Building a Head Start on Protection for Newborns: Six-Valent Group B Streptococcus Vaccine Passes Early Trial in Pregnant Women

During the first few weeks after birth, when newborns are at their most vulnerable, they may be exposed during delivery to group B Streptococcus (GBS) carried by the mother before they have had time to develop their own immune defenses. A randomized phase 1/2 study published in *Nature Medicine* showed that when Pfizer’s investigational six-valent vaccine GBS6 is administered during pregnancy, specific antibodies generated by the mother can cross the placenta to the infant, providing further clinical evidence for preventing early-onset infection.

The later stage of the study enrolled 216 pregnant women, who were randomly assigned to receive either 20 micrograms of GBS6 without an aluminum phosphate adjuvant or a placebo, and followed 209 infants. The results showed that the vaccine induced strong immune responses in pregnant women against different serotypes. Corresponding antibodies were also detected in the infants at birth, demonstrating that maternal antibodies had successfully crossed the placenta.

The central idea behind this approach is to temporarily turn the mother into an “antibody supplier” for the newborn. GBS6 targets six capsular polysaccharides in an effort to cover the major disease-causing serotypes. Compared with vaccinating newborns directly, immunization during pregnancy could allow protection to be present from the moment of birth, bridging the period before the infant’s own immune system has matured.

In terms of safety, adverse events occurred at similar frequencies in the vaccine and placebo groups. Pfizer’s summary of results for the same study, C1091002, also stated that pregnancy complications and infant birth outcomes were broadly similar between the two groups in the third stage of the study. The available data have not shown a clear new safety signal, but an early trial involving only several hundred participants remains insufficient to rule out rare risks.

C1091002 was a randomized, placebo-controlled, observer-blinded study that evaluated safety, tolerability, and immunogenicity in three stages. Participants included healthy nonpregnant women, pregnant women aged 18 to 40, and their infants. The study was conducted from January 2019 to March 2024 at 20 sites in three countries and was completed as planned. This publication subjects the immune-response data from pregnant women and infants to peer review.

Current GBS prevention relies mainly on screening during pregnancy and administering antibiotics during labor, while maternal vaccination could offer a different layer of prevention. However, increased antibody concentrations do not mean the vaccine has been proven to prevent sepsis, pneumonia, or meningitis. The study was also too small to reliably compare actual infection cases and cannot determine how long the antibodies persist.

GBS6 has therefore crossed the threshold for immunogenicity and preliminary safety, rather than reached the endpoint of clinical benefit. Further studies in larger and more diverse populations of pregnant women and infants are still needed to confirm the timing of vaccination, the protective antibody threshold, rare adverse events, and whether the vaccine can truly reduce invasive GBS disease in newborns.