A novel influenza A(H1N1) virus appeared in Mexico in March 2009. 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. 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. 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 questioned by a statement issued by the Canadian Agency for Drugs and Technologies in Health (CADTH). CADTH reported that the Canadian sentinel influenza surveillance system had found that persons who received the 2008–2009 seasonal TIV had higher rates of nH1N1 illness than did unvaccinated persons. The sentinel surveillance was an ongoing program that monitored the effectiveness of influenza vaccine in Canada annually. Although the data supporting the CADTH report of increased risk of nH1N1 infection for those receiving TIV had not been published, some Canadian provinces altered their recommendations for the timing of administration of seasonal TIV and nH1N1 vaccines.
With conflicting reports on seasonal TIV effectiveness from our neighbors to the South and to the North, the Centers for Disease Control and Prevention (CDC) examined available data for a case‐cohort analysis. The analysis used surveillance reports of persons aged >18 years with confirmed nH1N1 illness from 8 states. A survey of 2008 TIV uptake was used to estimate vaccine coverage for the population cohort. The overall vaccine effectiveness against nH1N1 virus infection was −10% (95% confidence interval [CI], −43% to 15%). The CDC’s conclusion was that current evidence did not suggest that seasonal influenza vaccine either decreases or increases the risk for acquiring nH1N1 illness; therefore, the recommendations were unchanged. The CDC report also cited experience in New York City schools that found an overall adjusted relative risk of 1.05 (95% CI, 0.91–1.20) for TIV‐vaccinated students with proven nH1N1 infections. A study of an outbreak of nH1N1 virus infections in military beneficiaries in San Diego, California, revealed a significantly higher proportion of individuals with laboratory‐confirmed cases than of individuals with negative cases had received the 2008 seasonal vaccine (which was usually the live attenuated influenza vaccine [LAIV]). The authors considered this to be a consequence of the fact that almost all members of the military are vaccinated and that LAIV is the vaccine most commonly administered to military personnel. Examination of persons with cultures obtained in the Central Texas Field Trial found that the 2008 seasonal influenza vaccine did not prevent or enhance infection with nH1N1 virus; LAIV was administered to 70% of >9000 seasonal vaccine recipients.