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ACAAI > Patients & Public > Resources > Ask the Allergist

What's in this year's flu vaccine?

Q. I thought the influenza vaccine changed each year. My nurse looked at this year’s influenza vaccine and found it has H1N1 again. Isn’t that swine flu, and why do we still need the same virus?

A. Your nurse is correct there is one strain of H1N1 (also known as swine flu) in this year’s vaccine. H1N1 is a subtype of Influenza A that has caused roughly half of all human influenza infections we’ve seen in the past several years. Influenza viruses are identified by two surface proteins: Hemagglutin (H) and Neuramidase (N). The strains H1, H2 and H3, and N1 and N2 are found in people. Different strains are found in birds and pigs. The strains found in animals can be spread to humans, as seen in the recent pig flu found in some human contacts. If a sufficient genetic shift occurs in an animal virus strain, it can then be transmitted person to person. Scientists closely monitor the strains infecting humans as well as animals, as crossover to humans may signal the next pandemic influenza. Yes, the human influenza strains do shift from year to year but it is very rare that all three strains in an influenza vaccine change in any given year. Therefore you will see repeating strains if you watch the vaccines' contents.

This year’s influenza vaccine does contain one H1N1 strain. This is the same pandemic strain that emerged in 2009. It also contains a H3N2 strain that differs from last year’s vaccine. The sole Influenza B strain contained in the 2012-2013 influenza vaccine is also different from last year. People need a yearly influenza vaccine for two reasons: immunity to influenza decreases over time, and the vaccines usually contain new strains each year.

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