Dose effect of influenza vaccine on protection against laboratory-confirmed influenza illness among children aged 6 months to 8 years of age in southern China, 2013/14-2015/16 seasons: a matched case-control study.

Background We conducted a matched case-control study in China during the 2013/14-2015/16 influenza seasons to estimate influenza vaccine effectiveness (VE) by dose among children aged 6 months to 8 years.Methods Cases were laboratory-confirmed influenza infections identified through the influenza-like illness sentinel surveillance network in Guangzhou. Age- and sex-matched community controls were randomly selected through the expanded immunization program database. We defined priming as receipt of ≥1 dose of influenza vaccine during the immediate prior season.Results In total, 4,185 case-control pairs were analyzed. Among children 6-35 months, VE for current season dose(s) across the three seasons during 2013/14-2015/16 were 59% (95% Confidence Interval: 44-71%), 12% (-11%,30%), 54% (32-69%); among unprimed children 6-35 months, VE for 1 vs 2 current season doses were 45% (8-67%) vs 65% (46-78%), -2% (-53%,32%) vs 19% (-11%,40%), and 37% (-24%,68%) vs 61% (32-78%). Among children aged 3-8 years, VE for current season dose(s) across study seasons were 62% (36-78%), 43% (22-58%), 32% (1-53%). VE for unprimed children receiving 1 dose only in current season was insignificant or lower than among all children.Conclusion Findings support utility of providing second dose ("booster dose") of seasonal influenza vaccine to unprimed children aged 6-35 months, and the need to study further dose effect of a booster dose among unprimed children aged 3-8 years in China.