**Country-level vaccination rate (at least one dose).** We include an estimate of the proportion of the country’s population that received at least one dose of a COVID-19 vaccine. Country level data was obtained at the time of data collection from Our World in Data (2024), which collected data from public official sources. The effect of the intervention could be smaller in countries with higher vaccination rate because the remaining unvaccinated sample may be particularly reluctant to receive the vaccine. It might also reflect the level of deployment of the vaccine as well as the general acceptance of the vaccine.

**Intervention start date.** We included the start month of the data collection. The dates were centered so that the starting month of the first intervention (i.e., January 2021) was set to month 0, and so on until month 16, corresponding to the start of the last intervention (May 2022). The intervention start date might also reflect the available knowledge about the virus and availability of the vaccine at the time of the intervention.

**GDP per capita PPP.** We also explore country wealth using gross domestic product (GDP) converted to international dollars using purchasing power parity (PPP) rates from the World Bank (2024). An international dollar possesses equivalent purchasing power relative to GDP as the U.S. dollar does within the United States. The GDP per capita has been shown to correlate with a large number of social and health outcomes across countries (Bloom & Canning, 2000), which may influence the effectiveness of the behaviorally informed messages targeting vaccine intentions.  When including this variable in the study-level moderator analysis, we center the data by subtracting the median GDP per capita PPP of the sample countries from the GDP per capita PPP of each country.

**Trust in healthcare workers.** We explore whether trust in healthcare worders (e.g., doctors and nurses) before the onset of the pandemic moderates impacts of message framings. We use data from the Wellcome Global Monitor (Wellcome Trust & Gallup, 2018), which assessed trust in healthcare workers with an item rating “ _How much do you trust each of the following?_ ” for “ _Doctors and nurses in this country_ ” on a 1-4 scale, with response options of “A lot”, “Some,” “Not much,” “Not at all”. Respondents could also respond with “Don’t know,” and “Refused”—such responses were dropped from our analysis. For ease of interpretation, we reverse-coded the responses, so higher values indicate stronger trust such that 1 = _Not at all_ and 4 = _A lot_ . We aggregate trust to the country level by calculating the mean response within each country. We then center country-level trust measures by subtracting the median country-level trust measure from each country-level means.

**Perceived vaccine safety.** We included perceived vaccine safety before the COVID-19 pandemic. We used data from the Wellcome Global Monitor (Wellcome Trust & Gallup, 2018). Perceived vaccine safety was measured by asking “Do you agree, disagree, neither agree nor disagree with the following statement? Vaccines are safe.” on a 1-5 scale, with response options of “Strongly agree,” “Somewhat agree,” “Neither agree nor disagree,” “Somewhat disagree,” “Strongly disagree,” “Don’t know,” and “Refused.” We reverse-coded the responses, so higher values indicate stronger agreement, such that 1 = _Strongly disagree_ and 5 = _Strongly agree_ . We aggregate perceived vaccine safety to the country level by calculating the mean response within each country. We then center country-level vaccine safety perceptions by subtracting the median country-level trust measure from each country-level mean.

**Perceived vaccine effectiveness.** We included perceived vaccine effectiveness before the COVID19 pandemic using data from the Wellcome Global Monitor (Wellcome Trust & Gallup, 2018). Perceived vaccine effectiveness was measured by asking “Do you agree, disagree, neither agree nor disagree with

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