Concurrent and cross-season protection of inactivated influenza vaccine against A(H1N1)pdm09 illness among young children: 2012-2013 case-control evaluation of influenza vaccine effectiveness

In 2012-2013, we examined 1729 laboratory-confirmed A(H1N1)pdm09 influenza cases matched 1:1 with healthy controls and estimated influenza vaccine effectiveness (VE) for trivalent inactivated influenza vaccine (IIV3) to be 67% (95% confidence interval = 58-74%) for ages 8 months to 6 years old. Among children aged 8-35 months old, VE for fully vaccinated children (73%, 60-81%) was significantly higher than VE for partially vaccinated children (55%, 33-70%). Significant cross-season protection from prior IIV3 was noted, including VE of 31% (8-48%) from IIV3 received in 2010-2011 against influenza illness in 2012--2013 without subsequent boosting doses.