The Food and Drug Administration has approved Pfizer’s Abrysvo as the first U.S. vaccine given during pregnancy to protect infants against respiratory syncytial virus, or RSV, from birth through six months of age. In the pivotal study, vaccination reduced the risk of severe RSV lower-respiratory-tract disease by 81.8% during the first 90 days after birth and by 69.4% through 180 days, according to the FDA’s Aug. 21 approval announcement.

The vaccine is approved as a single intramuscular dose at 32 through 36 weeks of gestation. That timing is narrower than the full gestational window studied because regulators observed a numerical imbalance in preterm births and are requiring Pfizer to conduct postmarketing studies. The approval creates a new prevention strategy for an infection that is a major cause of hospitalization in young infants.

Maternal vaccination is designed to protect infants before they can be immunized themselves

The biological strategy is straightforward: vaccinating a pregnant person raises RSV-neutralizing antibodies that cross the placenta and provide passive protection to the infant during the first months of life. Abrysvo contains stabilized prefusion F proteins from RSV A and RSV B strains, targeting a viral structure that is important for infection and that is highly vulnerable to neutralizing antibodies.

Pfizer’s U.S. approval release says the Phase 3 MATISSE trial enrolled more than 7,000 pregnant individuals and, counting their infants, generated data from more than 14,000 participants. The randomized, double-blind, placebo-controlled study was designed to determine whether immunization during pregnancy could prevent medically attended RSV lower-respiratory illness and severe disease in infants.

The registered MATISSE protocol describes an international study spanning both hemispheres so investigators could capture RSV seasons in different regions. Participants received vaccine or placebo during pregnancy, and their infants were followed for RSV outcomes through the first months of life.

The efficacy signal was strongest for severe disease

The trial’s peer-reviewed results were published in April in the New England Journal of Medicine. Severe medically attended RSV-associated lower-respiratory illness occurred substantially less often among infants born to vaccinated participants. The trial met its success criterion for severe disease, although the primary endpoint for all medically attended RSV lower-respiratory illness did not meet its prespecified statistical threshold at 90 days.

FDA’s approval analysis focused on the clinically important reduction in severe disease. Across the broader study population, severe lower-respiratory disease fell 81.8% within 90 days and 69.4% within 180 days. In the subgroup vaccinated during the now-approved 32-to-36-week window, FDA reported a 91.1% reduction in severe disease within 90 days and a 76.5% reduction through 180 days.

Those figures are consequential because the youngest infants have limited respiratory reserve and are among the groups most likely to require hospital care for RSV. A maternal vaccine can provide protection from the day of birth, avoiding the gap that would occur if prevention depended on an infant producing its own immune response after vaccination.

Preterm birth produced a meaningful safety question

The FDA label includes a warning about a numerical imbalance in preterm birth. In the safety studies, preterm delivery occurred in 5.7% of vaccine recipients compared with 4.7% of placebo recipients. Regulators say the available evidence is insufficient to establish or exclude a causal relationship. Preeclampsia occurred in 1.8% of vaccine recipients and 1.4% of placebo recipients.

That uncertainty explains why the U.S. indication is limited to vaccination at 32 through 36 weeks, rather than an earlier pregnancy window. FDA is requiring postmarketing studies to assess the potential preterm-birth signal and hypertensive disorders of pregnancy. The agency is balancing two risks: waiting too long could leave infants without sufficient transferred antibody before delivery, while vaccinating earlier could expose more pregnancies to a safety signal that remains unresolved.

Regulatory review outside the United States provides additional context. In July, a European Medicines Agency committee issued a positive opinion supporting the vaccine for maternal immunization and older adults. On Thursday, the European Commission approved Abrysvo for both uses, with maternal vaccination permitted between 24 and 36 weeks under the European authorization.

RSV prevention is entering a new era

The maternal approval comes during an unusually rapid expansion of RSV prevention. Earlier this year, FDA approved RSV vaccines for adults 60 and older. In July, the agency also approved nirsevimab, a long-acting monoclonal antibody designed to protect infants and some young children through an RSV season. These approaches are complementary rather than identical: one transfers vaccine-induced maternal antibodies before birth, while the other administers a laboratory-made antibody directly to the infant.

How these options are deployed will depend on recommendations from public-health authorities, availability, timing and family preferences. FDA approval establishes that Abrysvo may be used in pregnancy, but national immunization recommendations still require consideration by the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices.

The policy questions will include how to align vaccination with RSV seasonality, how to avoid unnecessary duplication with infant monoclonal-antibody prophylaxis, and how to ensure access for families most likely to experience severe disease. Hospitals and obstetric practices will also need systems to document maternal vaccination so pediatric clinicians know what protection an infant received before birth.

For now, the clinical milestone is clear. A vaccine administered late in pregnancy has demonstrated a large reduction in severe RSV disease during the months when infants are most vulnerable, and FDA has judged the benefit sufficient for approval while explicitly preserving safety monitoring around preterm birth. The result turns decades of RSV vaccine research into the first U.S. maternal immunization strategy aimed specifically at protecting newborns from the virus.