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BACKGROUND: Protection of newborns from infection can be achieved through maternal or vaccine-induced antibodies, but the factors influencing vaccine protection (correlate of protection) and subsequent infant immunity remain insufficiently understood. Further investigation is essential to optimize early-life vaccination strategies.OBJECTIVE: We sought to evaluate the impact of timing and sequence of hepatitis B vaccine (HBV) and Bacille Calmette-Guérin (BCG) vaccine within the first week of life on infant HBV immunogenicity and its relationship with maternal antibodies.METHODS: This was an exploratory analysis of a US National Institutes of Health-supported prospective, randomized study of systems biology signatures predictive of HBV immunogenicity in 2 geographically distinct cohorts in The Gambia (GAM; N = 720) and Papua New Guinea (PNG, N = 101) (clinicaltrials.gov NCT03246230). Healthy, hepatitis B surface antigen (anti-HBs)-negative mothers and their infants were enrolled and randomized into 4 vaccine groups: HBV alone, BCG alone, HBV + BCG, or no vaccination (delayed until no later than day of life [DOL]7). Blood samples were collected at birth (DOL0) and on a randomly assigned day (either DOL1, DOL3, or DOL7) with a maximum of 2 blood draws in the first week of life per ethics approval. All infants received catch-up vaccination within DOL7 and the recommended oral polio vaccine within 10 days of birth. Additional blood samples were collected at DOL30 and DOL128 to measure anti-HBs titers and assess immunogenicity.RESULTS: Demographic and clinical characteristics were balanced across vaccine groups, suggesting successful randomization in both cohorts. Similar infant growth trajectories and longitudinal anti-HBs titer trends were observed across all vaccine groups in both cohorts over the first 128 days of life. Maternal and neonatal baseline anti-HBs titers were strongly correlated (GAM: = 0.98; PNG: = 0.99). DOL0 titers predicted DOL30 titers (GAM: = 0.56; PNG: = 0.945) but not DOL128 titers. Exploratory pairwise comparisons showed that Gambian infants receiving BCG at birth and HBV at DOL7 had higher DOL30 titers than those vaccinated with HBV at DOL1 (Wilcoxon = .046), whereas in PNG, infants in the HBV + BCG group and the delayed group had higher DOL128 titers than those in the HBV group (Wilcoxon = .046; Wilcoxon = .019, respectively).CONCLUSIONS: This study demonstrates the feasibility of conducting large-scale neonatal immunogenicity studies in resource-constrained settings. Our observations underscore the importance of vaccine timing and maternal antibodies in shaping early-life vaccine-induced immunogenicity, offering valuable insights for neonatal vaccination schedules.