Timing is everything: optimizing maternal RSV immunization through ANC attendance patterns and timing in Viet Nam

Maternal immunization against respiratory syncytial virus (RSV) offers a promising strategy to reduce severe respiratory infections and hospitalizations among infants, particularly in low- and middle-income countries such as Viet Nam. Administered during antenatal care (ANC), success of maternal RSV immunization programs depends on alignment of ANC visits with vaccine timing recommendations. We conducted a retrospective cross-sectional study at 15 health facilities across levels of care in Viet Nam. Records from ANC registers for women attending at least one visit in 2023 were included. We analyzed gestational age at ANC visits, visit frequency, and attendance within two vaccine eligibility windows: the window used in Viet Nam (24–36 weeks’ gestation) and the World Health Organization (WHO)-recommended window (≥ 28 weeks' gestation). Among 7,349 women, ANC attendance and visit frequency varied by level of care. Women attending primary facilities had recorded ANC visits earlier (13 weeks’ gestation) and attended more frequently (median four visits) than those at secondary (38 weeks’, median one visit) and tertiary facilities (32 weeks’, median one visit). Attendance within RSV vaccine eligibility windows also differed by level of care. At primary facilities, 47% of women had at least one recorded visit during Viet Nam’s eligibility window, increasing to 88% under the WHO-recommended window. At secondary facilities, 26% had a recorded visit during Viet Nam’s window compared to 85% under the WHO window, while at tertiary facilities, corresponding proportions were 36% and 69%, respectively. Daily vaccine-eligible ANC attendance varied by level of care, with higher-level facilities receiving more eligible women. This study demonstrates the importance of aligning maternal RSV immunization strategies with ANC attendance patterns across levels of care in Viet Nam. Primary facilities, characterized by earlier and more frequent ANC engagement, are critical platforms for maternal vaccination. Adopting the wider WHO-recommended eligibility window, strengthening early ANC counseling, expanding public-sector access, and improving digital continuity of care are essential to achieving equitable national rollout.