Substantial Influenza Vaccine Effectiveness in Households With Children During the 2013-2014 Influenza Season, When 2009 Pandemic Influenza A(H1N1) Virus Predominated

Background. We examined the influenza vaccine effectiveness (VE) during the 2013-2014 influenza season, in which 2009 pandemic influenza A(H1N1) virus (influenza A[H1N1]pdm09) predominated. In 2 previous years when influenza A(H3N2) virus predominated, the VE was low and negatively affected by prior year vaccination. Methods. We enrolled and followed 232 households with 1049 members, including 618 children; specimens were collected from subjects with acute respiratory illnesses. The VE in preventing laboratory-confirmed influenza A(H1N1) pdm09 infection was estimated in adjusted models. Preseason hemagglutination-inhibition and neuraminidase-inhibition antibody titers were determined to assess susceptibility. Results. Influenza A(H1N1) pdm09 was identified in 25 households (10.8%) and 47 individuals (4.5%). Adjusted VE against infection with influenza A(H1N1) pdm09 was 66% (95% confidence interval [CI], 23%-85%), with similar point estimates in children and adults, and against both community-acquired and household-acquired infections. VE did not appear to be different for live-attenuated and inactivated vaccines among children aged 2-8 years, although numbers were small. VE was similar for subjects vaccinated in both current and prior seasons and for those vaccinated in the current season only; susceptibility titers were consistent with this observation. Conclusions. Findings, including substantial significant VE and a lack of a negative effect of repeated vaccination on VE, were in contrast to those seen in prior seasons in which influenza A(H3N2) virus predominated.