Viral Strains for Seasonal Flu Vaccine Well-Matched with Circulating Strains

Because influenza viruses frequently mutate, a new seasonal flu vaccine is developed each year in order to keep up with the circulating strains. Each year, the vaccine provides protection against three strains: two influenza A strains and one influenza B. (For more information about how the seasonal flu vaccine is created, see our Influenza article.) The flu vaccine for the 2010-2011 flu season provides protection against a 2009 H1N1 A strain, H3N2 A strain, and an influenza B strain.

The effectiveness of a seasonal flu vaccine depends on how well the strains used to create it match with circulating strains. The vaccine is developed based on predictions of which strains are most likely to circulate in the upcoming flu season; if different strains emerge, the vaccine may not provide adequate protection.

Of the circulating viruses that have been typed to this point, about 85% were influenza A and 15% were influenza B. Of the A strains identified, 74% were H3N2 and 26% were 2009 H1N1. These strains are very similar to those selected for the seasonal vaccine.

The Centers for Disease Control and Prevention maintain a weekly “FluView” report on their website. To find out more about the current flu season and influenza surveillance, click here to read the latest FluView report.

For information about seasonal flu vaccination, including a “Flu Vaccine Finder” you can use to find a flu clinic near you, visit CDC’s Seasonal Influenza page.