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The FDA approved Pfizer's bivalent respiratory syncytial virus (RSV) prefusion F protein vaccine (Abrysvo) to protect newborns from serious RSV-related outcomes.

Even though the vaccine was administered in trials starting at 24 weeks' gestation, the agency approved giving the shot at 32 to 36 weeks' gestation over a possible risk for preterm birth.

The vaccine is indicated for reducing the risk of RSV-related lower respiratory tract disease (LRTD) and severe LRTD in infants through 6 months of age.

In the phase III MATISSE study of about 7,000 pregnant patients, the vaccine showed an efficacy of 81.8% against severe LRTD in infants within 90 days after birth, and 69.4% within 180 days after birth, compared with placebo.

In a subgroup of about 3,000 pregnant people who got the shot at 32 to 36 weeks' gestational age, the vaccine reduced infants' risk of any medically attended LRTD by 34.7% within 90 days and 57.3% within 180 days. Risk for severe LRTD was reduced by 91.1% and 76.5%, respectively.

The numerical imbalance in preterm births in patients who received the vaccine -- 5.7% versus 4.7% for those who got placebo -- made it into the vaccine's prescribing information, which includes a "warning to inform" about the difference.

The warning also "informs healthcare providers to avoid the potential risk of preterm birth" by giving the vaccine within the recommended window of 32 to 36 weeks, according to the FDA. Clinical studies of the vaccine excluded those at greater risk for preterm birth.

An FDA advisory committee in May voted unanimously in favor of the vaccine's efficacy at preventing illness in infants, and despite the numerical imbalance, voted 10-4 in favor of its safety profile.

This is the same RSV vaccine from Pfizer that the FDA approved earlier this year for use in adults age 60 and up. Another RSV vaccine, the adjuvanted RSV prefusion F protein shot (Arexvy) made by GSK, was also approved for adults age 60 and up earlier this year, but the company for use in pregnancy after a turned up in trials in vaccine recipients.

As with the vaccine for seniors, Pfizer's vaccine would be given to pregnant people at a single intramuscular dose of 120 mcg.

Safety analyses were based on two trials that included about 7,400 individuals, half of whom received the vaccine. The most common side effects were pain at the injection site, headache, muscle pain, and nausea.

Preeclampsia occurred in 1.8% of pregnant people who got the vaccine compared with 1.4% of those who got placebo. Low birth weight and jaundice also occurred at a higher rate in the vaccine group than the placebo group, the FDA said.

FDA said it's requiring Pfizer to "conduct postmarketing studies to assess the signal of serious risk of preterm birth and to assess hypertensive disorders of pregnancy, including preeclampsia."

This summer, the FDA also approved a monoclonal antibody, nirsevimab (Beyfortus), to protect infants and at-risk young children against RSV. The single intramuscular injection is indicated for babies born during or entering their first RSV season, and for high-risk children up to age 2 years during their second season.

While the CDC's Advisory Committee on Immunization Practices has yet to weigh in, infectious disease experts told MedPage Today that infants whose mothers receive the RSV vaccine likely will not need to receive the monoclonal antibody.

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