The A/California/7/2009 (H1N1) virus is the pandemic strain that was used in the 2009 H1N1 monovalent vaccine. That virus has not yet undergone sufficient antigenic drift to warrant selection of a new strain for the vaccine. Note that a seasonal H1N1 strain from previous years will not be included in the vaccine. This change has been made because epidemiological evidence suggests that these viruses will probably not circulate at significant levels during the 2010-2011 northern hemisphere season. Although the vast majority of circulating influenza viruses in humans are related to the 2009 H1N1 pandemic strain, sporadic influenza A(H3N2) activity continues to be reported in several countries. This is the reason why an H3N2 component is part of the vaccine.
The selection of viruses for seasonal flu vaccines is based on which influenza viruses circulate during the previous season. Sample viruses are collected by 130 national influenza centers in 101 countries and data on disease trends are analyzed by the four World Health Organization (WHO) Collaborating Centers for Reference and Research on Influenza. Vaccine viruses are selected which will most likely protect against the main circulating viruses during the next influenza season. WHO makes recommendations about which specific virus strains should be included in the vaccine. Individual countries then decide which viruses will be included in the influenza vaccine.
Even though the 2009 H1N1 strain has not undergone significant antigenic changes, it’s important to be immunized again in anticipation of the next influenza season. That’s because immunity conferred by the vaccine isn’t particularly long lasting. As Adolfo Garcia-Sastre told me today*, even if influenza didn’t change, you would still have to be immunized every year to protect against infection.
*I recorded our conversation. Look for it at TWiV within the next few weeks.