effectiveness, aligning with prior research emphasizing the importance of message customization. This study also demonstrates the potential for public health authorities to use rapid online survey experiments to promptly identify vaccine hesitancy drivers (e.g., Badiani-Magnusson et al., 2022; Hoy et al., 2022, 2023; Lim et al., 2024; Margolies et al., 2023) and test messages in fast-evolving environments like the pandemic among a growing population within LMICs—social media users. There are over 5 billion smartphone users and over 3 billion Facebook daily users globally (Statista, 2023), underscoring the relevance of these findings despite our study samples not being nationally representative on dimensions like education. Furthermore, as we observe advances in the ability to deliver tailored communication to individuals through technologies like online social media, future research can also examine if and how messages tailored to personalized concerns and needs can increase vaccination intentions.

Our results highlight the importance of identifying and working with multiple trustworthy messengers, such as healthcare professionals, celebrities, and religious leaders, to convey messages effectively, given the consistently significant message impacts using experts and celebrities or religious leaders across countries. This aligns with prior research emphasizing the role of messengers in influencing attitudes and behaviors (Alatas et al., 2024; Huang & Sundar, 2022; Lu et al., 2021; Ruggeri et al., 2024). Interestingly, our results show that using multiple trusted messengers can bolster message effectiveness, while using experts (e.g., “national and international experts”) alone did not result in the consistent impacts on vaccination intentions across countries. Public health campaigns should consider partnering with these trusted and popular figures to amplify their messaging and increase its impact while future research can examine additional effects of consensus among multiple information sources.

Given the consistently significant impacts of dynamic social norms messages across countries, public health authorities in low- and middle-income countries could benefit from leveraging these social norms messages. Consistent with this, previous studies have shown that social norms messages focusing on descriptive norms (e.g., widespread vaccine acceptance) can effectively increase vaccination intentions (Ryoo & Kim, 2021; Sinclair & Agerström, 2021). Future research could explore strategies for creating injunctive social norms (Martinez et al., 2023) to complement dynamic norms in contexts where norms are not yet prominent to identify the best methods of promoting vaccination without triggering unintended consequences such as boomerang effects.

Our study also adds to the literature on message fatigue and health risk communication in dynamically evolving situations in that using diverse messages that shift contexts, such as endemic and variant framings, may help mitigate the fatigue and keep messaging relevant. These messages acknowledging the ongoing risks of COVID and highlighting that vaccination is protective even in contexts where disease is endemic were effective in increasing intentions, suggesting that nuanced information reflecting the experience of the public can be effective. This finding underscores the need for ongoing message adaptation and evolution to maintain engagement and effectiveness over time, especially in prolonged public health crises like the COVID-19 pandemic and slow-burning crises.

# **4.3. Study-Level and Individual-Level Moderators**

While the message framing focusing on endemic, safety, experts, efficacy, and prosocial, and variants showed moderate-to-high heterogeneity in message impacts across studies and countries, we found that country-level vaccination rates, intervention start dates, and GDP per capita PPP do not 25