Study: Investigational RSV Vaccine Elicited Neutralizing Antibody Responses in Pregnant Women

Results from a phase 2b trial showed that a bivalent RSV prefusion F protein-based vaccine was safe and led to higher neutralizing antibody responses in pregnant women.

A bivalent respiratory syncytial virus (RSV) prefusion F protein-based (RSVpreF) vaccine administered during pregnancy elicited neutralizing antibody responses with efficient transplacental transfer and without “evident safety concerns,” according to results from a phase 2b trial.

“In the United States, RSV is a common cause of hospitalization in infants, with an estimated annual incidence exceeding 2,000 cases of RSV pneumonia per 100,000 infants,” wrote researchers led by Eric Simões, MD, clinical professor of pediatrics-infectious diseases, University of Colorado School of Medicine, in The New England Journal of Medicine. “Currently, no specific therapy or vaccine to treat or prevent RSV infection is available.”

To determine the efficacy, immunogenicity, and safety of a bivalent RSVpreF vaccine in pregnant women and their infants, Simões and colleagues randomly assigned 406 women who were at 24 through 36 weeks’ gestation to receive either 120 or 240 µg of RSVpreF vaccine—with or without aluminum hydroxide—or a placebo. The study included 403 infants.

The team analyzed safety outcomes and the 50% titers of RSV A, B, and A/B neutralizing antibodies in maternal serum at delivery and in the umbilical cord blood. The researchers also calculated maternal-to-infant transplacental transfer ratios, according to the study.

In total, 327 (80.5%) women received the RSVpreF vaccine. Most postvaccination reactions were mild to moderate, although the incidence of local reactions was higher among participants who received RSVpreF vaccine containing aluminum hydroxide than among those who received the vaccine without aluminum hydroxide. The incidence of adverse events in the women and infants were similar in the vaccine and placebo groups.

The geometric mean ratios of 50% neutralizing titers between the infants of women in the vaccine group and of those of placebo recipients ranged from 9.7 to 11.7 among those with RSV A neutralizing antibodies and from 13.6 to 16.8 among those with RSV B neutralizing antibodies, noted researchers.

Results showed that the transplacental neutralizing antibody transfer ratios ranged from 1.41 to 1.67 with RSVpreF vaccines containing aluminum hydroxide and from 1.68 to 2.1 with those not containing aluminum hydroxide.

“The neutralizing titers were transferred across the placenta efficiently, and a post hoc analysis suggested that the transferred antibodies prevented medically attended RSV-associated lower respiratory tract illnesses in infants,” wrote investigators.

A phase 3 clinical trial is currently being conducted in response to the local injection reactions observed in women who received an RSVpreF vaccine containing the chemical compound. The trial will establish the efficacy of RSVpreF vaccines without aluminum hydroxide, according to Simões et al.

Reference: Simões EAF, Center KJ, Tita ATN, et al. Prefusion F protein-based respiratory syncytial virus immunization in pregnancy. N Engl J Med. 2022;386:1615-1626.