income context. We show that the direction of misalignment generally runs counter to what was previously observed in the case of routine immunization rates before the pandemic: Survey estimates systematically exceed vaccine coverage reported in administrative sources across most countries we study. This evidence suggests that the ongoing effort to reach widespread COVID19 immunization in LMICs relies on data that would imply (sometimes vastly) different policy conclusions depending on the data source consulted.

Second, our results suggest that phone surveys can be a suitable and reliable tool for COVID-19 vaccination research and possibly other health policy contexts, supporting the robustness of findings from a large body of research during COVID-19 that relied on these data (Kanyanda et al. 2021; Solís Arce et al. 2021; A. Reza et al. 2022; Wollburg, Markhof, et al. 2022; Wollburg et al. 2023). While survey estimates are somewhat sensitive to design choices, especially the selection of respondents to be interviewed, commonly feared and hard to ascertain non-sampling errors in phone surveys, such as mode effects, panel conditioning, and experimenter demand, appear not to affect estimates of vaccine uptake meaningfully. Careful survey design can limit the effects of sampling errors.

Third, our experimental results inform best practices for future phone survey-based research and policy beyond the COVID-19 context. Our results suggest that random respondent selection, wherever possible from an up-to-date roster of household members, can successfully mitigate errors of representation and should be preferred for collecting detailed, individual-level information on vaccine and health-related issues. This particularly applies where the information collected would be less salient to other household members such as (vaccine) attitudes and personal sentiments. Collecting proxy-reported information through a knowledgeable, purposively selected household member can constitute an alternative that is reasonably accurate for salient information such as vaccine uptake, is less challenging to implement from a survey design perspective, and is applicable to multi-topic surveys that require the selection of a broadly knowledgeable respondent.

In this sense, our findings also underscore the complementarity between phone and face-to-face surveys. The latter remain indispensable to provide a recent sampling frame for households and individuals which our results suggest are some of the mainstays of reliable estimates. At the same time, phone survey data can complement face-to-face data collection. For example, the usefulness of survey data for policy has been limited by its low frequency (every 2-3+ years) and the difficulty of conducting surveys in hard-to-access or conflict-affected areas (Cutts et al. 2016, DanovaroHoliday et al. 2021, Burton et al. 2009). Phone surveys offer the opportunity to improve on these limitations.

As phone surveys have been proposed as flexible vehicles for (health) data collection (Gourlay et al. 2021, Zezza et al. 2022, Glazerman et al. 2023) our findings may find direct application in future (vaccine) data collection efforts. Such efforts may include campaigns to catch up on (foregone) child immunization which saw the largest sustained decline in 30 years during the COVID-19 pandemic (WHO 2022, The Lancet Child & Adolescent Health 2021, Causey et al. 2021) or a large-scale rollout of the first Malaria vaccine (Wilyard 2022).

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