During the 2013/14–2015/16 influenza seasons in China, researchers conducted a matched case–control study to estimate influenza vaccine effectiveness (VE) by dose in children aged 6 months to 8 years. The cases were laboratory-confirmed influenza infections discovered through Guangzhou’s influenza-like illness sentinel surveillance network. The expanded immunization program database was used to randomly select age- and gender-matched community controls. They defined priming as receiving one dose of influenza vaccine during the previous season. A total of 4,185 case–control pairs were studied. Among children 6–35 months, the VE for current season dose(s) was 59 percent, 12 percent, and 54 percent across the three seasons from 2013/14 to 2015/16; among unprimed children 6–35 months, the VE for 1 vs 2 current season doses was 45 percent vs 65 percent, 2 percent vs 19 percent, and 37 percent vs 61 percent. Among children aged 3–8 years, the VE for current season dose(s) was 62 percent, 43 percent, and 32 percent, respectively, across study seasons.
The findings support the utility of administering a second dose of seasonal influenza vaccine to unvaccinated children aged 6–35 months, as well as the need to investigate the dose effect of a booster dose among unvaccinated children aged 3–8 years in China.