# **4. Discussion**

## **4.1. Findings Summary**

To identify the effect of behaviorally informed vaccine messaging on vaccine intentions, we conducted 28 randomized survey experiments about COVID-19 vaccination across 23 mostly low- or middle-income countries and territories, recruiting participants through social media. In this individual participant data meta-analysis, among the unvaccinated, we found that behaviorally informed messages significantly increased the odds of having the intention to get vaccinated by 1.28 times overall, with significant impacts of specific framings ranging from 1.16 times (variant framing) to 1.45 times (experts and religious leaders framing), compared to the control group (i.e., no message). The messages using experts and religious leaders showed the largest impacts, followed by endemic, messages with experts and celebrities, efficacy and pro-social, messages with experts, variants, and dynamic social norms. However, the safety message did not significantly increase vaccination intentions across studies. While some framings exhibited larger effects than others, it is important to interpret these impacts considering differences in implementation—as some framings (e.g., experts and safety) were used across more than a dozen studies while other framings were tested in fewer studies as the pandemic evolved (e.g., variants). In addition, it should be reiterated that our findings represent effects for populations who are active on social media and self-selected into a survey about COVID-19, and outcomes reflected selfreported intentions immediately following message exposure.

Messages using experts and celebrities, messages using experts and religious leaders, and dynamic social norms messages had consistent impacts across the countries and studies, indicated by non-significant heterogeneity in effect sizes. Conversely, the endemic, safety, experts, efficacy and prosocial, and variant messages showed moderate-to-high between-study heterogeneity in effect sizes, with impacts varying across studies and countries. Still, none of our tested study-level moderators, including country-level vaccination rates, intervention start dates, and GDP per capita PPP, significantly explained the heterogeneity. Marginally, safety messages have larger impacts in countries with higher vaccination rate, and expert messages have larger impacts when the intervention was conducted later. Lastly, we found most messages have consistent impacts across observable individual demographics, given the minimal individual-level heterogeneity by demographic characteristics, except larger impacts of expert and safety messages for younger people as well as efficacy and pro-social messages for women.

## **4.2. Implications for Behaviorally-Informed Communication in Practice**

When addressing hesitancy, messages focusing on the underlying concerns and drivers of vaccine acceptance can increase intentions to vaccinate. The observed heterogeneity in the effects of specific message framings suggests that the effectiveness of messages may vary based on contextual factors. Some messages, such as those involving experts and celebrities or religious leaders, showed consistent impacts across studies and countries, while others, such as safety messages, were not effective consistently across countries.

That some message impacts varied by context highlights the importance of testing messages with the target population before implementing communications widely. This also underlines the importance of developing messages tailored to specific contexts and populations to maximize their 24