Maternal GBS vaccine (GBS-AlpN) shows immunogenicity and acceptable safety in phase 2 trial

A phase 2, randomized, placebo-controlled study evaluating the investigational maternal vaccine GBS-AlpN found that the vaccine was immunogenic in pregnant individuals and resulted in high concentrations of vaccine-specific antibodies in infants at birth. The study also reported an acceptable safety profile among maternal participants and infants.

According to investigators, “GBS-AlpN had an acceptable safety profile and was immunogenic when administered to pregnant women, supporting its progression to phase 3 trials, with a flexible two-dose schedule allowing dosing intervals of 4–8 weeks.”

Study population and vaccination schedules

The multicenter study was conducted in Denmark, South Africa, and the United Kingdom. Eligible participants were healthy pregnant women with singleton pregnancies between 21 and 23 weeks of gestation at first vaccination. Participants received 1 or 2 doses of GBS-AlpN or placebo with injections at 22, 26, and 30 weeks’ gestation.

The primary endpoint measured concentrations of Alp-specific IgG in cord blood and early infant blood samples. Safety outcomes included maternal and infant adverse events.

Immunogenicity outcomes in infants

A total of 272 participants were randomly assigned across treatment groups. Investigators reported that “the highest serum IgG concentrations at birth were observed in group 1, followed by the other two dose vaccine groups (groups 2 and 3).” Lower IgG concentrations were reported in group 4 but remained “at least 21-fold higher than in group 5 (placebo only).”

No infants in the placebo arm achieved IgG concentrations greater than 1.0 μg/mL for any AlpN protein.

Safety findings

Maternal treatment-emergent adverse events occurred in 87% to 97% of participants, and investigators stated that “most events were mild–moderate.” Nine infant fatalities occurred during the trial period; investigators reported that “none were considered related to vaccination.”

Clinical implications for prevention of neonatal GBS

The authors noted that a maternal vaccine “could potentially reduce rates of infection in utero, early-onset and late-onset disease, miscarriages, and stillbirths.”

According to the discussion, “GBS-AlpN was shown to have an acceptable safety profile and be immunogenic, both in vaccinated participants and their infants.”

Further clinical development, including phase 3 evaluation, will determine the extent to which this approach may contribute to global GBS disease prevention.

Reference

1. Heath PT, Zuma-Gwala N, Helmig RB, et al. Immunogenicity and safety of a group B Streptococcus vaccine (GBS-AlpN) in pregnant women and their infants: a phase 2, multicentre, observer-blind, randomised, placebo-controlled study. The Lancet Infectious Diseases. Published online December 2025. doi:https://doi.org/10.1016/s1473-3099(25)00659-0