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Influenza Quick Facts


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. 78 910

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.

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 Influenza Table of Contents

« Return to Vaccines & Diseases Table of Contents


1 CDC. Influenza (flu) Viruses. Aug 13, 2018

2 CDC. Pandemic Influenza. Mar. 5, 2018

3 CDC. Flu Symptoms & Complications. Jun. 5, 2018

4 CDC. Update: Influenza Activity — United States, October 4, 2015–February 6, 2016. MMWR. Feb. 19, 2016 65(6);146–153

5 CDC. Influenza Viruses Isolated by WHO/NREVSS Collaborating Laboratories 2015-2016 Season

6 CDC How Flu Spreads. Oct 5, 2017

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

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

9 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.

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

11 CDC. Different Types of Flu Vaccines. July 10, 2018

12 CDC Selecting Viruses for the Seasonal Influenza Vaccine. May 4. 2016

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

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

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

16 CDC Frequently Asked Flu Questions 2017-2018 Influenza Season. Mar. 30, 2018

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

18 CDC Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2017–18 Influenza Season. MMWR. Aug. 25, 2017. 66(2);1–20

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