Composition and Role of the 2010-2011 Northern Hemisphere Seasonal Influenza Vaccine in the Post-Pandemic Period

Recent published studies indicate that seroprevalence against pH1N1 in populations in various regions following the 2009 pandemic waves ranges from 20%-50%, hence many people are still susceptible to infection. The situation is compounded by the fact that pH1N1 may continue to disproportionately affect younger age groups. Therefore, vaccination against pH1N1 will continue to play an important and integral role in the public health effort to prevent and control the spread of pH1N1, and to reduce excess morbidity and mortality due to pH1N1 in the post-pandemic period. Moreover, the trivalent vaccine will also provide protection against seasonal influenza A/H3N2 and B viruses that will likely co-circulate with pH1N1 during the winter months in the northern hemisphere.

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Key Points

• Who recommends that the following viruses be used for influenza vaccines in the 2010-2011 influenza season in the northern hemisphere:

• an A/California/7/2009 (H1N1)-like virus (pandemic H1N1 2009 virus; pH1N1)
• an A/Perth/16/2009 (H3N2)-like virus
• a B/Brisbane/60/2008-like virus.Recommendations are based on epidemiological, genetic and serological tests on influenza virus isolates collected through the WHO Global Influenza Surveillance Network between September 2009 and January 2010.

• On August 10, 2010, WHO announced that the world has moved into the post-pandemic period.
• During the post-pandemic period, pH1N1 appears to be taking on the behaviour of a seasonal virus and co-circulating with other seasonal influenza strains. However, pH1N1 will likely continue to affect younger age groups disproportionately as seen during the 2009 pandemic.
• Current influenza activity in temperate countries in the southern hemisphere (Argentina, Chile, New Zealand, Australia, and South Africa) provides us with information on possible influenza scenarios during the winter months in the northern hemisphere. Whereas seasonal influenza A/H1N1 viruses are undetectable in all five countries, influenza pH1N1, A/H3N2 and B viruses have been reported and their proportions in circulation in each country have been variable.
• The recommended trivalent seasonal vaccine for the northern hemisphere 2010-2011 season will provide protection against influenza pH1N1, A/H3N2 and B viruses.

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