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Selecting Viruses for the Seasonal Influenza Vaccine

The seasonal influenza vaccine is designed to protect against the influenza viruses research indicates are most likely to spread and cause illness among people during the upcoming flu season. Flu viruses are constantly changing, so the vaccine is updated each year based on which influenza viruses are making people sick, how those viruses are spreading, and how well the previous season’s vaccine protects against those viruses.

More than 100 national influenza centers in over 100 countries conduct year-round surveillance for influenza. This involves receiving and testing thousands of influenza virus samples from patients with suspected flu illness. The laboratories send representative viruses to five World Health Organization (WHO) Collaborating Centers for Reference and Research on Influenza, which are located in the following places:

  • Atlanta, Georgia, USA (Centers for Disease Control and Prevention, CDC);
  • London, United Kingdom (National Institute for Medical Research);
  • Melbourne, Australia (Victoria Infectious Diseases Reference Laboratory);
  • Tokyo, Japan (National Institute for Infectious Diseases); and
  • Beijing, China (National Institute for Viral Disease Control and Prevention).

In February of each year, WHO consults with experts from WHO Collaborating Centers, Essential Regulatory Laboratories, and other partners to review data generated by the worldwide network of influenza laboratories. Afterward, WHO makes recommendations for the composition of the seasonal influenza vaccine for the Northern Hemisphere.

The U.S. Vaccines and Related Biological Products Advisory Committee (VRBPAC) - which is a part of the U.S. Food and Drug Administration (FDA) – considers the WHO recommendations and makes a final decision regarding composition of seasonal flu vaccine for the United States.

2015-2016 Influenza Vaccine Composition

The WHO Vaccine Composition Meeting took place in Geneva, Switzerland on February 23-25, 2015 and the vaccine strain composition for the 2015-2016 influenza vaccines was announced on February 26, 2015.

On March 4, 2015, the Food and Drug Administration's Vaccines and Related Biologics Advisory Committee (VRBPAC) endorsed the WHO Northern Hemisphere flu vaccine strain selection for use in the production of upcoming U.S. 2015-2016 flu vaccines.

All of the 2015-2016 influenza vaccine is made to protect against the following three viruses:

  • an A/California/7/2009 (H1N1)pdm09-like virus
  • an A/Switzerland/9715293/2013 (H3N2)-like virus
  • a B/Phuket/3073/2013-like virus. (This is a B/Yamagata lineage virus)

Some of the 2015-2016 flu vaccine is quadrivalent vaccine and also protects against an additional B virus (B/Brisbane/60/2008-like virus). This is a B/Victoria lineage virus.

Vaccines that give protection against three viruses are called trivalent vaccines. Vaccines that give protection against four viruses are called quadrivalent vaccines.

More information about influenza vaccines is available at Preventing Seasonal Flu With Vaccination.

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