Quantifying maternal vaccination opportunities across gestational age windows: a real-world multi-country, longitudinal study.

Background: Maternal vaccination is a recognized strategy for reducing maternal and neonatal morbidity and mortality, including in low- and middle-income countries (LMICs). As new vaccines such as those for respiratory syncytial virus (RSV) and group B streptococcus (GBS) will be administered according to gestational age (GA), understanding real-world patterns of contact of pregnant women with antenatal care services (ANC) across pregnancy is essential to inform the most effective delivery strategies. Methods: We conducted a retrospective longitudinal, multisite analysis using anonymized data extracted from maternal health registers at ANC and delivery clinics in 6 LMICs: Bangladesh, The Gambia, Nepal, Nigeria, Tanzania, and Uganda. Eligible clinics contributed data from ANC and delivery services and access to routine data from January 2019 to December 2022. Retrospective, individual-level ANC attendance data from a total of 123,867 individuals were included in the study. These included the total number of ANC attendances per woman and GA at first and last ANC contact. The GA determination practices of each clinic were also recorded. Results: Across all sites, women had a median of 4 ANC contacts per pregnancy (IQR 2-5). An estimated 79·2% of women had at least one ANC contact during the 24-36-week GA window, falling to 66·6% in a 28-36 week GA window, and 46·5% at 32-36 weeks. When stratified by ANC contact frequency, the proportion with at least one contact in 24-36 weeks increased to 94·1% among women with ≥4 ANC contacts, indicating significantly greater vaccine delivery potential among those with more frequent ANC contacts. Conclusion: In these LMIC settings, the 24-36-week GA window presents opportunity for delivering maternal vaccination at high coverage. These findings underscore the need to align maternal vaccine delivery strategies with real-world ANC patterns to ensure equitable and timely vaccine access in LMICs.