In July 2023 Pfizer announced data from a Phase II study investigating its hexavalent capsular polysaccharide (CPS) conjugate Group B Streptococcus (GBS) vaccine candidate, GBS6. This is being developed for maternal administration to protect infants against invasive GBS disease. Results, published in The New England Journal of Medicine (NEJM) will inform a planned Phase III clinical development programme. They show that GBS6 generated “robust” maternal antibody responses against the 6 GBS CPS serotypes in the vaccine. These antibodies were “efficiently” transferred to infants at ratios of ~0.4-1.3 depending on GBS6 group.  


The study states that GBS is a “common cause” of sepsis and meningitis in newborns up to 89 days old, with the primary risk factor being exposure to maternal rectovaginal group B streptococcal colonisation during delivery. Additionally, ascending group B streptococcal infection in the mother can affect the foetus before delivery, causing intraamniotic infection, premature labour, or stillbirth. In many high-income countries, pregnancy screening is available, with intrapartum antibiotic prophylaxis more than 80% effective in the prevention of early-onset disease in infants (0 to 6 days of age). However, it is not as effective against late-onset disease (from 7 to 89 days) or prebirth sequelae associated with infection.  

The authors claim that a maternal vaccine, administered during pregnancy, could potentially prevent both early and late-onset disease and “may mitigate the need for intrapartum antibiotic prophylaxis” for “otherwise healthy women”.  

“Such a vaccine could be beneficial because intrapartum antibiotic prophylaxis may contribute to antimicrobial resistance and disrupt development of the infant microbiome.”  

Furthermore, it would be a “much-needed measure” against GBS infection for the “substantial percentage of pregnant women living in resource-limited community settings”.  


The vaccine is a hexavalent anti CPS/genetically detoxified diphtheria toxin cross reactive material (CRM) 197 glycoconjugate, developed to help prevent invasive GBS in newborns. Previously, Pfizer has successfully used polysaccharides conjugated to CRM in pneumococcal vaccines. GBS6 is designed to offer protection against the 6 “most prominent” serotypes, which collectively account for 98% of GBS disease worldwide.  

In trial 

The Phase II trial was divided into 3 stages: 

  1. Evaluated safety and immunogenicity in 66 healthy, nonpregnant individuals in South Africa. 
  2. The focus of the publication – evaluated safety and immunogenicity in 360 healthy pregnant individuals aged 18 to 40 years and their infants in South Africa.  
  3. Evaluation of a final formulation in 216 healthy pregnant individuals and their infants in South Africa, the US, and UK.  

The safety profile for mothers and infants was similar between the vaccine and placebo groups. Dr Annaliesa Anderson, SVP and CSO, Vaccine Research and Development, Pfizer, identifies “hope” in the recent findings.  

“Group B Streptococcus can cause potentially devastating diseases in infants, including sepsis, pneumonia, and meningitis. Annually, there are nearly 400,000 cases of infant disease and approximately 138,000 stillbirths and infant deaths worldwide due to GBS.”  

Dr Anderson hopes to prevent these cases and deaths with a “successfully developed and approved” vaccine. 

“Building on decades of expertise and knowledge in vaccines, we are committed to helping protect newborns and young infants through maternal immunisation.”

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