Advisers to the U.S. Food and Drug Administration (FDA) voiced support on May 18 for Pfizer’s maternal RSV vaccine even after several expressed concerns over premature births.
Outside experts on the Vaccines and Related Biological Products Advisory Committee unanimously said data from two trials run by Pfizer were sufficient to support the vaccine being effective at preventing RSV disease in infants. They also voted 10–4 in the affirmative when asked if the data from the trials were sufficient to demonstrate the safety of the vaccine.
The votes set up FDA clearance for what would be the first maternal vaccine for the respiratory syncytial virus, or RSV.
Pfizer’s vaccine, a recombinant protein subunit shot, targets the RSV A and RSV B subgroups in a 120 microgram dose that would be given to pregnant women in the second or third trimester. The goal would be to relay antibodies against RSV to fetuses, conferring protection that is supposed to last through the first year of life.
The vaccine was 81.8 percent efficacious through 90 days after birth against severe lower respiratory tract illness due to RSV. The efficacy dropped to 69.4 percent after another 90 days.
For any RSV-positive medically attended lower respiratory tract illness, the vaccine started at 57.1 percent efficacy and dropped to 51.3 percent over the same timeframe, according to data from a phase 3 trial that included about 3,500 vaccinated pregnant women.
The efficacy for medically attended lower respiratory tract illness from any cause was just 2.5 percent at 180 days and 5.1 percent at 360 days.
FDA staffers said in a briefing document that the trial showed “successful vaccine efficacy, and the advisers seconded that view with their unanimous vote.
But multiple experts expressed concerns about the risk of the vaccine causing premature births, a risk found in a similar product made by GlaxoSmithKline that prompted the company to halt testing in 2022.
In Pfizer’s trial, there were more premature deliveries in the vaccinated arm than the placebo arm—5.7 percent in the former versus 4.7 percent in the latter.
“That is hanging over this program,” Dr. Paul Offit, one of the FDA’s advisers, said during Thursday’s meeting.
