Causal mediation analysis of a randomised controlled trial in China: evaluating whether the pay-it-forward strategy increases HPV vaccine uptake by reducing vaccine delay intention and increasing vaccine confidence.

objectiveTo explore whether vaccine confidence and vaccine delay intention mediated the effect of the pay-it-forward intervention on human papillomavirus (HPV) vaccine uptake.

designThis secondary mediation analysis of a two-arm randomised controlled trial was conducted among female adolescents aged 15-18 years in Chengdu, China, from July 2022 to June 2023.

settingThis study was conducted in four residential areas representing diverse economic backgrounds in Chengdu.

participantsA total of 321 parents of girls aged 15-18 years who had not received the HPV vaccine participated in the study.

interventionParticipants were randomly allocated into two arms. Pay-it-forward participants received a community-contributed subsidy (47.7 USD) to support the HPV vaccination, along with educational postcards and an opportunity to donate to support others. In the standard-of-care arm, participants paid for their vaccination. PRIMARY AND SECONDARY OUTCOME MEASURES: Primary outcome was the receipt of the first HPV vaccine dose within a 3-month period following an intervention. Based on previous literature, we hypothesised that vaccine confidence and vaccine delay intention were potential mediators. Vaccine confidence was measured using the vaccine confidence index. Vaccine delay intention refers to the caregiver's preference to postpone HPV vaccination for their daughter until the preferred vaccine type becomes available, rather than accepting the immediately accessible HPV vaccine. Data on these mediators were collected via a self-administered online questionnaire conducted after the intervention but before vaccination.

resultsAmong urban participants, when compared with the standard-of-care arm, about 39% of the effect of the pay-it-forward intervention on vaccine uptake was mediated by a reduction in vaccine delay intention. Notably, vaccine confidence did not appear to mediate the effect of the intervention on vaccine uptake. Among suburban participants, no mediation effects were observed. In the suburban setting, caregivers who vaccinated their daughters showed poorer prior awareness of the HPV vaccine before participating in the trial compared with those who did not vaccinate their daughters (41.5% vs 21.1%; p=0.011).

conclusionOur findings indicate that among urban participants, the pay-it-forward may have effectively reduced vaccine delay intention, which was associated with an increased uptake of the HPV vaccine. However, in suburban areas, enhanced awareness might be a potential contributing factor to improved vaccine uptake, but further research is necessary to affirm this. TRIAL REGISTRATION NUMBER: Chinese Clinical Trial Registry: ChiCTR2200055542.