Moms who receive investigational RSV vaccine pass antibodies to infants

Infants born to mothers who were immunized with an investigational vaccine against respiratory syncytial virus — or RSV — received antibodies across the placenta, with no “evident safety concerns,” results from a phase 2b trial showed.

“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,” Eric A. F. Simões, MD, a clinical professor of pediatrics-infectious diseases at University of Colorado School of Medicine and Children’s Hospital Colorado in Aurora, Colorado, and colleagues wrote in The New England Journal of Medicine. “Currently, no specific therapy or vaccine to treat or prevent RSV infection is available.”

Simões and colleagues randomly assigned 406 women with a gestational age of 24 to 36 weeks to receive Pfizer’s investigational bivalent RSV prefusion F protein-based (RSVpreF) vaccine at 120 µg or 240 µg — with or without aluminum hydroxide — or a placebo. They analyzed safety outcomes and the titers of RSV A, B and A/B in the maternal serum at delivery and in the umbilical cord blood. The researchers also calculated transplacental transfer ratios.

In total, 327 (80.5%) women received a dose of the RSVpreF vaccine. Most postvaccination reactions were mild or moderate, although local reactions were more common in women who received vaccines containing aluminum hydroxide. Adverse events occurred at similar rates between the vaccine and placebo groups.

Among 379 mothers included in the immunogenicity analyses, the researchers found that the 50% geometric mean titer ratios in maternal serum ranged from 11 to 15.1 for RSV A antibodies and from 13.7 to 17.5 for RSV B antibodies. These responses were at least as high as those in nonpregnant people, the researchers said.

The geometric mean titer ratios in umbilical cord blood ranged from 9.7 to 11.7 and from 13.6 to 16.8 for RSV A and B antibodies, respectively.

For the 372 infants included in the immunogenicity analyses, the geometric mean transplacental 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,” Simões and colleagues wrote.

Moving forward, an ongoing phase 3 clinical trial will establish the efficacy of RSVpreF vaccines without aluminum hydroxide. The trial is being conducted in response to the local reactions observed in women who received an RSVpreF vaccine containing the chemical compound.

In March, Pfizer announced that the RSVpreF received breakthrough therapy designation from the FDA.

References:

• Pfizer granted FDA breakthrough therapy designation for respiratory syncytial virus (RSV) vaccine candidate for the prevention of RSV in infants from birth up to six months of age by active immunization of pregnant women. https://www.pfizer.com/news/press-release/press-release-detail/pfizer-granted-fda-breakthrough-therapy-designation. Published March 2, 2022. Accessed April 27, 2022.
• Simões EAF, et al. N Engl J Med. 2022;doi:10.1056/NEJMoa2106062.