<mark>and middle-income countries and territories to help inform policy makers’ approach to communicate for promoting uptake of the COVID-19 vaccine.</mark> While research has explored vaccine acceptance globally, including new research on COVID-19 vaccine hesitancy, evidence on crafting effective communications to promote COVID-19 vaccine acceptance in low- and middle-income countries is still limited (Batteux et al., 2022; Xia & Nan, 2023). The gap in experimental evidence providing insights into effective messaging in LMICs and non-Western, Educated, Industrial, Rich, and Democratic (WEIRD) contexts remains notable. Additionally, existing syntheses of effective communication for encouraging COVID-19 vaccination were largely systematic reviews (e.g., Batteux et al., 2022; Xia & Nan, 2023) and evidence synthesis (e.g., Ruggeri et al., 2024; Nan et al., 2022; van Bavel et al., 2020) that did not use meta-analytic methods, which could identify overall effect sizes, moderators, and sources of heterogeneity (Card, 2012).

Our research complements this work by quantifying and comparing messages’ overall effect sizes and moderators across countries using meta-analytic methods, which can help generate more precise policy recommendations about effective messaging to promote vaccine confidence. COVID-19 vaccine hesitancy may differ from hesitancy toward long-established vaccines like measles, mumps, and Rubella (MMR); seasonal influenza; and diphtheria, tetanus, and pertussis (DTaP); or newer ones like human papillomavirus (HPV), due to its rapid introduction and promotion on a large scale (e.g., Dombrádi et al., 2021).

The purpose of this research is to summarize the work completed in support of policy makers to evaluate the effectiveness of different messages to increase COVID-19 vaccination intentions by summarizing the findings of 28 survey experiments with 123,720 adults recruited from social media in 23 countries and territories between January 2021 and June 2022 using two-stage individual patient data (IPD) meta-analysis (MA) methods. Results from the meta-analysis show that, among unvaccinated survey respondents, behaviorally informed messages significantly increased the odds of vaccination intention by 1.28 times overall. Significant pooled effects of specific framings ranged from increasing odds of vaccination intentions by 1.16 times (framing that highlighted the shifting nature of the pandemic and need to protect against different variants of the virus) to 1.45 times (framing that leveraged messenger effects and endorsement of the vaccine by experts and religious leaders). While messages featuring experts with religious leaders or celebrities, and dynamic social norms had consistent impacts, others, such as endemic, safety, experts, efficacy and pro-social, and variant messages, showed moderate-to-high heterogeneity. This research underscores the importance of communication tailored to address different drivers of vaccine hesitancy and offers insights for handling future health crises with behavioral communication strategies leveraging rapid insights afforded by social media.

While this research offers evidence to help close the gap in understanding how to effectively promote vaccine intentions in primarily LMICS, the study has limitations that should be noted. In particular, this research leverages non-representative Facebook sampling that under-represents older, poorer, and less educated groups, and could reflect self-selection bias. In addition, this study examines impact of messaging on self-reported intentions instead of actual vaccination behavior. Still, this work offers new evidence to fill gaps in understanding message effects on vaccine confidence in non-WEIRD and primarily low- and middle-income countries that could inform future research that aims to address this study’s limitations and future public policy efforts.

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