Effectiveness of Monovalent 2009 Pandemic Influenza A Virus Subtype H1N1 and 2010–2011 Trivalent Inactivated Influenza Vaccines in Wisconsin During the 2010–2011 Influenza Season

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Abstract
Background. The 2009 influenza A virus subtype H1N1 (A[H1N1]pdm09) did not exhibit antigenic drift during the 2010–2011 influenza season, providing an opportunity to investigate the duration of protection after vaccination. We estimated the independent effects of 2010–2011 seasonal trivalent inactivated influenza vaccine (TIV) and A(H1N1)pdm09 vaccine for preventing medically attended influenza A virus infection during the 2010–2011 season. Methods. Individuals were tested for influenza A virus by real-time reverse transcription polymerase chain reaction (rRT-PCR) after a clinical encounter for acute respiratory illness. Case-control analyses compared participants with rRT-PCR–confirmed influenza A virus infection and test-negative controls. Vaccine effectiveness was estimated separately for monovalent pandemic vaccine and TIV and was calculated as 100 × [1 – adjusted odds ratio], where the odds ratio was adjusted for potential confounders. Results. The effectiveness of TIV against influenza A virus infection was 63% (95% confidence interval [CI], 37%–78%). The effectiveness of TIV against A(H1N1)pdm09 infection was 77% (95% CI, 44%–90%). Monovalent vaccine administered between October 2009 and April 2010 was not protective during the 2010–2011 season, with an effectiveness of −1% (95% CI, −146% to 59%) against A(H1N1)pdm09 infection. Conclusions. Monovalent vaccine provided no sustained protection against A(H1N1)pdm09 infection during the 2010–2011 season. This waning effectiveness supports the need for annual revaccination, even in the absence of antigenic drift in A(H1N1)pdm09.

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