANT NOTE: NVIC encourages you to become fully informed about Influenza and the Influenza vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.

IMPORTANT NOTE: NVIC encourages you to become fully informed about Influenza and the Influenza vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.

Food & Drug Administration (FDA)

  • Adjuvanted Influenza Vaccine Product Inserts & Licensing Information
  • Live Trivalent and Quadrivalent Intranasal Influenza Vaccine Product Insert & Licensing Information
  • Attenuated Trivalent and Quadrivalent Injectable Influenza Vaccine Product Insert & Licensing Information

Centers for Disease Control (CDC)

National Institute of Allergy & Infectious Diseases (NIAID)

NIAID on Influenza

Vaccine Reaction Symptoms & Ingredients

NVIC’s Ask 8, If You Vaccinate webpage contains vaccine reaction symptoms and you can download a Know the Facts to Stay Healthy This Flu Season brochure.

Search for Vaccine Reactions

NVIC hosts MedAlerts, a powerful VAERS database search engine. MedAlerts examines symptoms, reactions, vaccines, dates, places, and more.

Reporting a Vaccine Reaction

Since 1982, the NVIC has operated a Vaccine Reaction Registry, which has served as a watchdog on VAERS. Reporting vaccine reactions to VAERS is required by federal law under the National Childhood Vaccine Injury Act of 1986. If your doctor will not report a reaction, you have the right to report a suspected vaccine reaction to VAERS.

IMPORTANT NOTE:NVIC encourages you to become fully informed about Influenza and the Influenza vaccine by reading all sections in the Table of Contents, which contain many links and resources such as the manufacturer product information inserts, and to speak with one or more trusted health care professionals before making a vaccination decision for yourself or your child. This information is for educational purposes only and is not intended as medical advice.

« Return to Vaccines & Diseases Table of Contents

What is Influenza?     

Influenza is a viral infection that produces fever, chills, sore throat, muscle aches, and cough that lasts a week or more.19  People tend to use the term "flu" to describe any kind of respiratory or gastrointestinal illness, such as colds or diarrhea and vomiting that resemble “influenza-like-illness” (ILI) symptoms. But influenza is usually associated with more severe illness and lasts longer than the common cold and, normally, influenza does not cause vomiting or diarrhea in adults.

Influenza viruses are RNA genome viruses in the Orthomyxoviridae family. Influenza A viruses infect humans, animals and birds and influenza B and C viruses mainly infect humans, while influenza type D infects cattle. According to the WHO, “influenza virus undergoes high mutation rates and frequent genetic reassortment (combination and rearrangement of genetic material) leading to variability in HA (haemagglutinin) and NA (neuraminidase) antigens.”20

Influenza A viruses are found in ducks, chickens, pigs, horses, whales and seals. Wild birds are the primary natural reservoir for influenza A viruses and often cause asymptomatic or mild infection in birds but can become virulent in both wild and domestic poultry (chickens, turkeys). Pigs can be infected with swine, human and bird (avian) viruses and sometimes those viruses recombine and create new influenza viruses.21 22 Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus and can be further broken down into different strains, while influenza B viruses are not divided into subtypes but can be broken into lineages and strains.23

Because influenza viruses are constantly mutating and there are different strains and subtypes that are more or less prevalent among human populations from year to year, outbreaks and epidemics occur in certain geographical areas or countries. Occasionally, an influenza strain will emerge to cause an influenza pandemic that spreads globally and is usually associated with more severe disease and increased mortality.24 Historically, influenza pandemics with higher rates of complications and death have involved type A influenza strains like the one that caused the 1918-19 influenza pandemic.25

About 80 percent of all respiratory infections that occur during the “flu season” are not type A or type B influenza because there are many other viruses and bacteria that can cause respiratory “influenza-like illness” (ILI). ILI infection symptoms are similar to influenza symptoms and only lab tests can confirm whether an individual has been infected by influenza or an ILI.26 27

The vast majority of people recover from influenza without any complications and develop immunity to future infection with the same strain or a related influenza strain that may prevent illness symptoms or make illness less severe. However, there is an increased risk for serious complications and death for the elderly and those with compromised immune systems or who are suffering from diabetes, kidney dysfunction, heart disease, and other chronic health issues.28

Between the 1976/1977 and 2006/2007 flu seasons, the Centers for Disease Control (CDC) estimated that, depending upon the influenza type and strain circulating in a given year, influenza-related deaths in the United States range between a low of 3,000 and a high of 49,000.29 In 2016, the CDC updated their numbers and estimated that influenza-associated deaths between 2010-2011 and 2013-2014 averaged between a low of 12,000 and a high of 56,000.30 However, these numbers are only estimates because the CDC does not collect influenza-related death information in persons older than 18 years of age. Currently, the CDC acknowledges that it is unknown how many people in the U.S. die every year from influenza-related complications.31

Is Influenza contagious?   

A viral infectious disease, influenza is contagious and can last three to 14 days if there are no complications.32 Most healthy adults are thought to be infectious and able to shed influenza virus and transmit it others beginning one day before symptoms develop and for five to seven days after becoming ill. It is estimated that some children may be infectious for longer than seven days.33  Severely immunocompromised children and adults may shed influenza virus for weeks or months.34

Both vaccinated and unvaccinated persons can be infected with and shed and transmit influenza virus in respiratory secretions35 and wild-type influenza virus has also been shed and identified in stool.36 Vaccinated and unvaccinated individuals can transmit influenza to others but be asymptomatic and have no apparent clinical symptoms themselves.37 38

Influenza viruses are transmitted through the air by droplets when infected persons cough, sneeze or talk. These droplets can end up in the mouths or noses or inhaled into the lungs of others near a person with influenza.39 Less frequently, a person might also become infected when touching an item or surface with influenza virus on it and then touching their own nose or mouth.40

To avoid transmitting influenza virus – or other types of influenza-like respiratory infections - to others, people who know they are sick should stay home until they are well. Frequent hand washing with soap and water can help prevent the spread of influenza and other viruses. If soap and water is not available, alcohol-based hand sanitizers can also be used. Eating utensils, dishes, linens and other personal items used by those who are sick should not be shared without thorough washing. Surfaces that are frequently touched should be cleaned and disinfected at home, school and work, especially if used by someone who is ill.41

What is the history of influenza in America and other countries?

The name Influenza originated in 15th century Italy from the belief that the epidemic of respiratory illness was “influenced” by the stars. While the first documented global influenza pandemic appears to have occurred in 1580, ancient Greek literature traces reports of possible influenza as far back as 412 BC.42 43

It is likely that seasonal influenza outbreaks and epidemics have occurred yearly in different parts of the world throughout recorded history. However, because influenza rarely caused significant mortality and morbidity on a global scale, influenza reports in the historical literature likely focused on influenza pandemics and not seasonal outbreaks.

Influenza pandemics occur when a new strain of influenza emerges and has a global impact on human populations because most people - or certain age groups within a population - do not have natural immunity to the new influenza strain. While influenza outbreaks or epidemics typically affect the elderly and those with chronic health issues, influenza pandemics tend to also impact younger people, both healthy individuals and those with chronic illness.44

Between 1700 and the 1918-19, historical literature documented at least four influenza pandemics, each occurring between 40 and 60 years apart.45

The 1918-19 influenza pandemic is thought to have originated in the U.S., even though it is often referred to as the “Spanish Flu,” and it was the first flu pandemic of the 20th century. Caused by a type A influenza strain related to swine (pigs), the 1918-19 pandemic is estimated to have resulted in 20 to 50 million deaths worldwide, including more than 600,000 deaths in the U.S.

The 1957-58 Asian flu pandemic began in February of 1957 in Singapore and continued on to Hong Kong before spreading globally. It is estimated that approximately 1.1 million excess deaths occurred worldwide during this pandemic and there was a noticeable increase in the influenza-related respiratory mortality rate among school aged children and young adults.46 Ten years later, in comparison, the 1968-69 Hong Kong flu pandemic was fairly mild, with 33,800 U.S. deaths being attributed to it.47

In 1976, a type A influenza virus (H1N1/swine flu) was isolated from two soldiers at Fort Dix, New Jersey and approximately 200 more soldiers were subsequently infected. The fear of a repeat of the influenza pandemic of 1918-19 prompted the pre-emptive development of a flu vaccine. The projected swine flu epidemic did not occur and the outbreak was limited to New Jersey, with no additional cases detected after February 1976.48

In April 2009, a new influenza type A H1N1 (swine flu) strain that was first identified and has been confirmed to have originated in Mexico near a pig farm was detected in the United States.49 By April 26, 2009, public health officials from the Centers for Disease Control (CDC) and the U.S. Secretary of Homeland Security declared a national public health emergency.50 The quick declaration of a public health emergency put the production of experimental pandemic H1N1 influenza vaccines on a fast track and vaccine manufacturers were given liability protection, while there was a nationwide promotion for everyone to get vaccinated.51 52 Eventually the CDC recommended that Americans get two flu shots in the 2009-2010 flu season: a seasonal flu shot and a pandemic H1N1 flu shot.53

The 2009 influenza pandemic was mild compared to previous pandemics like the one in 1918-19. The CDC estimated there were more than 60 million cases in the U.S. and 12,469 deaths directly attributable to the 2009 H1N1 “swine flu” virus.54

Can influenza cause injury and/or death?      

The influenza virus is constantly mutating and this frequent change makes it difficult to know exactly which type A and B strains will be circulating during the upcoming flu season.55 56  One particular strain of influenza may be predominant early in a flu season, while another different strain may emerge later in the season.  Timing, severity and duration of the flu season varies widely from year to year, depending the prevalent circulating influenza strains that are associated with either mild, moderate or severe illness.57

Official estimates of annual influenza-associated deaths in the United States have varied widely during the last half century.58 59 In the past, the CDC has estimated between 3,000 and 49,000 influenza-related deaths occur every year in the U.S., but the actual number is unknown because influenza-related deaths for persons over age 18 are not required to be reported to the CDC.60 61 62 In 2016, the CDC updated their numbers and estimated that influenza-associated deaths between 2010-2011 and 2013-2014 averaged between a low of 12,000 and a high of 56,000.63 However, the CDC also acknowledged that the number of persons in the U.S. die who die every year from influenza-related complications is unknown.64

A 2005 article published in the British Medical Journal asked the question: "Are U.S. Flu Death Figures More PR Than Science?"65 The author analyzed the U.S. Vital Statistics Mortality Data, which has been recorded for more than a century by the National Center for Health Statistics, and noted that from 1900 to 2010 the mortality rates for influenza deaths have been dropping and do not closely align with CDC influenza-related mortality estimates.66 Counting death certificates listing influenza as the cause of death could provide more accurate data. However CDC officials maintain this to be a gross underestimation of seasonal influenza’s true impact.” 67 68

The impact of influenza-related illness on people also varies widely. A 2014 study published in The Lancet found that three-quarters of people confirmed with seasonal and pandemic influenza were asymptomatic, meaning they showed no symptoms of illness at all.69 70  People who do not show symptoms of influenza Illness, whether they have been vaccinated or not, can still transmit infection to others.71

However, for some people who get influenza, serious complications develop and can be life-threatening. These complications may include bacterial pneumonia, ear infections, sinus infections, dehydration, sepsis, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.72

Who is at highest risk for getting influenza? 

Public health officials state that, while anyone can get sick with influenza, those most at risk for complications are pregnant women, the elderly, individuals with specific chronic medical conditions (i.e., HIV/AIDS, asthma, diabetes, heart or lung diseases), young children under age five, and health care workers.73 74 Due to the fact that influenza viruses are continually changing, the severity of symptoms associated with influenza infections and the prevalence of related complications varies from season to season.75 76

What is the history of influenza vaccine use in America?

The 1918-19 influenza pandemic at the beginning of the 20th century stimulated research on the influenza virus and, in 1933, influenza type A was isolated in ferrets. In 1936, influenza type B was isolated and an Australia scientist discovered that the virus could be grown in embryonic hen eggs. These discoveries fueled an interest in the development of an influenza vaccine that would reduce mortality in future epidemics and pandemics.77 78

The first vaccine for influenza was developed in 1938 and given to U.S. soldiers during World War II. A 1944 study of the new influenza vaccine determined that, while helpful in reducing illness with a temperature above 99 degrees F, it did not appear to have an impact on clinical outcomes. In 1947, further evaluation of the influenza vaccine found no difference in health outcomes between those who were vaccinated and those who were not vaccinated.79 80

Early flu vaccines contained only inactivated influenza virus type A (monovalent) but, by 1942, there was a bivalent vaccine containing both Influenza type A and influenza type B. This early vaccine caused localized and systemic reactions, especially in children. Despite little evidence of its effectiveness, the influenza vaccine was licensed for use in the U.S. in 1945.81 82 83 84

When the predicted “Asian” flu pandemic materialized in the 1957-1958 flu season, production of a vaccine against this pandemic influenza strain was initiated quickly in hopes it would limit mortality and reduce severity of the illness for vaccinated persons. Approximately 40 million doses of the vaccine were administered to people in the U.S.

However, due to the lack of effectiveness and limited availability, public health officials reported “the vaccine had no appreciable effect on the trend of the pandemic.”85 The failure was presumed to be primarily related to the lack of availability of the vaccine and, by 1960, health officials started recommending routine flu shots every year for the elderly and certain high risk groups. 86

A review of this “annual flu shot” recommendation four years later found little evidence that annual vaccination of seniors and others thought to be at high risk for influenza had any appreciable impact on influenza-related mortality rates. A 1968 double-blind randomized study conducted by CDC officials and published by the World Health Organization came to similar conclusions and even suggested that “attention should be redirected towards finding a more efficacious means of protection.”87 Yet, despite studies demonstrating that the influenza vaccine was ineffective, government vaccine policy recommendations for annual flu shots continued.

In early 1976, two cases of H1N1 “swine flu” were confirmed in the U.S. and public health officials working with the pharmaceutical industry made the decision to start manufacturing a vaccine out of concerns that this new strain of type A influenza could start a pandemic similar to the 1918-19 influenza pandemic. The U.S. Congress approved $137 million dollars for vaccine production with the goal that nearly all Americans would be vaccinated before the flu season started.88 

Mass production of the swine flu vaccine did not start until the U.S. Congress gave in to demands from drug companies lobbying for a product liability shield to block vaccine injury lawsuits for any harm caused by swine flu shots.89 This decision called into question the safety of the swine flu vaccine and public support for the mass vaccination program began to wane.

The swine flu vaccination program began in October 1976 and, within two weeks public concerns about safety were highlighted when three senior citizens died after getting vaccinated at the same clinic. By December 1976, there had been numerous reports of people becoming paralyzed from Guillain-Barre Syndrome (GBS) that developed after getting swine flu shots and, with no evidence of an impending influenza pandemic, the swine flu vaccination program was cancelled.90

Despite this setback, seasonal flu vaccine production continued and, in 1978, the first trivalent influenza vaccine was licensed for use in the U.S. after scientists identified two different influenza A strains circulating simultaneously. The new inactivated trivalent influenza vaccine contained two strains of influenza A and one strain of influenza B virus.91

The first live attenuated influenza vaccine (LAIV), FluMist, was approved by the FDA in 2003. A trivalent vaccine administered as a nasal spray, FluMist was approved for use in healthy children and adults ages five to 49 years of age.92 This live virus flu vaccine was not recommended for individuals with a history of asthma or other respiratory illness, any underlying chronic illness, including those with immune or metabolic dysfunction, persons with a history of Guillain-Barre Syndrome, children or adolescents on aspirin therapy, pregnant women, or anyone with an egg allergy. 

Importantly, FluMist was not recommended for people coming in close contact with immunocompromised individuals “because of the theoretical risk that a live, attenuated vaccine virus could be transmitted to the immunosuppressed person and cause disease.”93 There are documented cases in the medical literature of vaccine virus infection, shedding and transmission with FluMist and other live attenuated virus vaccines.94 95

On April 26, 2009, public health officials declared a national public health emergency after the discovery of a new Influenza A (H1N1) strain was first identified in Mexico and then in the U.S.96 A new pandemic H1N1 swine flu vaccine was quickly licensed and made available to the public in October of 2009, but a 2011 study of the effectiveness of the 2009 pandemic H1N1 swine flu vaccine found it had an overall effectiveness of only 56%.97

The influenza vaccine market has grown considerably since 2009 with the introduction of several new types of influenza vaccines along with new delivery methods, including vaccines than use insect and animal cells for production instead of chicken eggs.

A “high-dose” influenza vaccine targeting those over age 65 years was approved by the FDA in 2009 and became available in the U.S. for the 2010-2011 flu season.98 This vaccine contains four times the amount of antigen as other flu vaccines and is supposed to stimulate a stronger immune response that will produce more antibodies and, theoretically, give the elderly better protection from getting sick with influenza.99

In 2012, the FDA approved the first quadrivalent influenza vaccine, containing two type A and two type B influenza viruses.100 FluMist Quadrivalent, the live attenuated influenza vaccine, became available for the 2013-2014 flu season, and several quadrivalent inactivated injectable vaccines followed soon afterward. Quadrivalent vaccines add another strain of type B influenza B virus to the traditional trivalent vaccines, which contain two strains of type A influenza virus and one strain of type B influenza virus, with the goal of improving flu vaccine effectiveness.

The first cell-based influenza vaccine, Flucelvax, was approved by the FDA in 2012, using canine (dog) kidney cells101 instead of chicken embryos to produce the vaccine.102 In 2013, Flublok, a recombinant influenza vaccine using armyworm caterpillar cells instead of chicken embryos for production, was approved for use in adult over 18 years of age.103

On June 26, 2014, the CDC recommended giving FluMist to healthy children between two and eight years of age instead of inactivated injectable influenza vaccines but, two years later, in June 2016, the CDC withdrew its recommendation based on data that showed the vaccine was very ineffective in preventing influenza.104 105 On February 21, 2018, the CDC reinstated a new formulation of FluMist in advance of the 2018-2019 flu season. Vaccine efficacy studies on this new formulation of FluMist have not been completed and it is unknown whether it will offer any protection from influenza. The CDC voted against recommending FluMist over the available inactivated injectable influenza vaccines and would be considered an option if appropriate.106

In 2015, the FDA approved FLUAD, a trivalent influenza vaccine containing MF 59, a squalene oil adjuvant that hyper-stimulates the immune system to produce more antibodies. FLUAD is approved  for adults over the age of > 65.107 The licensing of  FLUAD was fast-tracked by the FDA despite concerns over the use of the squalene adjuvant and its association with immune and neurological disorders.108

Methods of how influenza vaccines are administered to people have also changed, as well. In 2012, the first intradermal influenza vaccine (administered between skin layers rather than into the muscle) was approved for use.109 In 2014, the first jet injector (vaccine delivery device using high-pressure) to administer influenza vaccine became available.110

As the influenza vaccine market expanded, so did recommendations for use by the CDC’s Advisory Committee on Immunization Practices (ACIP).

In 1984, the CDC recommended annual flu shots for high risk individuals, which included adults over the age of 65; any person with a chronic illness or a metabolic disorder; persons living in nursing homes or other long term care facilities, as well as health care personnel. At that time, pregnancy was not considered to be a high risk factor for severe illness or complications from influenza.  The ACIP committee stated in 1984 that, “Pregnancy has not been demonstrated to be a risk factor for severe influenza infection, except in the largest pandemics of 1918-1919 and 1957-1958.”111

However, in 1997, the CDC updated influenza vaccine recommendations to include pregnant women in their second or third trimester, which was considered an off-label use of the vaccine because the FDA had not licensed influenza vaccine for use by pregnant women.112 The CDC’s recommendation in 1997 was based on information contained in a small number of documents from the 1918-1919 and 1957-1958 influenza pandemics, along with a few case reports and small studies reporting an increase in influenza-related hospitalizations in pregnant women.113

Between 1999 and 2010, the ACIP annual recommendations for the seasonal flu vaccine quickly expanded to include more and more target populations. Infants and children, aged six months to 23 months, were added in 2004.114 The presence of thimerosal, a mercury-containing preservative present in all multi-vial flu vaccine vials, was discussed by the ACIP due to a 1999 recommendation for removal from all vaccines routinely administered to children.115 However, in 2004 the CDC stated that, “the benefits of influenza vaccination outweigh the theoretical risk, if any, for thimerosal exposure through vaccination,” and there was no recommendation made for infants, children or even pregnant women to receive a thimerosal-free influenza vaccine.116

By 2010, the ACIP’s recommendation was that every person six months and older, including pregnant women at any stage of pregnancy, should get an annual flu shot. The only contraindications were for persons with a history of hypersensitivity or anaphylaxis to eggs or any other influenza vaccine ingredient, or history of Guillain-Barre Syndrome (GBS).117

In 2011, the CDC began recommending that individuals who had previously developed hives following exposure to eggs should get influenza vaccine.118 By 2016, egg allergies were no longer considered a contraindication to flu vaccine.119

Currently, a history of GBS or a severe allergy to a vaccine component or history of a life threatening allergic reaction to a previous flu shot are the only CDC approved official contraindications (medical reasons for not getting vaccinated) to receiving influenza vaccine.120 However, the CDC also states that influenza vaccination should be postponed if a child or adult has “moderate or severe acute illness with or without a fever,” which is listed as a “precaution” for receipt of any vaccine.121

Despite the ever-growing number of influenza vaccines available in the U.S. market, influenza vaccination rates have not improved very much over the years. In 2003-2004, the CDC increased research efforts to determine just how well seasonal flu vaccine works in preventing cases of influenza in vaccinated persons.

Since 2004, the seasonal influenza vaccine has failed to prevent influenza in vaccinated persons more than half the time, demonstrating a low of 10 percent effectiveness in 2004/20015 to a high of 60 percent effectiveness in 2010/2011. The average effectiveness of influenza vaccines over the past 14 flu seasons was less than 41 percent.122

The Cochrane Collaboration’s 2014 review of the medical literature on influenza vaccine noted bias in the publication of influenza vaccine research on effectiveness and safety:

“An earlier review of 274 influenza vaccine studies in all age groups (including most of the studies in this review) showed an inverse relationship between risk of bias and the direction of study conclusions. Conclusions favourable to the use of influenza vaccines were associated with a higher risk of bias. In these studies, the authors made claims and drew conclusions that were unsupported by the data they presented. In addition, industry-funded studies are more likely to have favourable conclusions, to be published in significantly higher-impact factor journals and to have higher citation rates than non-industry-funded studies. This difference is not explained by either their size or methodological quality (Jefferson 2009a). Any interpretation of the body of evidence in this review should be made with these findings in mind.”123

The Cochrane review also concluded that recommendations for routine use of influenza vaccine as a routine public health measure was not supported by the published evidence base and stated,

The results of this review provide no evidence for the utilisation of vaccination against influenza in healthy adults as a routine public health measure. As healthy adults have a low risk of complications due to respiratory disease, the use of the vaccine may only be advised as an individual protective measure.” 124

The 2018 Cochrane review of influenza vaccination in healthy adults found that the flu vaccine may only have a modest impact on reducing the number of cases of influenza and influenza-like illness but the data was insufficient to determine whether vaccination had any impact on working days lost or on reducing serious complications of the flu during influenza season.125

Who should not get the Influenza (flu) vaccines?

Different influenza vaccines are licensed by the FDA and approved for use in different groups of people according to a person’s age, state of health and personal history of allergies or reactions to previous vaccinations. By federal law, pharmaceutical companies producing vaccines for release in the US. must publish information that accompanies vials of vaccine shipped to public health clinics and doctors’ offices that contain information about the vaccine’s ingredients, safety and effectiveness data from pre-licensure clinical trials, contraindications and precautions, reported vaccine adverse events, age use recommendations and more.

Prior to receiving an influenza vaccination or any vaccination, NVIC encourages consumers to read information contained in the vaccine manufacturer package insert carefully.

Note: There are certain influenza vaccines that are licensed for use by people in certain   age groups. For example, high-dose flu shots are not licensed for use by people under age 65 years and flu shots administered intradermally are not licensed for use by children under age 18 years. Refer to the specific vaccine’s product insert in Influenza Quick Facts for additional information.

According to most manufacturer’s package inserts for influenza vaccines, children younger than six months of age and people with severe, life-threatening allergies to influenza vaccine or any ingredient in the vaccine should not receive flu shots.

Most influenza vaccine package inserts with the exception of Flucelvax, using Madin Darby canine kidney cells for production, and Flublok, using armyworm cells for production, list an allergy to egg and egg protein as a contraindication to vaccination because most influenza vaccines are made using chicken eggs.

However, in 2016, the CDC’s Advisory Committee on Immunization Practices (ACIP) revised flu shot recommendations for people with egg allergies and stated that individuals with an allergy to egg can receive any type of flu vaccine, whether manufacturers use chicken eggs for production or not. CDC officials state that individuals who develop hives from egg products may be vaccinated without any special precautions, while those who have experienced a severe anaphylactic reaction (one involving respiratory distress, angioedema or use of epinephrine) should be monitored in a setting where there is a health care professional trained to recognize and quickly treat an anaphylactic reaction. While CDC officials state that allergic reactions can occur in individuals who are allergic to eggs, they consider it to be rare and not serious enough to warrant a contraindication.126 127

Currently, a history of GBS or a severe allergy to a vaccine component or history of a life threatening allergic reaction to a previous flu shot are the only CDC approved official contraindications (medical reasons for not getting vaccinated) to receiving inactivated influenza vaccines.128 However, the CDC also states that influenza vaccination should be postponed if a child or adult has “moderate or severe acute illness with or without a fever,” which is listed as a “precaution” for receipt of any vaccine. 129

Live Nasal Spray Flu Vaccine (FluMist): A new formulation of live FluMist nasal spray influenza vaccine will be available for the 2018-2019 flu season as an option when appropriate.130 The reintroduction of FluMist reverses a 2016 CDC decision recommending against the use of this vaccine which was determined to be ineffective at preventing influenza.131 FluMist is available in the U.S. and, due to the possibility of vaccinated persons shedding and transmitting live vaccine strain influenza virus to others, recently vaccinated persons are advised to avoid close contact with immune-compromised individuals for at least 21 days.

  • Individuals who should not get a live nasal spray influenza vaccine:
    • Children younger than two years old
    • Adults 50 years and older
    • People with a history of a severe allergic reaction to any component of the vaccine or to a previous dose of any influenza vaccine
    • People who are allergic to eggs
    • Children two years through 17 years of age who are receiving aspirin therapy or aspirin-containing therapy.
    • Pregnant women
    • People with weakened immune systems (immunosuppression)
    • Children two to four years old who have asthma or have had a history of wheezing in the past 12 months
    • People who have taken influenza antiviral drugs within the previous 48 hours
    • People who care for severely immunocompromised persons who require a protective environment (or otherwise avoid contact with those persons for 7 days after getting the nasal spray vaccine).

Individuals who should talk to their doctor before getting the live nasal spray flu vaccine:

There are also other “warnings and precautions” for the nasal spray flu vaccine. You should talk to your doctor if you have:

  • Asthma: Children and adults of any age with asthma might be at increased risk for wheezing after getting the nasal spray flu vaccine.
  • A chronic condition like lung disease, heart disease, kidney or liver disorders, neurologic/neuromuscular, or metabolic disorders. The safety of the live nasal spray flu vaccine has not been established in people with underlying medical conditions that place them at high risk of serious flu complications. See People at High Risk of Developing Flu–Related Complications.
  • If you ever had GBS (a severe paralyzing illness). Talk to your doctor about your GBS history.
  • If you have gotten any other vaccines in the past 4 weeks, or if you are not feeling well.

Can influenza be prevented and are there treatment options?

To decrease the risk of becoming infected with influenza or other viruses, it is important to avoid contact with people who are ill. Frequently washing hands with soap and water or using alcohol-based hand rubs helps to reduce the risk of infection. Covering your nose and mouth with a tissue and properly disposing of it, as well as keeping surfaces at home, work and school clean can also help to prevent spread of influenza and other types of infections.132

Eating a well-balanced diet, reducing stress, staying properly hydrated and getting enough sleep, Vitamin D and exercise can help reduce the risk of becoming sick.133 134

When you are sick, it is important to stay away from others until they are well to limit your risk of becoming sick, too. The CDC recommends staying home for at least 24 hours after a fever is gone (without the use of medications to reduce fever) except for medical services or other necessary outings.135

Treatments for influenza may include options such as natural home remedies or physician prescribed medications.136 137 138 139 NVIC encourages all consumers to carefully research the potential risks and benefit of any treatment option being considered in order to make educated decisions.

Who is at highest risk for suffering complications from influenza?

The CDC lists the following persons as being at increased risk for complications from influenza: 140

  • Asthma;
  • Neurological and neurodevelopmental conditions including disorders of the brain, spinal cord, peripheral nerve, and muscle such as cerebral palsy, epilepsy (seizure disorders), stroke, intellectual disability (mental retardation), moderate to severe developmental delay, muscular dystrophy, or spinal cord injury;
  • Chronic lung disease (such as chronic obstructive pulmonary disease COPD and cystic fibrosis);
  • Heart disease(such as congenital heart disease, congestive heart failure and coronary artery disease);
  • Blood disorders (such as sickle cell disease);
  • Endocrine disorders (such as diabetesmellitus);
  • Kidney disorders;
  • Liver disorders;
  • Metabolic disorders (such as inherited metabolic disorders and mitochondrial disorders);
  • Weakened immune system due to disease or medication (such as people with HIV or AIDS, or cancer, or those on chronic steroids);
  • People younger than 19 years of age who are receiving long-term aspirin therapy;
  • People who are morbidly obese (body mass index more than 40) Calculate Body Mass Index or BMI here.

What is influenza (flu) vaccine? 

There are many different kinds of influenza vaccines available in the U.S. NVIC encourages consumers to read the vaccine manufacturer's package insert information carefully before receiving influenza vaccine or any vaccine.

Standard-Dose Inactivated Flu Vaccine –

  • The most common flu vaccine is the inactivated (killed) influenza vaccine, which is prepared from the fluids of chick embryos inoculated with a specific type(s) of influenza virus. 141 The strains of flu virus in the vaccine are inactivated with formaldehyde and preserved with thimerosal, which is a mercury derivative.142 (There is a limited supply of thimerosal-free or influenza vaccine with trace amounts of thimerosal and it is supplied in single dose vials, which do not require a preservative). This type of inactivated influenza vaccine is administered by injection into the muscle and contains either three (trivalent) or four (quadrivalent) influenza virus type A and type B strains.143 One trivalent flu vaccine, Afluria, is given by jet injector, a medical device that uses high pressure to administer the vaccine.144 145

High-Dose Trivalent Flu Vaccines -

  • The high-dose trivalent influenza vaccine, Fluzone, is approved for adults age 65 and older and contains four times the amount of antigen than the standard flu vaccine.146 This vaccine is designed to hyper-stimulate the immune system to produce a stronger antibody response in the elderly.147 High-dose Fluzone is the only high-dose trivalent flu vaccine currently available in the U.S.

Recombinant Flu Vaccines -

  • The recombinant flu vaccine, approved for use in 2013, is manufactured through genetic engineering.148 It is produced using insect (armyworm) cells. Flublok is the only recombinant flu vaccine currently licensed in the U.S. and one of two influenza vaccines using alternatives to chicken eggs for production.149,150

Cell-Based Flu Vaccines -

  • Cell-Based flu vaccines differ from standard egg-based flu vaccines because animal cells are used to grow the influenza virus.151 Currently only one cell-based flu vaccine, Flucelvax, is available in the U.S. Licensed in 2012, it is prepared from influenza virus grown in Madin Darby canine kidney cells. This vaccine is approved for use by adults and children age 4 and older.152

Trivalent Flu Vaccine, Adjuvanted -

  • In 2015, the FDA approved Fluad, the first adjuvanted trivalent flu vaccine containing a squalene oil adjuvant (MF59).153 This vaccine was approved for fast track licensure by the FDA, despite limited data on safety and immunogenicity, with approval based on a single clinical trial of about 1,000 healthy adults over the age of 65. 154 This vaccine will be available for the first time to adults over the age of 65 for the 2016-2017 flu season.155

Intradermal Flu Vaccine –

  • The intradermal flu vaccine was approved for use in 2012. It is an inactivated injectable influenza vaccine that is injected into the skin instead of the muscle.156 There is one quadrivalent intradermal flu vaccine available, Fluzone Intradermal, and it is licensed for use by adults aged 18 to 64. This vaccine is prepared from influenza viruses propagated in chicken embryos (unhatched baby chicks) and inactivated by formaldehyde. The intradermal flu vaccine does not contain thimerosal.157

Nasal-Spray Flu Vaccine FluMist -

  • A live-virus nasal flu vaccine, FluMist, was licensed by the FDA in June 2003 and approved use was limited to healthy people between the ages of five and 49.158 It was subsequently approved by the FDA for use in children as young as two years of age but with precautions.159 In 2014, CDC officials recommended FluMist be the influenza vaccine given to children between two and eight years old but, in 2016, the CDC recommended that FluMist not be given to children or adults of any age and the stated reason was that the vaccine was ineffective.160 However, in February 2018, the CDC approved a new formulation of FluMist as an option for the 2018-2019 flu season despite a lack of vaccine efficacy studies.161 FluMist is prepared by introducing influenza viruses into eggs where they multiply. FluMist is a live virus vaccine and does not contain any preservatives.162

The majority of Influenza vaccines were initially designated as Category B or C pharmaceutical products. This means that adequate and well-controlled studies on pregnant women were not conducted prior to licensure of influenza vaccines and it is not known whether the vaccines can cause fetal harm when administered to a pregnant woman or can affect fertility and the reproduction capacity of a woman. 

However, in 2015, the FDA removed pregnancy categories, which were replaced it with a Pregnancy and Lactation Labeling Rule.163 This rule affects all influenza vaccine products submitted after June 30, 2015. As new language is phased in, information on risks associated with vaccinating while pregnant will appear in 8.1 of each vaccine’s product insert under Risk Summary. NVIC encourages pregnant women to read this information carefully prior to receiving influenza vaccine or any other vaccine.

Below are links to the U.S. Food & Drug Administration’s (FDA) website for the most current legally-required licensing information published in manufacturer product package inserts for influenza vaccines available in the U.S.  It is important to understand and read this information carefully prior to receiving a vaccine. Vaccine product package inserts contain important information about ingredients, contraindications, precautions, reported adverse reactions, safety and effectiveness data from pre-licensure clinical trials, use recommendations and more.

Quadrivalent Vaccines - Nasal

  • FluMist by MedImmune, LLC

Quadrivalent Vaccines – Injected

  • AFLURIA QUADRIVALENT by Seqirus Pty Ltd.
  • Fluarix Quadrivalent by GlaxoSmithKline Biologicals
  • Flucelvax Quadrivalent by Seqirus, Inc.
  • FluLaval Quadrivalent by ID Biomedical Corporation of Quebec
  • Fluzone by Sanofi Pasteur Inc.

Trivalent Vaccines - Nasal

  • FluMist by MedImmune, LLC

Trivalent Vaccines - Injected

  • AFLURIA by Seqirus Pty Ltd.
  • FluLaval by ID Biomedical Corporation of Quebec
  • Fluarixby GlaxoSmithKline Biologicals
  • FLUAD by Seqirus, Inc.
  • Flublokby Protein Sciences Corporation
  • Flucelvaxby Seqirus, Inc.
  • Fluvirin by Seqirus Vaccines Limited
  • Fluzoneby Sanofi Pasteur, Inc.

How effective is influenza vaccine?

Like all vaccines, the influenza vaccine only gives a temporary artificial immunity and, in the case of flu shots, that temporary artificial immunity is confined to the influenza virus strains contained in the vaccine. The only way to get more complete and longer lasting immunity to a strain of type A or B influenza is to recover from the illness. Natural immunity to a particular strain of influenza can be protective against severe illness symptoms if that strain or a closely related strain circulates in the future.

However, the flu vaccine only provides temporary immunity to selected type A and B strains and those strains may or may not be prevalent each year. Since 2010, U.S. public health officials have directed doctors to give every American over six months old a flu shot every year, whether or not they are healthy or at high risk for influenza complications.164 165

Every year, public health officials at the World Health Organization (WHO) and U.S. Centers for Disease Control and Prevention (CDC) try to guess which three or four influenza strains are most likely to be circulating and causing illness in the U.S. the following year to determine which strains will be included in vaccine manufactured for the upcoming flu season. Despite elaborate influenza monitoring systems by the WHO and U.S.166 and scientists worldwide to select the strains to be targeted in the following year’s flu vaccine, studies by the U.S. Flu Vaccine Effectiveness Network have shown that, since 2005, the flu vaccine has never been more than 60 percent effective.

The CDC reported in February 2018 that between 2004/2005 and 2017/2018, the influenza vaccine was less than 50 percent effective in ten out of 14 flu seasons. In the 2014-2015 flu season, the influenza vaccine was only 19 percent effective.167

In the fall of 2017, scientists reported that the H3N2 influenza virus strain mutated during the 2014-2015 season and, although the 2016-2017 seasonal flu vaccine was updated to include the mutated strain, another mutation occurred that season in manufacturing labs when the H3N2 strain was grown in chicken eggs to produce the vaccine. There is emerging evidence that H3N2 influenza viruses cannot be grown in chicken eggs without adaptive mutations occurring.168 The 2017/2018 influenza vaccine reportedly was only 10 percent effective in the southern hemisphere because the mutated H3N2 strain was the one making most people sick but it was not in the vaccine.169

A 2005 study on the impact of flu vaccination on mortality in adults 65 years of age and older determined that an increase in flu vaccine coverage after 1980 in senior adults had no impact on reducing mortality rates in any age group.170 A 2011 review of existing research determined that the inactivated influenza vaccine had a pooled efficacy of 59 percent for adults 18 to 65 years of age for eight out of 12 seasons. Similar data for inactivated influenza vaccine for adults over 65 years of age and children between two and 17 years of age was lacking and require additional study. This same review found that live-attenuated influenza vaccine (LAIV), had a pooled efficacy of 83 percent in nine of the 12 seasons analyzed for children aged six months to seven years. Similar data for LAIV efficacy for children aged eight to 17 years was lacking and requires additional study.171 

In 2018, the Cochrane Collaboration published a review of medical literature on the effects of the influenza vaccination in the elderly and concluded that:

“The available evidence relating to complications is of poor quality, insufficient, or old and provides no clear guidance for public health regarding the safety, efficacy, or effectiveness of influenza vaccines for people aged 65 years or older.”172

In 2016, CDC officials advised against use of the live attenuated nasal influenza vaccine based on effectiveness data collected between 2013 and 2016 that showed LAIV vaccine to be only three percent effective against any influenza virus in children ages two to 17.173 However, in February 2018, CDC officials voted to approve that a new formulation of LAIV be added as an option when appropriate for the upcoming 2018-2019. Vaccine efficacy studies on whether or not this new formulation of FluMist have not been completed and this approval was based on data analysis provided by the manufacturer.174
The Cochrane Collaboration’s 2014 review of the medical literature on influenza vaccine noted bias in the publication of influenza vaccine research on effectiveness and safety:

“An earlier review of 274 influenza vaccine studies in all age groups (including most of the studies in this review) showed an inverse relationship between risk of bias and the direction of study conclusions. Conclusions favourable to the use of influenza vaccines were associated with a higher risk of bias. In these studies, the authors made claims and drew conclusions that were unsupported by the data they presented. In addition, industry-funded studies are more likely to have favourable conclusions, to be published in significantly higher-impact factor journals and to have higher citation rates than non-industry-funded studies. This difference is not explained by either their size or methodological quality (Jefferson 2009a). Any interpretation of the body of evidence in this review should be made with these findings in mind.” 175

The Cochrane review also concluded that recommendations for routine use of influenza vaccine as a routine public health measure was not supported by the published evidence base and stated,

The results of this review provide no evidence for the utilisation of vaccination against influenza in healthy adults as a routine public health measure. As healthy adults have a low risk of complications due to respiratory disease, the use of the vaccine may only be advised as an individual protective measure.” 176

The 2018 Cochrane review of influenza vaccination in healthy adults found that the flu vaccine may only have a modest impact on reducing the number of cases of influenza and influenza-like illness but the data was insufficient to determine whether vaccination had any impact on working days lost or on reducing serious complications of the flu during influenza season.177

Can influenza vaccine cause injury and death?

The most common influenza vaccine reactions, which begin within 12 hours of vaccination and can last several days are: fever, fatigue, painful joints and headache.

One of the most serious documented influenza vaccine reactions is Guillain-Barre Syndrome (GBS). 178  An immune mediated painful and disabling neurological disorder that can occur after viral infection or vaccination, GBS involves inflammation of the peripheral nervous system and can cause temporary or permanent paralysis that may lead to death. 179 GBS usually develops within two to four weeks of vaccination.

Characterized by muscle weakness, unsteady gait, numbness, tingling, pain, GBS can cause paralysis of the face or one or more limbs. It can take several months for recovery or leave the affected person with chronic health problems and disability.180 The mortality rate with GBS occurs mainly in the elderly and those who develop severe complications.181

Brain and nerve disorders such as brain inflammation and encephalopathy, Acute Disseminated Encephalomyelitis (ADEM), optic neuritis, partial facial paralysis, and brachial plexus neuropathy, as well as vasculitis, have also been reported following influenza vaccinations. There have been few biological mechanism studies conducted and public health officials state that a causal relationship between influenza vaccine and GBS has not been definitively established. 182The CDC states that: 183

“Some studies have found a possible small association of injectable flu vaccine with Guillain-Barré syndrome (GBS). Overall, these studies estimated the risk for GBS after vaccination as fewer than 1 or 2 cases of GBS per one million people vaccinated. Other studies have not found any association. GBS also, rarely, occurs after flu illness. Even though GBS following flu illness is rare, GBS is more common following flu illness than following flu vaccination. GBS has not been associated with the nasal spray vaccine.”

Adult influenza vaccine injury claims are now the leading claim submitted to the federal Vaccine Injury Compensation Program (VICP), with GBS as the leading alleged injury.184 185

In 2015, a published Canadian study186 found that the effectiveness of the flu vaccine decreased when a person received the shot two years in a row. As a result of these negative findings that implied the vaccine increased a person’s susceptibility to influenza, study authors recommended further research be completed.187

In 2017, a Centers for Disease Control (CDC) funded study reported that women vaccinated in the early part of their pregnancy with a flu vaccine containing the pandemic H1N1 (H1N1pdm09) strain and who also had been previously vaccinated the prior season with a  H1N1pdm09-containing flu vaccine had a greater risk of miscarriage in the first four weeks following vaccination. The CDC is in the process of conducting further research among women who were pregnant during the 2012-2013 through 2014-2015 flu season and eligible to receive the flu vaccine. Results of this study are expected in late 2018 or 2019.188

Who is at highest risk for complications from influenza vaccine?

The Institute of Medicine has acknowledged that there is individual susceptibility to vaccine reactions for genetic, biological and environmental reasons but that vaccine providers cannot accurately predict prior to a vaccine’s administration who will suffer complications, injury or death from vaccination. 189 However, a person who has previously had a serious reaction to a vaccination or is acutely or chronically ill should become informed about all potential risks associated with vaccination and discuss any concerns with a trusted health care professional before receiving influenza vaccine or any other vaccine.

Currently, a history of GBS or a severe allergy to a vaccine component or history of a life threatening allergic reaction to a previous flu shot are the only CDC approved official contraindications (medical reasons for not getting vaccinated) to receiving inactivated influenza vaccines.190 However, the CDC also states that influenza vaccination should be postponed if a child or adult has “moderate or severe acute illness with or without a fever,” which is listed as a “precaution” for receipt of any vaccine. 191

Is Flu Vaccine Recommended for Children? 

One consideration with the recommended widespread administration of annual flu shots to all children over six months of age is interference with the acquisition influenza antibodies acquired after recovery from type A or B influenza infections. The question of whether it is better for healthy children, who rarely suffer complications from flu, to get the flu and develop permanent immunity to that flu strain or it is better for children to get vaccinated every year to try to suppress all flu infection in early childhood is a question that has yet to be adequately answered by medical science.

Although in the past the flu vaccine has not been recommended for healthy children, today vaccination of children between the ages of 6 months and 18 years is strongly recommended by the Advisory Committee on Immunization Practices (ACIP) of the CDC192 and New Jersey now requires influenza vaccine for daycare and kindergarten entry.193

Is influenza vaccine safe during pregnancy?

In years past, pregnancy was also a contraindication to flu vaccine but, today, the Advisory Committee on Immunization Practices (ACIP) of the Centers for Disease Control (CDC) recommends flu vaccine for all pregnant women.194

Initially, most Influenza vaccines were classified as Category B or C drugs, which means that adequate and well-controlled studies on pregnant women have not been conducted and it is not known whether these vaccines can cause fetal harm when administered to a pregnant woman or if they can affect reproduction capacity.195 

In 2015, the FDA removed pregnancy categories due to concerns of confusion and oversimplification and replaced it with the Pregnancy and Lactation Labeling Rule.196 This rule affects all influenza vaccine products submitted for approval after June 30, 2015. As new language is phased in, information on risks associated with vaccinating while pregnant will appear in 8.1 of each vaccine’s product insert under Risk Summary. NVIC encourages consumers read this information carefully prior to receiving a vaccine.

Prior to the FDA licensing of all influenza vaccines, drug companies did not test the safety and effectiveness in pregnant women197 and little data is available on biological responses to these vaccines that could affect pregnancy and birth outcomes.198

Pregnant women should also be aware that the multi-dose flu vaccine contains Thimerosal, which is a mercury derivative. Mercury is toxic to the brain and has been found to be associated with brain damage and developmental delays in babies whose mothers were exposed to high levels of mercury during pregnancy.199,200

In December 2016, Congress signed the 21st Century Cures Act into law. This new law protects vaccine manufacturers from lawsuits in civil court if an FDA licensed vaccine given to a pregnant woman causes the injury or death of her unborn child in the womb.201 202 As a result, policies relating to compensating vaccine injuries sustained by an unborn child in the womb are being developed.

In 2017, a Centers for Disease Control (CDC) funded study reported that women vaccinated in the early part of their pregnancy with a flu vaccine containing the pandemic H1N1 (H1N1pdm09) strain who also had been previously vaccinated during the prior flu season with a  H1N1pdm09-containing flu vaccine had a greater risk of miscarriage in the first four weeks following vaccination. The CDC is in the process of conducting further research among women who were pregnant during the 2012-2013 through 2014-2015 flu season and eligible to receive the flu vaccine. Results of this study are expected in late 2018 or 2019203. Despite concerns regarding the safety of flu vaccination during pregnancy, the CDC continues to recommend the flu vaccine for all pregnant women at any point during pregnancy204.

What about mercury in the influenza vaccine?

In 1999, the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA)205 directed the vaccine manufacturers to take mercury out of all childhood vaccines.

In October 2001, the Institute of Medicine issued a report that said it is "biologically plausible" that mercury-containing vaccines could cause injury to the brain but there have been “too few scientific studies conducted to prove conclusively that mercury in vaccines has caused brain damage.”206

Nevertheless, the Institute of Medicine recommended that drug companies take all mercury out of all vaccines and over-the-counter drugs.

In compliance with this recommendation a preservative-free vaccine formulated for children ages 6 to 35 months, with only a trace amount of thimerosal, is available in a limited amount. It is distinguished by a pink syringe plunger rod in the pre-filled syringe or in a single dose vial.  All multi-dose vials of influenza vaccine contain thimerosal.

Many influenza vaccines in the U.S. also contain the mercury preservative, Thimerosal, in amounts above federal safety guidelines. Thimerosal free influenza vaccine is also licensed in the U.S. and it is advisable to request these vaccines in advance from your healthcare provider, if your preference is the Thimerosal free version. Click here to determine which vaccines are thimerosal free.

What questions should I ask my doctor about the influenza vaccine?      

NVIC’s If You Vaccinate, Ask 8! webpage downloadable brochure suggests asking eight questions before you make a vaccination decision for yourself, or for your child. If you review these questions before your appointment, you will be better prepared to ask your doctor questions.  Also make sure that the nurse or doctor gives you the relevant Vaccine Information Statement (VIS) for the vaccine or vaccines you are considering well ahead of time to allow you to review it before you or your child gets vaccinated. Copies of VIS for each vaccine are also available on the CDC's website and there is a link to the VIS for influenza vaccine on NVIC's “Quick Facts” at the top of this page. 

Due to the brevity of the VIS, it is also a good idea to read the vaccine manufacturer product insert that can be obtained from NVIC’s Influenza Quick Facts above, doctor or public health clinic to get additional information.  Federal law requires drug companies marketing vaccines to include certain kinds of vaccine benefit, risk and use information in product information inserts that may not be available in other published information, like the VIS.  

Other questions that may be useful to discuss with your doctor before getting the influenza vaccine are: 

  • If other vaccines in addition to influenza vaccine are scheduled for my child at this office visit, am I allowed to modify the schedule so fewer vaccines are given at once?
  • What should I do if my child has a high fever or appears very ill after vaccination?
  • What other kinds of reaction symptoms should I call to report after influenza vaccination?
  • If the influenza vaccine doesn’t protect my child, do I have any other options for preventing influenza infection?

It also is important to be able to recognize a vaccine reaction and seek immediate medical attention if the reaction appears serious, as well as know how to make a vaccine reaction report to federal health officials at the Vaccine Adverse Reporting System (VAERS). NVIC’s Report  Vaccine Reactions—It’s the Law webpage can help you file a vaccine reaction report yourself to VAERS if your doctor fails or refuses to make a report.

NVIC's Influenza Commentaries and Video Collection

NVIC Influenza Video Playlist

View the collection of video resources within the player below for more information on influenza and the influenza vaccine.

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72 CDC. Influenza – Flu symptoms & Complications. Jun. 5, 2018

73  CDC. People at High Risk of Developing Flu–Related Complications. Jan. 23, 2018

74 WHO. Influenza (Seasonal) Jan. 31, 2018

75 CDC. Frequently Asked Flu Questions 2016-2017 Influenza Season. Sept. 11, 2017

76 Dennis B. Flu vaccines: a mixture of hard science and good fortune. The Guardian Jan. 18, 2015.

77 Potter CW. A History of Influenza. J. Appl. Microbiol 2001; 91(4): 572-579.

78  CDC. Influenza. Epidemiology and Prevention of Vaccine-Preventable Diseases (The Pink Book). 13th ed. 2015.

79 Hannoun C. The Evolving History of Influenza Viruses and Influenza Vaccines. Expert Rev Vaccines 2013;12(9):1085-1094.

80 Biondi E.A , Aligne A.C. Flu Vaccine for All: A Critical Look at the Evidence. Medscape Pediatrics Dec. 21, 2015.

81 Hannoun C. The Evolving History of Influenza Viruses and Influenza Vaccines. Expert Rev Vaccines 2013;12(9):1085-1094.

82 Stanley WM. The Preparation and Properties of Influenza Virus Vaccines Concentrated and Purified by Differential Centrifugation. J Exp Med 1945; 81(2): 193–218.

83 Hampson AW. Vaccines for Pandemic Influenza. The History of our Current Vaccines, their Limitations and the Requirements to Deal with a Pandemic Threat. Ann Acad Med Singapore 2008; 37:510-517.

84 Biondi E.A , Aligne A.C. Flu Vaccine for All: A Critical Look at the Evidence. Medscape Pediatrics Dec. 21, 2015.

85 Henderson DA, Courtney B et al. Public health and medical responses to the 1957-58 influenza pandemic. Biosecur Bioterror 2009;7:265-273.

86 Langmuir AD, Henderson DA, Serfling RE. The epidemiological basis for the control of influenza. Am J Public Health 1964; 54:563-571.

87 Schoenbaum SC, Mostow SR et al. Studies with inactivated influenza vaccines purified by zonal centrifugation. 2. Efficacy. Bull World Health Organ 1969; 41:531-535. 

88 Sencer DJ, Millar JD.  Reflections on the 1976 Swine Flu Vaccination Program. Emerg Infect Dis 2006; 12(1): 29–33.

89 Neustadt RE, Fineberg HV. The Swine Flu Affair: Decision-Making on a Slippery Disease. Washington (DC): National Academies Press (US) 1978.

90 Dowdle WR. The 1976 Experience. J Infect Dis 1997; 176 (Supplement 1): S69-S72.

91 Hannoun C. The Evolving History of Influenza Viruses and Influenza Vaccines. Expert Rev Vaccines 2013;12(9):1085-1094.

92 FDA. June 17, 2003 Approval Letter - Influenza Virus Vaccine Live, Intranasal. June 2003.

93 CDC. Using Live, Attenuated Influenza Vaccine for Prevention and Control of Influenza: Supplemental Recommendations of the Advisory Committee on Immunization Practices (ACIP) MMWR Sept. 26, 2003, 52(RR13);1-8

94 Mallory RM, Yi T, Ambrose CS. Shedding of Ann Arbor strain live attenuated influenza vaccine virus in children 6-59 months of age. Vaccine 2011; 29(26): 4322-4327.

95  Fisher BL. The Emerging Risk of Live Virus and Virus Vectored Vaccines: Vaccine Strain Virus Infection, Shedding and Transmission. National Vaccine Information Center November 2014.

96 The White House. Press Briefing On Swine Influenza. Apr. 26, 2009.

97 Griffen MR, Monto AS et al. Effectiveness of Non-Adjuvanted Pandemic Influenza A Vaccines for Preventing Pandemic Influenza Acute Respiratory Illness Visits in 4 U.S. Communities. PloS One2011; 6(8): e23085.

98 FDA. Approval Letter - Fluzone High-Dose. Dec. 23, 2009.

99 CDC. Fluzone High-Dose Seasonal Influenza Vaccine. Dec. 14, 2017.

100 Lowes R. First Quadrivalent Flu Vaccine Approved, Debuts Fall 2013. Medscape Medical News Mar 1, 2012.

101 FDA. Flucelvax Package Insert – Seqiris, Inc. April 2016.

102 CDC. Cell-Based Flu Vaccines. Nov 7, 2016.

103 CDC. Flublok Seasonal Influenza (Flu) Vaccine. Oct. 3, 2017.

104  Stobbe M. Ouch! Flu spray fails again, panel urges shot instead. Associated Press June 22, 2016.

105 CDC.  ACIP votes down use of LAIV for 2016-2017 flu season. June 22, 2016.

106 Walker, M. ACIP Reinstates FluMist for 2018-2019 Flu Season. MEDPAGE TODAY Feb. 21, 2018

107 CDC. FLUAD™ Flu Vaccine With Adjuvant. July 11, 2016.

108 NVIC Questions FDA Fast Tracking of Squalene Adjuvanted Flu Vaccine The Vaccine Reaction Sept. 16, 2016.

109 CDC. Intradermal Influenza (Flu) Vaccination. Dec. 14, 2017

110 CDC. Flu Vaccination by Jet Injector. Oct. 12, 2017

111 CDC. Recommendation of the Immunization Practices Advisory Committee (ACIP) Prevention and Control of Influenza MMWR May 18, 1984; 33(19).

112  Fisher BL. Vaccination During Pregnancy: Is It Safe? National Vaccine Information Center Nov. 9, 2013.

113 CDC. Prevention and Control of Influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Apr. 25, 1997; 46(RR-9):1-25.

114 CDC. Prevention and Control of Influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR May 28, 2004; 53(RR06);1-40.

115 CDC. Recommendations Regarding the Use of Vaccines That Contain Thimerosal as a Preservative. MMWR Nov. 5, 1999; 48(43):996-998.

116 CDC. Prevention and Control of Influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR May 28, 2004; 53(RR06);1-40.

117 CDC. Prevention and Control of Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR Aug. 6, 2010; 59(RR08):1-62.

118 CDC. Prevention and Control of Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2011. MMWR Aug. 26, 2011; 60(33):1128-1132.

119 CDC. Flu Vaccine and People with Egg Allergies. Dec. 28, 2017

120  CDC. Seasonal Influenza Vaccine Safety - A Summary for Clinicians: Contraindications and Precautions. Oct. 3, 2017.

121  CDC. Vaccine Recommendations and Guidelines of the ACIP: Contraindications and Precautions. July 18, 2018

122 CDC. Seasonal Influenza Vaccine Effectiveness 2005-2018. Feb. 15, 2018.

123 Demicheli V, Jefferson T et al. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews Mar. 13, 2014. 

124 Ibid.

125 Demicheli V, Jefferson T et al. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews. Feb 1, 2018

126 CDC. Prevention and Control of Seasonal Influenza with Vaccines Recommendations of the Advisory Committee on Immunization Practices — United States, 2016–17 Influenza Season MMWR Aug. 26, 2016; 65(5):1–54.

127 CDC. Flu Vaccine and People with Egg Allergies Dec. 28, 2017

128  CDC. Seasonal Influenza Vaccine Safety - A Summary for Clinicians: Contraindications and Precautions. Oct. 3, 2017.

129  CDC. Vaccine Recommendations and Guidelines of the ACIP: Contraindications and Precautions. July 18, 2018

130 Walker, M. ACIP Reinstates FluMist for 2018-2019 Flu Season. MEDPAGE TODAY Feb. 21, 2018

131 CDC. ACIP votes down use of LAIV for 2016-2017 flu season. Jun. 22, 2016

132 CDC. Personal NPIs: Everyday Preventive Actions Aug. 2, 2017

133 Mercola.com. How to Prevent the Flu -- as Easy as 1, 2, 3... Dec. 9, 2009.

134 Urashima M, Segawa T et al. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren Am J Clin Nutr May 2010; 91(5):1255-1260.

135 CDC. Nonpharmaceutical Interventions (NPIs) – At Home. Feb. 6, 2018

136 WebMd. Home Remedies for Fast Flu Relief. Nov. 3, 2016.

137 DerSarkissian C. 12 Natural Treatment Tips for Colds and Flu. WebMd Jun. 14, 2017

138 Malerba L. Homeopathy: A Time-Tested Treatment for the Flu. The Huffington Post May 25, 2011.

139 CDC. What You Should Know About Flu Antiviral Drugs. Jun. 20, 2018.

140 CDC. People at High Risk of Developing Flu–Related Complications. Jan. 23, 2018.

141 CDC. How Influenza (Flu) Vaccines Are Made. Jun. 27, 2018.

142 CDC. Thimerosal in Flu Vaccine. Nov. 2, 2017.

143 CDC. Quadrivalent Influenza Vaccine. Dec. 14, 2017.

144 CDC  Flu Vaccination by Jet Injector Oct. 12, 2017.

145 FDA.  Afluria Package Insert Mar. 14, 2017

146 CDC. Fluzone High-Dose Seasonal Influenza Vaccine Dec. 14, 2017.

147 Simonsen L, Reichart T, Viboud C et al., Impact of Influenza Vaccination on Seasonal Mortality in the US Elderly Population Arch Intern Med 2005;165(3):265-272.

148 CDC. Recombinant Flu Vaccines. Jun. 27, 2018.

149 CDC. Flublok Seasonal Influenza (Flu) Vaccine. Oct. 3, 2017.

150 FDA. Flublok Quadrivalent Influenza Vaccine Package Insert. Jun. 29, 2018

151 CDC. Cell-Based Flu Vaccines. Nov. 7, 2016.

152 FDA. Flucelvax Package Insert. April 2016.

153 FDA. FLUAD Influenza Vaccine Package Insert. March 2017.

154 NVIC Questions FDA Fast Tracking of Squalene Adjuvanted Flu Vaccine The Vaccine Reaction Sept. 16, 2016.

155 CDC. FLUAD™ Flu Vaccine With Adjuvant. July 11, 2016.

156 CDC. Intradermal Influenza (Flu) Vaccination. Dec. 14, 2017.

157 FDA. Fluzone Quadrivalent Intradermal Influenza Vaccine Package Insert. May 10, 2016.

158 FDA. June 17, 2003 Approval Letter - Influenza Virus Vaccine Live, Intranasal. June 17, 2003.

159 FDA. September 19, 2007 Approval Letter - Influenza Virus Vaccine Live, Intranasal. Sept. 19, 2007.

160 CDC. ACIP votes down use of LAIV for 2016-2017 flu season. June 22, 2016.

161 Walker, M. ACIP Reinstates FluMist for 2018-2019 Flu Season. MEDPAGE TODAY Feb. 21, 2018

162 FDA. FluMist Quadrivalent Influenza Vaccine Package Insert. Mar. 17, 2017.

163 FDA. Pregnancy and Lactation Labeling (Drugs) Final Rule. Jun. 28, 2018.

164   CDC Prevention and Control of Influenza with Vaccines – Recommendations of the Immunization Practices Advisory Committee (ACIP), 2010.  MMWR Aug. 6, 2010. 59(RR08);1-62

165 Mayo Clinic Flu shot: Your best bet for avoiding influenza. Sept. 19, 2017.

166 World Health Organization. Global Influenza Surveillance and Response System (GISRS). 2018.

167 CDC. Seasonal Influenza Vaccine Effectiveness, 2005-2018. Table: Adjusted Vaccine Effectiveness for Influenza Seasons from 2005-2018. Feb. 15, 2018.

168  Perelman School of Medicine. H3N2 mutation in last year’s flu vaccine responsible for lowered efficacy. Medical Xpress Nov. 6, 2017.

169  Dwyer D. Q&A: Why the flu vaccine isn’t always effective. Boston Globe Dec. 7, 2017.

170 Simonsen L, Reichert TA, Viboud C, et al. Impact of Influenza Vaccination on Seasonal Mortality in the US Elderly Population. Arch Intern Med 2005;165(3):265-272.

171 Osterholm MT, Kelley NS et al. Efficacy and effectiveness of influenza vaccines: a systematic review and meta-analysis. Lancet Infect Dis 2012;12:36-44.

172 Demicheli V, Jefferson T, et al. Vaccines for preventing influenza in the elderly. Cochrane Database of Systematic Reviews. Feb. 1, 2018

173 CDC. ACIP votes down use of LAIV for 2016-2017 flu season. June 22, 2016

174 Schnirring, L. CDC vaccine panel brings back FluMist for 2018-19 season. CIDRAP Feb. 21, 2018

175 Demicheli V, Jefferson T et al. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews Mar. 13, 2014.

176 Ibid.

177 Demicheli V, Jefferson T et al. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews. Feb 1, 2018

178  Kwong JC, Vasa PP et al. Risk of Guillain Barre Syndrome after seasonal influenza vaccinations and influenza health care encounters: a self-controlled study. The Lancet Infectious Diseases 2013; 13(9): 769-776.

179  GBS Flu Vaccine Reaction Leaves Nurse A Quadriplegic. National Vaccine Information Center Oct. 31, 2011.

180 CDC. Guillain-Barré syndrome and Flu Vaccine. Nov. 2, 2017.

181 Van den Berg B, Bunschoten C et al. Mortality in Guillain-Barré Syndrome. Neurology 2013; 80(18): 1650-1654

182 Institute of Medicine Committee to Review Adverse Effects of Vaccines. Adverse Effects of Vaccines Evidence and Causality. Influenza Vaccine. pp 293-420.  Washington, D.C. The National Academies Press 2012.

183  CDC. Flu Vaccine Safety Information: Do flu vaccines cause any side effects? Oct. 5, 2017.

184 Department of Health and Human Services. Advisory Committee on Childhood Vaccines (ACCV) Meeting. Mar. 3, 2011.

185 HRSA. Vaccine Injury Compensation Program Data and Statistics. Aug 8, 2018

186 Skowronski DM, Chambers C et al. Interim estimates of 2014/15 vaccine effectiveness against influenza A(H3N2) from Canada's Sentinel Physician Surveillance Network, January 2015. Euro Surveill. 2015 Jan 29;20(4).

187 Yourex-West H, McIntosh S, Canadian study finds flu shot could increase risk of getting sick. Global News. Jan. 30, 2015.

188 CDC. Flu Vaccination & Possible Safety Signal. Sep. 13, 2017

189  Institute of Medicine Committee to Review Adverse Effects of Vaccines. Adverse Effects of Vaccines: Evidence and Causality: Evaluating Biological Mechanisms of Adverse Events (p. 57-102), Increased Susceptibility (p. 82). Washington, DC: The National Academies Press 2012.

190  CDC. Seasonal Influenza Vaccine Safety - A Summary for Clinicians: Contraindications and Precautions. Oct. 3, 2017.

191  CDC. Vaccine Recommendations and Guidelines of the ACIP: Contraindications and Precautions. July 18, 2018.

192 CDC. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2016–17 Influenza Season. MMWR Aug. 26, 2016; 65(5);1–54.

193 Alfonsi S. N.J. Mandatory Flu Shots for Preschoolers Cause Outrage. ABC News Oct. 17,2008.

194 CDC. Flu Vaccine Safety and Pregnancy. Oct. 3, 2017

195 U.S. Dept. of Health & Human Services. FDA Pregnancy Categories. Sept. 29, 2017

196 FDA. Pregnancy and Lactation Labeling (Drugs) Final Rule. Jun. 28, 2018

197 Gruber MF. Maternal Immunization: US FDA Regulatory Considerations (Abstract). Vaccine 2003; 21(24): 3487-3491.

198 Christian LM, Iams JD et al. Inflammatory Responses to Trivalent Influenza Virus Vaccine Among Pregnant Women. Vaccine 2011; 29(48): 8982-8987.

199 Ayoub D Yazbak FE. Influenza vaccination during pregnancy: A critical Assessment of the Recommendations of the Advisory Committee on Immunization Practices (ACIP). Journal of American Physicians & Surgeons 2006; 11(2): 41-47.

200 Elce D, Celik A. Genotoxicity of thimerosal in cultured human lymphocytes with and without metabolic activation sister chromatid exchange analysis proliferation index and mitotic index. Toxicology in Vitro June 2008; 22(4): 927-934.

201 Businesswire. National Vaccine Information Center (NVIC) Calls 21st Century Cures Act “A Wolf in Sheep’s Clothing” and Urges Presidential Veto to Protect Public Health. NVIC Press Release: Dec. 8, 2016.

202 Feemster KA. Advisory Commission on Childhood Vaccines Maternal Immunization Working Group Draft Recommendations. ACCV June 7, 2013.

203 CDC. Flu Vaccination & Possible Safety Signal. Sep. 13, 2017

204 CDC. Flu Vaccine Safety and Pregnancy. Oct. 3, 2017

205 FDA. Thimerosal in Vaccines. Recent and Future FDA Action. Feb. 2, 2018.

206 Institute of Medicine Committee to Review Adverse Effects of Vaccines. Immunization Safety Review: Thimerosal - Containing Vaccines and Neurodevelopmental Disorders. Washington, D.C. The National Academies Press 2001.


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