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Little is known about vaccine effectiveness (VE) against nonmedically attended A(H3N2) influenza illness during 2014–2015 when the vaccine component appeared to be a poor match with circulating strains.Forty-three eligible school influenza outbreaks in Beijing, China, from November 1, 2014, to December 31, 2014, were included in this study. The VE of 2014–2015 trivalent inactivated influenza vaccine (IIV3) was assessed in preventing laboratory-confirmed influenza among school-age children through a case-control design, using asymptomatic controls. Influenza vaccination was documented from a vaccination registry. VE was estimated adjusting for age group, sex, rural versus urban area, body mass index, chronic conditions, onset week and schools through a mixed effects logistic regression model.The average coverage rate of 2014–2015 IIV3 among students across the 43 schools was 47.6%. The fully adjusted VE of 2014–2015 IIV3 against laboratory-confirmed influenza was 38% [95% confidence interval (CI): 12%–57%]. Receipt of previous season’s (2013–2014) IIV3 significantly modified VE of the 2014–2015 IIV3; children who received both 2013–2014 and 2014–2015 vaccinations had VE of 29% (95% CI: −8% to 53%), whereas VE for children who received 2014–2015 IIV3 only was 54% (95% CI: 8%–77%).VE for 2014–2015 IIV3 against A(H3N2) illness identified in schools was modest. Children who did not receive the prior season’s vaccine with a homologous A(H3N2) component may have enjoyed greater protection than repeated vaccinees.