frequency phone surveys were typically carried out by National Statistics Offices and are more plausibly representative than convenience web surveys.

Overall, the estimates of COVID-19 vaccine hesitancy from the HFPSs were more conservative than other published estimates (Figures 9 and 10). There are various possible explanations for the differences. The first relates to the timing of data collection. Most of the existing literature is based on surveys carried out in mid to late 2020 (e.g., de Figueiredo and Larson 2021, Gallup 2021, Lazarus et al. 2021, Solis Arce et al. 2021, and Wouters et. A. 2021), whereas the HFPS data included in our sample were collected from end2020 through August 2021. It is plausible to believe COVID-19 vaccine hesitancy generally declined during this period as multiple vaccines became available and widespread vaccination was safely rolled out in high-income countries.

Another possible reason for the differences is the variation in how the vaccine hesitancy question was framed and the response options available. For example, the HFPS question in countries without general access to the vaccine at the time of data collection was <mark>“When a vaccine to protect you from COVID-19 is available to you, are you planning to be vaccinated?” whereas other studies included the phase “at no cost to you” or descriptors about the quality of the vaccine such as “COVID-19 vaccine proven safe and effective”) (Gallup 2021, Lazarus et al. 2021). There was even more variation across the studies in the answer options available to the respondents. One study relied on a 5-point Likert scale of agreement with the statement “I</mark> f a COVID-19 vaccine is proven safe and effective and is available, I will take it”: <mark>‘completely disagree’, ‘somewhat disagree’, ‘neutral/no opinion’, ‘somewhat agree’ and ‘completely agree” (Lazarus 2021). Others relied on a 4-point Likert scale to the question such as “y</mark> es, definitely,” “yes, probably,’ “no, probably not,” and “no, definitely not” (Africa CDC, Facebook 2021, de Figueiredo and Larson 2021, Wouters et al. 2021) <mark>, whereas the HFPS mostly relied on three answer options (yes, not sure, no). It is possible that “probably not” is not the same as “unsure,” although it is not known how this might bias the respondents’ answers and cross-study comparisons.</mark> Finally, variations in survey modality may have created biases. Among the published studies included for comparison, many studies were based on data from commercial online sample providers, often using quotas sampling to ensure an appropriate distribution in terms of gender, age, and region (Africa CDC, de Figueiredo and Larson, 2021, Facebook 2021, Gallup 2021, Lazarus et al. 2021, and Wouters et. A. 2021), while others were based on convenience samples (Anjorin et al. 2021, Asadi Faezi et al 2021, Bono 2021, Wong 2021), or a mix of methods (Solis-Arce et al. 2021 and Sallam 2021). There was also wide variety in the survey mode, including online, computer-assisted telephone, and face-to-face surveys.

<mark>It is not possible to know the extent to which each of the above-mentioned potential sources of bias affect the comparisons of the findings from the HFPS with the others. For example, it was possible to match the Facebook survey results for the month and year of the HFPS for 20 countries, thus taking away any differences in the timing of the survey. Nevertheless, the HFPS estimates were still lower than those reported by Facebook (Figure 9, Panel D), and this could be due to differences in question-and-answer wording and/or the sample frame.</mark> 18