THURSDAY, March 14, 2024 (HealthDay News) — Pregnant women receiving the respiratory syncytial virus (RSV) prefusion F protein-based maternal vaccine (RSVPreF3-Mat) have offspring with lower risks for any and severe medically assessed RSV-associated lower respiratory tract disease; however, the vaccine may increase the risk for preterm birth, according to a study published in the March 14 issue of the New England Journal of Medicine.

Ilse Dieussaert, from GSK in Wavre, Belgium, and colleagues conducted a phase 3 trial involving pregnant women aged 18 to 49 years to examine the efficacy and safety of RSVPreF3-Mat. Women were randomly assigned to receive either RSVPreF3-Mat or placebo between 24 weeks 0 days and 34 weeks 0 days of gestation. Enrollment and vaccination were stopped early after the observation of a higher risk for preterm birth in the vaccine group.

The analyses included 5,328 pregnant women and 5,233 infants; 3,426 infants in the vaccine group and 1,711 in the placebo group were followed from birth to 6 months of age. The researchers found that 16 and 24 infants in the vaccine and placebo groups, respectively, had any medically assessed RSV-associated lower respiratory tract disease (vaccine efficacy, 65.5 percent), and eight and 14, respectively, had severe medically assessed RSV-associated lower respiratory tract disease (vaccine efficacy, 69.0 percent). Preterm birth occurred in 6.8 and 4.9 percent of infants in the vaccine and placebo groups, respectively (relative risk, 1.37).

“The increased risk of preterm birth in the vaccine group was largely seen in low- and middle-income countries, was limited to a defined period in the trial, and remains unexplained,” the authors write.

The study was funded by GlaxoSmithKline Biologicals, which is developing RSVPreF3-Mat

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