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

<b>Background</b>: 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. <b>Methods</b>: 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. <b>Results</b>: The proportion of women with HAI titers ≥1:40 was significantly greater in vaccinees (63%) compared to placebo-recipients (22%; p <b>Conclusion</b>: 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.