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Influenza Disease & Vaccine Information

Find the Information You Need to Make an Informed Vaccine Decision


Influenza: The Disease

Influenza is a viral infection that produces fever, chills, sore throat, muscle aches, and cough that lasts between a few days to 2 weeks. 1  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.2

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. Learn more about influenza

Influenza Vaccine

There are many 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. The majority of influenza vaccines were initially designated as Category B or C pharmaceutical products. While these categories are no longer used, the information remains in the product insert and 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.  Learn more about Influenza vaccine

Influenza Quick Facts


  • Over 70 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. 3 4
  • 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. 5 6 7 8  Continue reading quick facts

Influenza Vaccine

  • 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.9
  • 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.10 The CDC reports that between 2004/2005 and 2018/2019, overall influenza vaccine effectiveness ranged from 10 percent (2004/2005) to 60 percent (2010/2011) and was less than 50 percent effective in 11 out of 15 flu seasons. Continue reading quick facts...

« Return to Vaccines & Diseases Table of Contents


1 CDC Influenza – Flu symptoms & Complications. Aug. 31, 2020

2 CDC Key Facts About Influenza (Flu) Sep. 13, 2019

3 CDC Interim Estimates of 2019–20 Seasonal Influenza Vaccine Effectiveness — United States, February 2020 MMWR Feb. 21, 2020; 69(7);177–182

4 CDC Influenza Viruses Isolated by WHO/NREVSS Collaborating Laboratories 2019-2020 Season

5 CDC Personal NPIs: Everyday Preventive Actions Aug. 26, 2019

6 Mercola.com Is the Flu Vaccine Really ‘Working Well’ This Year? Mar. 6, 2019

7 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.

8 CDC Nonpharmaceutical Interventions (NPIs) – At Home. Aug. 26, 2019

9 CDC Selecting Viruses for the Seasonal Influenza Vaccine. Sep. 4, 2018

10 CDC Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2020–21 Influenza Season MMWR Aug. 21, 2020. 69(8);1–24

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