Early this week, the U.S. Food and Drug Administration announced its approval of vaccines for the 2012 – 2013 influenza season.

In a post from last year’s influenza season, The Disease Daily explained how viruses are chosen to make the seasonal flu vaccine. To refresh your memory, the WHO Global Influenza Surveillance Network (GISN) monitors the spread and trends in influenza throughout the year to determine what strains are most predominant and likely to cause the most illness. The GISN is composed of five WHO Collaborating Centers for Reference and Research on Influenza, over 100 national influenza centers across the globe, and about 15 laboratories. Twice a year, a committee meets to recommend what strains are included in the vaccine. There are three components to the seasonal influenza vaccine and each component protects against one of the three types of influenza viruses that are transmitted among humans. This year, Influenza A/California/7/2009 (H1N1)-like virus, A/Victoria/361/2011 (H3N2)-like virus and B/Wisconsin/1/2010-like virus were deemed likely to cause the most illness and were thus included in the 2012-2013 flu vaccines.

Influenza vaccines can either be injected into the muscle, an intradermal injection (the vaccine is injected into the skin instead of the muscle), or a nasal spray. The shots are inactivated vaccines, meaning that they contain a killed virus. The nasal spray is a live, but significantly weakened, virus.

The vaccine’s effectiveness depends on a few factors: how well recipients respond to the vaccine and how well the vaccine is matched to the viruses in circulation during a given flu season. If the vaccine is not a good match for the viruses in circulation, it may still offer some protection. Because different strains of influenza viruses are related, antibodies created through vaccination often offer some protection against different strains (see this Disease Daily post about antigenic shift and drift).

Illness from influenza can range in severity. According to the CDC, the range of deaths over the past 31 flu seasons has been between 3,000 and 49,000. “The best way to prevent influenza is by getting vaccinating each year,” states Karen Midthun, MD, director of the Center for Biologics Evaluation and Research at the FDA.

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