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Influenza Vaccine and Disease

Quick Facts


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  • Influenza, often referred to as “flu,”“ is an infectious respiratory disease caused by type A or type B influenza viruses, which are present in the mucus membranes and secretions of the nose, throat and lungs. There are other viruses and bacteria associated with “flu-like” symptoms and it is estimated that about 80 percent of all flu-like illness that occurs every year during the “flu season” is not type A or type B influenza. Only lab confirmation can detect whether flu-like symptoms, including serious complications like pneumonia, are caused by influenza viruses or other types of viral or bacterial organisms.
  • Influenza viruses are primarily spread through coughing and sneezing but can also be transmitted by touching or using items that have been handled by an infected person and then touching one’s own mouth, or nose.
  • It is also possible for vaccine strain influenza to be transmitted by persons, who have recently received live attenuated influenza virus nasal vaccines. Vaccine strain viruses can be shed in body fluids for seven or more days after administration of live attenuated virus vaccines.
  • 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 death.
  • Effective ways to stay well during the flu season and help to prevent spread of influenza or other infections all year-around include washing hands with soap and water frequently; covering mouth while coughing; staying home when sick until recovered and avoiding contact with those, who are sick until they are well; staying hydrated and eating nutritious food; lowering stress and getting plenty of exercise, sleep and vitamin D.  View NVIC's 3 minute flu prevention video to the right for healthy tips.

Influenza Vaccine

  • Three types of influenza vaccines are available in the U.S.: inactivated influenza vaccine, recombinant influenza vaccine and live attenuated influenza vaccine. Inactivated influenza vaccine consists of two types: Intramuscular influenza vaccine which is injected into the muscle, and intradermal influenza vaccine, which is injected into the skin. Inactivated, injectable influenza vaccines packaged in multi-dose vials contain the mercury preservative thimerosal but inactivated influenza vaccines in single dose vials are thimerosal-free or contain trace amounts of the mercury preservative. Recombinant influenza vaccines differ from inactivated influenza vaccines as they are produced through genetic engineering by growing the influenza virus inside insect cells instead of chicken eggs. The live attenuated nasal influenza vaccine is inhaled through the nose and does not contain thimerosal.
  • In 2016, the CDC’s Advisory Committee on Immunization Practices (ACIP) advised against use of the live attenuated nasal influenza vaccine due to lack of effectiveness of the vaccine based on data collected between 2013 and 2016.
  • There are several different influenza vaccine products licensed in the U.S. marketed by different pharmaceutical companies. 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 flu shots given during the current flu season.
  • The CDC’s Advisory Committee on Immunization Practices (ACIP) recommends that all Americans six months of age or older get a flu shot every year throughout life. The CDC states that babies between six and eight months old should receive two doses of influenza vaccine one month apart.
  • Reported moderate reactions to influenza vaccine 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. Reported serious complications include brain inflammation, convulsions, Bell’s palsy, limb paralysis, neuropathy, shock,  wheezing/asthma and other breathing problems. Guillain Barre Syndrome (GBS) is a disabling neurological disorder that involves temporary or permanent paralysis that can lead to death and has been causally related to influenza vaccinations.
  • As of October 3, 2016, there had been 2,954 claims filed in the federal Vaccine Injury Compensation Program (VICP) for injuries and deaths following Influenza vaccination, including 109 deaths and 2,845 serious injuries.
  • Using the MedAlerts search engine, as of June 30, 2016, there have been more than 128,194 reports of reactions, hospitalizations, injuries and deaths following influenza vaccinations made to the federal Vaccine Adverse Events Reporting System (VAERS), including 1,270 related deaths, 10,780 hospitalizations, and 2,377 related disabilities. In 2013 the Federal Advisory Commission on Childhood Vaccines (ACCV) voted to add GBS to the Vaccine Injury Table within the federal Vaccine Injury Compensation Program (VICP).

Food & Drug Administration (FDA)

Centers for Disease Control (CDC)

National Institute of Allergy & Infectious Diseases (NIAID)

Vaccine Reaction Symptoms & Ingredients

Our Ask 8, If You Vaccinate webpage contains vaccine reaction symptoms and more. Calculate vaccine ingredients for potential toxic exposures & print a vaccination plan with the Vaccine Ingredients Calculator.

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 the law. 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.

Influenza & Influenza Vaccine

The following information is provided to help you understand the disease influenza and the influenza vaccine so you can make an informed decision regarding use of this vaccine. This information is not intended to serve as medical advice but as background information that you can use to educate yourself.

Whenever making a major health care decision for yourself or your child, especially one that involves use of a pharmaceutical product such as a vaccine, obtaining information from many different sources and consulting health care professionals you trust is important. NVIC encourages you to become fully informed about influenza and Influenza vaccine and speak with one or more trusted health care professionals before making a decision about vaccination.

It is important to be equally concerned and knowledgeable about the risks of Influenza disease as we are about the risks of influenza vaccine. Both influenza and the influenza vaccine carry risks. Influenza disease has the potential to cause seizures, brain damage, and even death, just as the vaccine can.

Most of America’s medical community believes that the risk of serious injury or death from influenza is greater than the risk of injury or death which can be caused by Influenza vaccine. However, recognition of and concern about the risks of Influenza disease does not diminish our need and responsibility to acknowledge the need to minimize influenza vaccine risks. The challenge today is for parents, physicians, scientists, manufacturers and health officials to recognize the risks of both the disease and the vaccine and work to protect the health and well being of every individual.

If your doctor does not support your informed health choices, consider consulting another doctor, who will work as a partner with you and respect whatever decision you make. Below are links to specific topics regarding Influenza and Influenza vaccine for ease of use. However, the topics are interrelated and the content comprehensive.  No topic link below should be treated in a stand-alone fashion and NVIC recommends that all the information be used to reach a vaccination decision.

What is the flu?

Influenza is a respiratory infection that produces fever, chills, sore throat, muscle aches, and cough that lasts a week or more. The flu can be deadly for the elderly and those with compromised immune systems or who are suffering from diabetes, kidney dysfunction and heart disease. According to the Centers for Disease Control (CDC), each year about 36,000 Americans, mostly in these high risk groups, reportedly die from flu complications. However, there is controversy about whether the majority of the 36,000 reported deaths are caused by complications of true influenza or may be due to viral and bacterial infections which can be mistakenly attributed to influenza.

Who is at highest risk for getting the flu?

Those most at risk for getting the flu include children under the age of two, pregnant women, seniors, individuals residing in crowded spaces, and those living with chronic illness.

Influenza Type A and Type B continually change which accounts for most of the changes in the viruses from season to season. Due to these changes, it is impossible to predict whether a flu season will be mild or severe and whether the available flu vaccine will offer protection to those most at risk of developing the flu.

Who is at increased risk for serious complications due to the flu?

The Advisory Committee on Immunization Practices (ACIP) lists the following persons as being at increased risk for complications from influenza: anyone 65 years of age or older; nursing home residents or residents of other long-term care facilities; anyone with chronic lung or heart disorders, chronic metabolic diseases (like diabetes), kidney dysfunction, or blood disorders; anyone who is immune suppressed by medication (steroids, chemotherapy, etc) or by AIDS/HIV ; children or adolescents on long-term aspirin therapy due to possible risk of developing Reye syndrome; and women who will be in the second or third trimester of pregnancy during flu season.

What is the flu vaccine?

There are several types of flu vaccines available on the market. NVIC encourages consumers read the influenza vaccine manufacturer's package insert information carefully prior to receiving a vaccine.

Standard-Dose Inactivated Flu Vaccine –

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

High-Dose Trivalent Flu Vaccines -

  • The high-dose trivalent Flu vaccine is approved for adults age 65 and older and contains four times the amount of antigen than the standard flu vaccine. The goal of this vaccine is to produce a stronger immune response as research showed standard seasonal flu vaccines to be ineffective in the elderly. High-dose Fluzone is only one high-dose trivalent Flu vaccine currently available on the market in the U.S.

Recombinant Flu Vaccines -

  • The recombinant flu vaccine, approved for use in 2013, is manufactured through genetic engineering. It is made by isolating the protein of an influenza virus and mixing it with insect cells. This combination is injected into the insect cells to replicate. Flublok is the only recombinant flu vaccine currently available on the market and it is derived from the fall armyworm, an insect related to moths, caterpillars and butterflies. It is the only flu vaccine that does not use chicken eggs or the influenza virus in its production.

Cell-Based Flu Vaccines -

  • Cell-Based flu vaccines, approved for use in 2012, differ from standard egg-based flu vaccines as they use animal cells to grow the influenza virus. Currently only one cell-based flu vaccine, Flucelvax, is available in the U.S., and it is prepared from a virus grown in Madin Darby Canine Kidney cell. This vaccine is a quadrivalent vaccine approved for adults and children age 4 and older.

Trivalent Flu Vaccine, Adjuvanted -

  • In 2015, the FDA approved the first Adjuvant trivalent flu vaccine, Fluad, made from MF59 (squalene). This vaccine was approved for accelerated licensure by the FDA despite limited data on safety and immunogenicity and approval was based on a single clinical trial of about 1,000 healthy adults over the age of 65. This vaccine will be available for the first time to adults over the age of 65 for the 2016-2017 flu season.

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. There is one intradermal Flu vaccine available, Fluzone Intradermal, a quadrivalent vaccine, approved for adults ages 18-64. This vaccine is prepared from influenza viruses propagated in embryonated chicken eggs and inactivated by formaldehyde. The Intradermal flu vaccine does not contain thimerosal.

Nasal-Spray Flu Vaccine FluMist -

  • A live-virus nasal flu vaccine, FluMist, was approved for use in June 2003 and its approved use was limited to healthy people between the ages of 5 and 49. It was subsequently approved by the FDA for use in children as young as two years of age but with precautions. However, in 2016, the CDC’s Advisory Committee on Immunization Practices (ACIP) advised against use of the live attenuated nasal influenza vaccine for the 2016-2017 flu season due to lack of effectiveness of the vaccine. FluMist is prepared by introducing influenza viruses into eggs where they are allowed to multiply. Fluid from the eggs is processed and sucrose, potassium phosphate and monosodium glutamate (MSG) are added as stabilizers. The antibiotic Gentamicin is also added during the manufacturing process. FluMist is a live virus vaccine and does not contain any preservatives.

Nasal Spray Vaccine Note: The Nasal Spray Vaccine is not currently recommended by the ACIP due to lack of effectiveness.

The majority of Influenza vaccines are designated as Category B or C. This 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. However, in 2015, the FDA removed pregnancy categories due to concerns of confusion and oversimplification and replaced it with the Pregnancy and Lactation Labeling Rule . This rule affects all influenza vaccine products submitted after June 30, 2015, while products currently on the market will maintain the previous FDA Pregnancy Category designations but will gradually be phased in. NVIC encourages consumers read this information carefully prior to receiving a vaccine.

Below are current links for the 2016/2017 influenza vaccine manufacturer's package inserts that have been licensed for use in the U.S.  As a consumer, it is important to understand and read this information carefully prior to receiving a vaccine. These links are directly from the U.S. Food & Drug Administration's website, which makes available to the public package insert information from vaccine manufacturers.  Inserts contain important information regarding drug interactions, contraindications, adverse reactions, warnings and precautions.    

Quadrivalent Vaccines - Nasal

Quadrivalent Vaccines – Injected

Trivalent Vaccines - Injected

What are contraindications to the flu vaccine?

Among high risk factors listed by the CDC and the vaccine manufacturers are anyone who:  

(1) is sick with a fever;
(2) has an egg allergy; 
(3) has a mercury allergy; 
(4) has a history of Guillain-Barre syndrome. 

If immunosuppressed individuals receive the flu vaccine they may not get an adequate protective antibody response.  

Is Flu Vaccine Recommended for Children? 

One consideration with the mass use of flu vaccine in healthy children is the removal of natural antibodies to flu which are obtained from natural infection. 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 CDC and New Jersey now requires influenza vaccine for daycare and kindergarten entry.

Is the 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 women more than 14 weeks pregnant.

Influenza vaccines are 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. 

The package inserts published by the flu vaccine manufacturers state that "Animal reproduction studies have not been conducted with influenza virus vaccine. It is also not known whether influenza virus vaccine can cause fetal harm when administered to a pregnant woman. Although animal reproductive studies have not been conducted, the prescribing health care provider should be aware of the recommendations of the Advisory Committee on Immunization Practices. The ACIP states that if used during pregnancy, administration of influenza virus vaccine after 14 weeks of gestation may be preferable to avoid coincidental association of the vaccine with early pregnancy loss."

In 2015, the FDA removed pregnancy categories due to concerns of confusion and oversimplification and replaced it with the Pregnancy and Lactation Labeling Rule. This rule affects all influenza vaccine products submitted for approval after June 30, 2015. Influenza vaccine products currently on the market will maintain the previous FDA Pregnancy Category designations but will eventually be phased into the new rule. NVIC recommends careful review of package insert for information on pregnancy, lactation and reproduction.

Pregnant women should be aware that the 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.

What about mercury in the vaccine?

In 1999, the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA) 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."

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. All adult formulations still 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.

How effective is influenza vaccine? 

Like all vaccines, the flu vaccine only gives a temporary immunity to the virus strains or closely related virus strains contained in the vaccine. The only way to get natural and permanent immunity to a strain of flu is to recover naturally from the flu. Natural immunity to a particular strain of flu can be protective if that strain or closely related strains come around again in the future. However, because the vaccine only provides temporary immunity to selected strains and those strains may or may not be prevalent each year, doctors say you have to get a flu shot every year.

Every year, federal health agency officials try to guess which three or four flu strains are most likely to be prevalent in the U.S. the following year to determine which strains will be included in next year's flu vaccine. If they guess right, the vaccine is thought to be 70 to 90 percent effective in temporarily preventing the flu of the season in healthy persons less than 65 years old. For those over 65 years old, the efficacy rate drops to 30 to 40%  but the vaccine is assumed to be 50 to 60% effective in preventing hospitalization and pneumonia and 80% effective in preventing death from the flu. When health officials do not correctly predict which flu strains will be most prevalent and the vaccine's effectiveness is much lower for that year.

However, according to a 2011 review of existing research, inactivated influenza vaccine had a pooled efficacy of 59% for adults 18 to 65 years of age for 8 out of 12 seasons. Similar data for inactivated influenza vaccine for adults over 65 years of age and children between 2-17 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% in 9 of the 12 seasons analyzed for children aged 6 months to 7 years. Similar data for LAIV efficacy for children aged 8-17 years was lacking and requires additional study.

In 2016, the CDC’s Advisory Committee on Immunization Practices (ACIP) advised against use of the live attenuated nasal influenza vaccine(LAIV) due to lack of effectiveness of the vaccine based on data collected between 2013 and 2016.  Data collected showed the LAIV to be 3 percent effective against any flu virus in children ages 2-17.

Throat, respiratory, gastrointestinal and ear infections caused by bacteria or other kinds of viruses are not prevented by getting an annual flu shot.

Can influenza vaccine cause injury and death?

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

The most serious reaction that has been associated with flu vaccine is Guillain-Barre Syndrome (GBS) which occurs most often within two to four weeks of vaccination. GBS is an immune mediated nerve disorder characterized by muscle weakness, unsteady gait, numbness, tingling, pain and sometimes paralysis of one or more limbs or the face. Recovery takes several months and can include residual disability. Less than 5 percent of GBS cases end in death.

Brain and nerve disorders such as encephalopathy, optic neuritis, partial facial paralysis, and brachial plexus neuropathy as well as vasculitis have also been reported following the flu vaccine, although a definite causal relationship has not been established.

Adult influenza vaccine injury claims are now the leading claim submitted to the federal Vaccine Injury Compensation Program.

Questions to Ask Doctors about 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.

It is also a good idea to read the vaccine manufacturer product insert that can be obtained from your doctor or public health clinic because 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. 

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 measles infection?

Under the National Childhood Vaccine Injury Act of 1986, doctors and all vaccine providers are legally required to give you vaccine benefit and risk information before vaccination; record serious health problems following vaccination in the permanent medical record; keep a permanent record of all vaccines given, including the  manufacturer’s name and lot number; and report serious health problems, injuries and deaths that follow vaccination to VAERS.

Remember, if you choose to vaccinate, always keep a written record of exactly which shots/vaccines you or your child have received, including the manufacturer’s name and vaccine lot number. Write down and describe in detail any serious health problems that develop after vaccination, and keep vaccination records in a file you can access easily.

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

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

To view the entire video collection, click the hamburger menu in the upper left corner of the video player above. This will expand a full list of videos. You may also open the video player in full screen mode for optimal display.


NVIC Press Releases, Statements and Commentaries

Statements and Commentaries

Press Releases

Additional Bibliography of References

Medical Literature

Media Articles

This portion of our website is also in the process of being updated as new information on influenza vaccines in use this season is made known.  Please continue to check back for the latest information.

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