How Did the 2008–2009 Seasonal Influenza Vaccine Affect the Pandemic?

A novel influenza A(H1N1) virus appeared in Mexico in March 2009 [1]. The prototype strain, A/California/09/2009 (H1N1), was identified in April 2009, and the virus was found to have surface antigens that are distinct from those of the circulating seasonal influenza A(H1N1) virus. An early survey of antibody prevalence suggested that many persons 60 years of age might be expected to have preexisting antibody titers to nH1N1 virus that could be protective and that a few younger persons had lower titers that may be boosted by immunization with seasonal influenza vaccines for both 2007–2008 and 2008–2009 [2]. This information suggested that seasonal trivalent influenza vaccine (TIV) might benefit some older adults but not children. In fact, early reports from Mexico found “partial protection” for persons who had received TIV [3]. Because several months would be required to produce a vaccine against the nH1N1 virus, vaccination with TIV was recommended as soon as it was available. This recommendation was ques-