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BackgroundChina has low seasonal influenza vaccination rates among children and older adults. This quasi-experimental pragmatic trial examined the effectiveness of a pay-it-forward intervention on influenza vaccination compared to standard-of-care (user-paid vaccination) among children and older people in China. Pay-it-forward offered a free influenza vaccine from a local group and an opportunity to donate financially to support future individuals.MethodsAt each of the three study sites, participants were recruited into the standard-of-care arm first until expected sample size was reached and then recruited into the pay-it-forward arm. The primary outcome was vaccine uptake. Secondary outcomes included vaccine confidence and costs. Regression methods compared influenza vaccine uptake and vaccine confidence between the two arms.ResultsAmong 300 participants enrolled, 55/150(36.7%) in the standard-of-care arm 111/150(74.0%) in the pay-it-forward arm received an influenza vaccine. People in the pay-it-forward arm were more likely to receive the vaccine compared to people in the standard-of-care arm (adjusted odds ratio (aOR)= 6.7, 95%CI [2.7, 16.6] among children; aOR=5.0, [2.3, 10.8] among older adults). People in the pay-it-forward arm had greater confidence in vaccine safety, importance, and effectiveness. In the pay-it-forward arm, 107/111 (96.4%) of participants donated money for subsequent vaccinations, and 19 of 60 invited (31.7%) created postcard messages. The pay-it-forward arm had a lower economic cost per person vaccinated ($45.60) than the standard-of-care arm ($64.67).ConclusionsPay-it-forward was effective in improving influenza vaccine uptake and public engagement. Our data have implications for pro-social interventions to enhance influenza vaccine uptake in countries where influenza vaccines are available for a fee.Trial registrationChiCTR2000040048Main point summaryPay-it-forward substantially increased influenza vaccine uptake among children and older adults compared to standard of care user-paid vaccination.