A phase 3 trial of a maternal vaccine candidate for respiratory syncytial virus (RSV) has been stopped early because the risk for preterm births is higher in the candidate vaccine group than in the placebo group.
By the time enrollment was stopped on February 25, 2022 because of the safety signal of preterm birth, 5328 pregnant women had been vaccinated, about half of the intended 10,000 enrollees. Of these, 3557 received the candidate vaccine RSV prefusion F protein–based maternal vaccine, and another 1771 received a placebo.
Data from the trial, sponsored by GSK, were immediately made available when recruitment and vaccination were stopped, and investigation of the preterm birth risk followed. Results of that analysis, led by Ilse Dieussaert, IR, vice president for vaccine development at GSK in Wavre, Belgium, are published online on March 13 in The New England Journal of Medicine.
“We have discontinued our work on this RSV maternal candidate vaccine, and we are closing out all ongoing trials with the exception of the MAT-015 follow-on study to monitor subsequent pregnancies,” a GSK spokesperson said in an interview.
The trial was conducted in pregnant women aged 18-49 years to assess the efficacy and safety of the vaccine. The women were randomly assigned 2:1 to receive the candidate vaccine or placebo between 24 and 34 weeks’ gestation.
Preterm Births
The primary outcomes were any or severe medically assessed RSV-associated lower respiratory tract infection in infants from birth to 6 months and safety in infants from birth to 12 months.
According to the data, preterm birth occurred in 6.8% of the infants in the vaccine group and in 4.9% of those in the placebo group (relative risk [RR], 1.37; 95% CI, 1.08-1.74; P = .01). Neonatal death occurred in 0.4% in the vaccine group and 0.2% in the placebo group (RR, 2.16; 95% CI, 0.62-7.56; P = .23).
To date, only one RSV vaccine (Abrysvo, Pfizer) has been approved for use during pregnancy to protect infants from RSV-associated lower respiratory tract infection.
“It was a very big deal that this trial was stopped, and the new candidate won’t get approval.” said Aaron E. Glatt, MD, chair of the Department of Medicine and chief of Infectious Diseases and Hospital Epidemiologist at Mount Sinai South Nassau in Oceanside, New York.
Only One RSV Vaccine Approved in Pregnancy
Dr. Glatt pointed out the GSK vaccine is like the maternal vaccine that did get approved. “The data clearly show that there was a slight but increased risk in preterm labor,” Dr. Glatt said, “and while not as clearly shown, there was an increase in neonatal death in the group of very small numbers, but any neonatal death is of concern.”
The implications were disturbing, he added, “You’re giving this vaccine to prevent neonatal death.” Though the Pfizer vaccine that was granted approval had a very slight increase in premature birth, the risk wasn’t statistically significant, he pointed out, “and it showed similar benefits in preventing neonatal illness, which can be fatal.”
Dr. Glatt said that there is still a lingering concern with the approved vaccine, and he explained that most clinicians will give it closer to the end of the recommended time window of 34 weeks. “This way, even if there is a slight increase in premature term labor, you’re probably not going to have a serious outcome because the baby will be far enough along.”
A difference in the incidence of preterm birth between the experimental vaccine and placebo groups was predominantly found in low- and middle-income countries, according to Dieussaert’s team, “where approximately 50% of the trial population was enrolled and where the medical need for maternal RSV vaccines is the greatest.”
The RR was 1.56 (95% CI, 1.17-2.10) for low- and middle-income countries and 1.04 (95% CI, 0.68-1.58) for high-income countries.
“If a smaller percentage of participants from low- and middle-income countries had been enrolled in our trial, the RR for preterm birth in the vaccine group as compared with the placebo group might have been reduced in the overall trial population,” they reported.
The authors explained that the data do not reveal the cause of the higher risk for preterm birth in the vaccine group.
“We do not know what caused the signal,” the company’s spokesperson added. “GSK completed all the necessary steps of product development including preclinical toxicology studies and clinical studies in nonpregnant women prior to starting the studies in pregnant women. There were no safety signals identified in any of the earlier parts of the clinical testing. There have been no safety signals identified in the other phase 3 trials for this vaccine candidate.”
Researchers did not find a correlation between preterm births in the treatment vs control groups with gestational age at the time of vaccination or with particular vaccine clinical trial material lots, race, ethnicity, maternal smoking, alcohol consumption, body mass index, or time between study vaccination and delivery, the GSK spokesperson said.
The spokesperson noted that the halted vaccine is different from GSK’s currently approved adjuvanted RSV vaccine (Arexvy) for adults aged 60 years or older.