Increasing child vaccination rates is a critical step toward mitigating the spread of COVID-19. Both distrust in expert sources and concern about the safety and efficacy of vaccines may contribute to parent vaccine hesitancy. The present study is the first to test the effectiveness of building trust and providing evidence supporting child COVID-19 vaccines in recommendation messages for parents. Based on dual-processing theories, emphasis on source trustworthiness and pro-vaccine evidence may each be particularly effective when the other is not present. It was hypothesized that these two approaches would have main and interaction effects on perceived message effectiveness and pro-vaccine beliefs. A between-subjects 2 (trust-building appeal vs. no trust-building appeal) X 2 (pro-vaccine evidence vs. no pro-vaccine evidence) online survey experiment was conducted in December 2021 and January 2022 with United States parents/guardians of children <18 years old (n = 401). As hypothesized, trust and pro-vaccine evidence each had significant simple main effects on both outcomes. Analysis of variance showed a significant negative interaction effect of trust and pro-vaccine evidence on perceived message effectiveness [F(3, 394) = 6.47; η2 = 0.02, p = 0.002; 95% CI (0.01, 0.11)], supporting the dual-processing hypothesis. The interaction effect on pro-vaccine beliefs was also negative but not significant [F(3, 394) = 2.69; η2 = 0.01; p = 0.102; 95% CI (0.00, 0.03)]. Either highlighting evidence supporting vaccines or building trust in expert sources can influence parent vaccine support. Messages which include strong evidence supporting recommended behaviors may influence recommendation acceptance even among those with lower trust in expert sources and establishing trust may reduce the need to describe available evidence.
