The FDA seems to be satisfied that Pfizer's RSV vaccine is safe and effective for administration in pregnant women to protect infants from severe disease.
The agency’s confidence was revealed in briefing documents filed ahead of an advisory committee meeting set for Thursday. The Vaccines and Related Biological Products Advisory Committee will consider whether immunization with Pfizer’s Abrysvo is safe and effective in protecting infants aged birth through six months from RSV lower respiratory tract disease and severe RSV disease when administered during the second or third trimester of pregnancy.
This is the third advisory meeting in recent months to discuss RSV vaccines and the second for Abrysvo. Pfizer’s shot was endorsed by the same committee in February for preventing lower respiratory tract disease caused by RSV in older adults, although the FDA flagged some safety concerns, including Guillain-Barré syndrome. The same committee reviewed and endorsed GSK’s rival RSV vaccine candidate Arexvy, which has since become the first shot approved in the older population.
Now, it’s time for the committee to review the proposed maternal use for Pfizer’s candidate. The safety data underscore what's at stake in the development of these RSV vaccines, which have long eluded the pharma industry: At least one infant died from respiratory illness due to RSV in the placebo group in one of Pfizer's studies.
Pfizer said that Abrysvo has the potential to reduce global infant mortality due to RSV if approved, according to the company's briefing documents.
Five studies were submitted for consideration in the company's biologics license application. The FDA said review of the evidence—which covered 4,144 participants—did not reveal any significant safety concerns. The most commonly reported safety event was pain at the injection site.
The FDA did flag a 1% higher rate of preterm births in the vaccine arm of one study, at a rate of 5.7% preterm birth compared with 4.7% in the placebo group. The agency called this a numerical imbalance that did not appear to be statistically significant.
One infant was born preterm at 36 weeks and five days, 86 days after the maternal vaccination. While this was considered related to receiving the shot, the child had a normal birth outcome and there were no complications, according to the FDA.
Pfizer also successfully demonstrated that Abrysvo is effective in preventing RSV lower respiratory tract disease and severe RSV disease in infants when administered at 24 to 36 weeks gestation.
The shot was 82% effective at preventing lab-confirmed severe medically attended lower respiratory tract illness caused by RSV at 90 days after birth and 69% effective in protecting at 180 days. At preventing lab-confirmed disease altogether, Abrysvo was 57% effective through 90 days after birth. This measure was “clinically meaningful,” according to Pfizer, but did not meet the study's primary endpoint.
Pfizer also examined the vaccine’s efficacy against RSV-associated medically attended lower respiratory tract illness, finding it was 45% effective and 41% effective through 210 and 360 days after birth, respectively. Abrysvo was also 68% effective at preventing RSV-associated hospitalizations through 90 days post-birth and 57% effective through 180 days. This effect did not last beyond 360 days after birth.
The FDA is expected to deliver a decision on Abrysvo's use in the maternal population by Aug. 21. The vaccine was granted priority-review designation earlier this year. A decision in the older population is expected by May 31. In total, 25,017 people have received the shot across all populations, Pfizer noted.
GSK previously paused work on a maternal RSV vaccine over a safety red flag and has yet to restart. Pfizer has another challenger in AstraZeneca and Sanofi’s long-acting antibody nirsevimab, which is administered after birth but could also win approval this year.