Postcard reminders for HPV vaccination mainly primed parents for providers' recommendations

Parent-targeted reminders increase human papillomavirus (HPV) vaccination rates. Our postcard reminders increased HPV vaccine initiation (receipt of at least one dose) by 60% among girls, but not substantially among boys. To select a complementary intervention that would maximize the effect of reminders, we assessed the percentage increase in HPV vaccine initiation from reminders increasing preventive care visits or increasing the likelihood of vaccine receipt during preventive care. We conducted a secondary analysis of the postcard reminders from a quasi-experimental, factorial designed, feasibility trial conducted in 2013-2014 among 2,470 11- to 17-year-old girls enrolled in Florida's Medicaid or Children's Health Insurance Programs. We used log-linear models and a 4-way decomposition method to estimate the mediating and interacting effects of preventive visits on the effect of postcards on HPV vaccination. After receiving a preventive visit, 76% (269/356) of girls remained unvaccinated. Nearly half of the effect of postcards on HPV vaccination was mediated by preventive visits (44%; 95% CI = 9.2% to 78.0%). Postcards increased HPV vaccine initiation mainly by increasing the likelihood of HPV vaccine receipt during preventive visits (pure indirect effect = 32.3%; 95% CI = 1.5% to 63.1%), and also by increasing the occurrence of preventive visits (mediated interaction = 11.3%; 95% CI = 1.5 to 21.1). Reminders likely increased vaccination rates during well visits by priming parents for HPV vaccine conversations with their child's health care provider. Thus, reminder effectiveness may be increased if timed closely to preventive care visits and/or combined with interventions that improve provider recommendations. Trial Registration: ClinicalTrials.gov, NCT04208269, Registered 23, December 2019 - Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT04208269.