King JP, et al. - Recent observational studies in the United States indicated less efficacy of live attenuated influenza vaccine (LAIV) vs inactivated influenza vaccine (IIV) against clinical influenza infection caused by A(H1N1)pdm09 in children. Herein, researchers report the serologic responses to LAIV vs IIV among children aged 5–17 years during the 2013–2014 influenza season. Factors associated with vaccine failure were also determined. Children who received LAIV exhibited minimal hemagglutination inhibition (HI) antibody response, including those seronegative at baseline. Notably, LAIV recipients had significant increased risk of A(H1N1)pdm09 infection compared to IIV recipients.