Influenza vaccination of pregnant women protects them over two consecutive influenza seasons in a randomized controlled trial

Background: We assessed the persistence of hemagglutinin inhibition (HAI) antibodies and the vaccine efficacy (VE) of trivalent inactivated influenza vaccine (IIV3) following vaccination of a cohort of pregnant South African women during a second influenza season. Methods: A cohort of women who participated in a randomized placebo-controlled trial on the safety, immunogenicity and efficacy of IIV3 in 2011 had HAI titers measured in 2012 and were monitored for influenza illness until the end of 2012. Results: The proportion of women with HAI titers ≥1:40 was significantly greater in vaccinees (63%) compared to placebo-recipients (22%; p < 0.001). VE in 2012 was 63.8% (95% confidence interval [95%CI]: −33.7%, 90.2%); combined VE for 2011 and 2012 was 58.3% (95%CI: 0.2%, 82.6%). Conclusion: The majority of women who received IIV3 during pregnancy had HAI titers above the putative threshold for protection against influenza illness one year after vaccination and showed a trend towards protection against influenza disease.