Effectiveness of seasonal trivalent influenza vaccine for preventing influenza virus illness among pregnant women: a population-based case-control study during the 2010-2011 and 2011-2012 influenza seasons.

BACKGROUND Although vaccination with trivalent inactivated influenza vaccine (TIV) is recommended for all pregnant women, no vaccine effectiveness (VE) studies of TIV in pregnant women have assessed laboratory-confirmed influenza outcomes. METHODS We conducted a case-control study over 2 influenza seasons (2010-2011 and 2011-2012) among Kaiser Permanente health plan members in 2 metropolitan areas in California and Oregon. We compared the proportion vaccinated among 100 influenza cases (confirmed by reverse transcription polymerase chain reaction) with the proportions vaccinated among 192 controls with acute respiratory illness (ARI) who tested negative for influenza and 200 controls without ARI (matched by season, site, and trimester). RESULTS Among influenza cases, 42% were vaccinated during the study season compared to 58% and 63% vaccinated among influenza-negative controls and matched ARI-negative controls, respectively. The adjusted VE of the current season vaccine against influenza A and B was 44% (95% confidence interval [CI], 5%-67%) using the influenza-negative controls and 53% (95% CI, 24%-72%) using the ARI-negative controls. Receipt of the prior season's vaccine, however, had an effect similar to receipt of the current season's vaccine. As such, vaccination in either or both seasons had statistically similar adjusted VE using influenza-negative controls (VE point estimates range = 51%-76%) and ARI-negative controls (48%-76%). CONCLUSIONS Influenza vaccination reduced the risk of ARI associated with laboratory-confirmed influenza among pregnant women by about one-half, similar to VE observed among all adults during these seasons.