Table 2 Interview Protocol

Pregnant women: pregnant women waiting for their scheduled antenatal care visits at the clinics were approached by research members and asked if they willing to participate in the study. If they were willing, they were taken to a private room/office designated by the hospital for confidential consenting and interviewing. After allowing time for consent review and answering questions, the study team recorded each interview.

 • Inclusion criteria: Women aged 15–40 · Women in any trimester; Patient at health facility included in the study; Be willing to converse with others in a focus group format (only for message testing phase); Able to provide informed consent (If participant is illiterate, procedures to ensure full understanding of the research and consent process will be implemented according to international and federal guidelines).

 • Exclusion criteria: have previously participated in this study; Those who do not or cannot provide consent; Failure to meet other inclusion criteria.

 • Interview Topics: Interview guide for women included discussions on the following topics: (a) have they had a checkup in the last year; (b) have they received any vaccines that they can recall; (c) if they have received vaccines, they will be asked about their understanding of the vaccines they received [e.g., do they know what the vaccine prevents against, did they get the vaccine just because their parent/doctor told them to]; (d) comfort discussing sensitive topics with their doctor and parents; (e) awareness of maternal vaccines; (f) information from peers about maternal vaccines [friends’ vaccination status, anecdotal side effects, discussions on social media, reasons to get it/not get it]; (g) discussions with parents/guardians about maternal vaccines; and (h) motivating factors to be vaccinated.

HCPs: providers working at the antenatal care were contacted ahead of time to arrange interviews for times that would work best for them. During this phase of data collection, HCP were on a nationwide strike. To mitigate the effects of delayed data collection, study team members organized interviews outside of clinic.

 • Inclusion Criteria: Currently working at the selected study sites; Current physician, nurse, nurse midwife, community health worker; Able to provide informed consent.

 • Exclusion Criteria: Those who do not or cannot provide consent; Failure to meet other inclusion criteria.

 • Interview Topics: The semi-structured interview guide for providers included discussions on the following topics: (a) proportion of patients they estimate have received or refused maternal vaccines; (b) times at which they recommend maternal vaccine; (c) practices regarding immunization history verification (e.g. immunization information system); (d) barriers or reasons for refusal cited by parents/patients; (e) perceived ability and methods used to address these barriers/refusals; (f) comfort discussing vaccine recommendations with their patients; (g) existing efforts of reminder/recall for maternal vaccinations; and (h) knowledge of Tdap vaccine effectiveness and safety.

Participant observation and Facility Profiles: Non-structured observation of pregnant women and HCPs were also conducted within the clinic. The research staff took detailed field notes to examine patient-administration, patient-patient, patient-provider relationships, dynamics of provider-government officials, and provider-provider, provider-patient, and provider-administration interactions within each of the selected sites. Interview notes and observations notes were used to edit the guide as needed. Notes about each facility, e.g. patient flow, vaccine storage and supply chain, etc. were also typed up over the course of interviews.