Rationale: The highest rates of coronavirus disease (i.e., COVID-19) vaccine hesitancy in the US are among young adults (YAs) aged 18-25. Our preliminary studies show that social norms - perceptions of peers' vaccination attitudes/behaviors - are most strongly related to YAs' vaccine intentions/uptake. Most YAs underestimate the perceived importance of vaccination and their peers' intentions to be vaccinated. The proposed research will develop and test an intervention to correct misperceived norms for vaccination hesitancy and uptake.

Methodology: Rapid prototyping with 20 unvaccinated YAs will help refine the content and design of the online intervention. Then, a diverse national sample (N=600) of unvaccinated YAs will be randomized to treatment or an attention-matched control. The treatment condition will receive personalized normative feedback (PNF) designed to correct normative misperceptions for vaccine hesitancy and uptake.

Normative feedback will be derived from the US Census Bureau's Household Pulse Survey. Follow-up surveys will be administered at 1, 2, 3, and 6 months to assess key outcomes including vaccine uptake, intentions, and reasons for vaccine hesitancy.

Aims and Data Analysis:

• Aim 1: Develop and refine a PNF intervention for vaccine hesitancy/uptake with user feedback from YAs. Rapid analysis of qualitative data will involve looking for themes in responses. Changes will be made iteratively to refine intervention content, design, and delivery.
• Aim 2: Evaluate intervention efficacy for increasing vaccine uptake and reducing time to first vaccine dose, relative to control, over the following year.
• Aim 3: Examine mediators (changes in perceived norms) and moderators (intellectual humility, identification with other people and young adults) of intervention efficacy. A longitudinal moderated mediation model will be examined.

Impact: Findings will clarify the causal role of psychological determinants of vaccine hesitancy (social norms, intellectual humility, group identification). If preliminary intervention efficacy is supported, this intervention could be a low-cost, and easily disseminated strategy to promote YAs' vaccine uptake and contribute to public health efforts to address the COVID-19 pandemic.

Inclusion Criteria:

• 18-24 years old (at screening)
• Reside in the United States.
• Have not received a COVID-19 vaccine (at screening)
• Pass attention checks.

Exclusion Criteria:

• Not meeting inclusion criteria.
• Not fluent in English.
• Not providing consent.
• Unwilling to participate.