2.1.2. Study Setting and Sampling Criteria

The study was implemented in the DRC (May–June 2021), Mozambique (July–August 2021), and Nigeria (October 2022) by research teams comprising qualitative researchers and project staff with expertise in immunization. The selection of study states/provinces, districts, health facilities, and communities was done stepwise with health system managers at each level. As a qualitative study, we did not use sample size calculations, but did aim to interview enough respondents to achieve theoretical saturation on our research questions. The first sub-national unit, states or provinces, were selected with the project funder and national immunization managers, to ensure that this project's eventual interventions were implemented in provinces in need of an immunization technical partner. Within those states/provinces, we used survey or administrative data to rank districts by their proportion of un- and under-vaccinated children. Among those districts with the highest proportion of un- and under-vaccinated children, we purposefully selected districts that were accessible to our study team during the study period (all countries), represented a mix of both urban and rural districts (DRC), and were free from security concerns (Nigeria). The number of districts selected in each country depended on available financial and human resources to implement the study, resulting in seven districts (health zones) in the DRC, six in Mozambique, and three in Nigeria. Within the selected districts, we worked with district-level managers to select health facilities that represented typical cases of that district, and then selected one community linked to each facility with high proportions of zero-dose or missed communities. We worked with community leaders and CHWs to identify mothers of zero-dose and under-vaccinated children in selected communities.
