Pfizer Group B strep vaccine for infants returns encouraging mid-stage trial results

Pfizer on Wednesday said its experimental vaccine targeting the potentially deadly bacterial disease Group B Streptococcus returned strong mid-stage clinical trial results, a promising step as the drug inches toward potential approval.

Pfizer is among several drugmakers racing to develop the world’s first shot targeting Group B strep disease, which is linked to nearly 150,000 infant deaths worldwide each year, especially in lower-income countries.

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The Food and Drug Administration in September granted breakthrough therapy designation to Pfizer’s vaccine, which is intended to expedite the development and review of the shot. 

Pfizer’s single-dose shot generated antibodies that may provide infants with meaningful protection against the disease, according to the data released Wednesday from a phase two clinical trial.

The jab is administered to expectant mothers, who pass vaccine-induced antibodies to their fetuses. One of the company’s vaccines targeting respiratory syncytial virus also uses that maternal vaccination method. 

Pfizer’s encouraging phase two trial results provide hope that maternal vaccination against the disease, also known as GBS, could help prevent thousands of cases in babies. 

The results will also help the company plan its phase three clinical trials on the shot, which are typically required before the FDA approves a drug.

The Bill & Melinda Gates Foundation, which supported the phase two trial, provided an additional $100 million grant to Pfizer last year that will fund late-stage trials and help facilitate the delivery of shots to lower-income countries following a potential approval.

GBS disease is caused by a common and usually harmless bacteria that many adults carry in their bodies.

But an expectant mother can pass that bacteria to a newborn during labor and delivery, which can cause severe infections during the baby’s first few weeks or months of life.  

About 1 out of every 4 women carries GBS bacteria, according to the Centers for Disease Control and Prevention. 

Infants with GBS infections can experience symptoms including fever and difficulty breathing.

Some infants can experience invasive GBS infections, which cause more serious complications such as pneumonia, infections in the bloodstream, and meningitis, or the inflammation of tissues surrounding the brain and spinal cord.

There are 10 different GBS serotypes, meaning distinct variations of the bacteria that causes the disease. Pfizer’s vaccine targets six of the most prominent serotypes, which collectively account for 98% of GBS disease cases worldwide. 

Pfizer’s trial followed 360 healthy pregnant individuals in South Africa. The mothers were randomly assigned to receive a single shot at three different dosage levels, with or without a specific adjuvant, or a placebo. 

The trial found that Pfizer’s shot generated robust antibodies against the six GBS serotypes in mothers. Those antibodies were “efficiently transferred” to infants at ratios between 0.4 and 1.3, depending on the dose. 

That means some infants received only a fraction of antibodies from their mothers, while others received higher antibody levels than even what their mothers had. 

Pfizer said those antibody transfer levels are associated with a reduced risk of GBS disease. That conclusion was based on a parallel natural history conducted in South Africa.

The safety profile for both mothers and infants appeared to be similar between the vaccine and placebo groups, according to the trial results, suggesting that the shot was generally well tolerated during the phase two trial.

Reactions among mothers following vaccination were generally mild or moderate and short in duration. Between 2% to 8% of participants who received the shot reported fever, compared with 5% in the placebo group, according to the results. 

Around 45% to 70% of pregnant individuals who received the vaccine experienced more adverse reactions such as headache and vomiting. But the placebo group wasn’t much different, with more than 60% of expectant mothers experiencing those adverse events. 

About 62% to 75% of infants in the vaccine group and 74% of those in the placebo group experienced adverse events, including upper respiratory tract infection. There were three infant deaths in the vaccine group and two in the placebo group.

The study authors determined that no adverse events or deaths among infants were related to the vaccine.

The results come as Pfizer braces for a continued decline in Covid-related sales this year.