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Key Stats for the 2012-2013 Flu Season

By Alex B. Berezow

The CDC has released more data about the U.S. influenza season. (See figure for the current snapshot of flu activity across the country for the first week of January.)

There are three types of influenza: A, B and C. Influenza A is associated with pandemics, and so it is by far the scariest. Currently, influenza A (subtype H3N2), influenza A (subtype H1N1) and influenza B cause seasonal flu. (Note: Seasonal H1N1 and the form of H1N1 which caused a pandemic in 2009 were different strains.) This season, H3N2 has been the predominant virus, but there have also been many laboratory confirmations of influenza B.

Each year, the injectable flu vaccine contains three viruses: a strain of type A H3N2, a strain of type A H1N1, and a strain of type B. The exact strains chosen usually vary from year-to-year, based on the predictions of epidemiologists and public health officials. As of October 1, 2012, the CDC has characterized 521 influenza samples taken from patients:

-327 were influenza A (H3N2), strain A/Victoria/361/2011-like (which is covered by the vaccine)
-177 were influenza B; of these, 118 were strain B/Wisconsin/1/2010-like (which is covered by the vaccine), and 59 were from the B/Victoria lineage (which is not covered by the vaccine)
-17 were influenza A (H1N1), strain A/California/7/2009-like (which is covered by the vaccine)

The CDC reports that the vaccine is 62% effective, so it is a very good idea to get your flu shot if you haven't done so already. Also, for some more good flu information about how the circulating viruses change each season, check out this piece in USA Today.

Source: Centers for Disease Control and Prevention. "2012-2013 Influenza Season Week 1 Ending January 5, 2013." FluView.

Source: Centers for Disease Control and Prevention. "Vaccine Virus Selection for the 2012-2013 Influenza Season."

Source: Centers for Disease Control and Prevention. "Early Estimates of Seasonal Influenza Vaccine Effectiveness — United States, January 2013." MMWR 62 (Early Release): 1-4.

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