Public distrust in official sources of health information and uncertainty about novel guidelines may discourage engagement in recommended disease prevention practices. This dissertation examines how trust in recommendation sources and behavior-specific beliefs influenced prevention behaviors throughout the COVID-19 pandemic. Study 1, a longitudinal nationally representative survey study of U.S. adults, found main and interactive effects of trust in public health sources and behavioral beliefs in their prediction of three recommended COVID-19 prevention behaviors (facemask wearing, social distancing, vaccination) in May/Jun. 2020 (T1), Jul. 2020 (T2), and Apr./Jun. 2021 (T3). Study 2, an online survey experiment conducted in Dec. 2021/Jan. 2022, examined the main and interactive effects of building trust and providing pro-behavior evidence in messages recommending child COVID-19 vaccines to U.S. parents. Trust cues and evidence each had main effects on persuasive outcomes [perceived message effectiveness (PME) and pro-behavioral beliefs]. There were negative interaction effects of trust and evidence, which were significant for PME and not significant for beliefs. Study 3, a mixed-methods case study, documents the process through which youth participatory action research (YPAR) produced local data-informed campaign messages and contributed to quantitative survey development. Study 4 uses the YPAR-informed survey instrument in a population-level longitudinal study of U.S. young adult bivalent booster uptake conducted in Nov./Dec. 2022 (T1) and Feb./Mar. 2023 (T2). A new measure (“EVT”) captured exposure to, valence of, and trust in twelve potential information sources about COVID-19 vaccination. EVT and behavioral beliefs had significant main effects on subsequent booster intentions and behavior longitudinally and cross-sectionally. Interactive effects of EVT and behavioral beliefs were in the hypothesized (negative) direction across analyses but were only significant in the T2 cross-sectional analysis. Overall, results from quantitative studies reflect the independent importance of both exposure to recommendations from trusted sources and evidence supporting pro-behavioral beliefs on recommendation following in a complex media environment and unprecedented public health crisis. Further, this dissertation offers a model for integrating participatory action research with population-level research. Future work should explore how new iterations of this participatory model can maximize the impact of communication interventions locally and more broadly.