Pfizer’s Abrysvo During Pregnancy Shows Strong Real-World Protection for Infants, New U.S. Data Confirms

The protection was even more pronounced in the first month after birth, when transferred maternal antibodies are at their peak. For infants 0 to 30 days old, effectiveness against RSV-associated ARI hospitalization was measured at 73.8% (95% CI, 10.9%–93.7%) .

A Crucial First U.S. Data Point, Despite Limitations

While the study’s findings are strong, it is important to interpret them within the context of its design. Unlike nationwide studies from the UK, this was a single-region, hospital-based retrospective analysis.

The smaller sample size resulted in wide confidence intervals for the effectiveness estimates. For the primary outcome of 67.6% effectiveness against ARI hospitalization, the true population effect could plausibly be as low as 33.2% or as high as 85.4% . The study confirmed the vaccine’s benefit but with less statistical precision than larger national analyses.

The analysis covered RSV seasons from the vaccine’s rollout through the 2024–25 and 2025–26 seasons , providing a multi-season look at protection.

How the U.S. Data Compares to UK Real-World Studies

These results are consistent with and reinforce evidence from larger national programs in the United Kingdom, where maternal RSV vaccination was also widely implemented.

• A national study in Scotland, published in November 2025 in The Lancet Infectious Diseases, reported 82.2% effectiveness (95% CI, 75.1–87.3%) against RSV-associated LRTD hospitalization in infants. The study estimated that 219 infant RSV hospital admissions were averted during the 2024–25 season alone .
• An evaluation of England’s program by the UK Health Security Agency (UKHSA) found that vaccination given at least four weeks before delivery provided nearly 85% protection against RSV infection in newborns, with strong effectiveness also demonstrated for premature infants .
• A combined analysis from the BronchStop study in England and Scotland found a 72% reduction in infant hospitalizations when mothers were vaccinated more than 14 days before delivery .

The somewhat higher effectiveness figures from the UK (broadly in the 72%–82% range) are not in conflict with the Pittsburgh findings. The UK studies were prospective, population-based analyses with vastly larger sample sizes, leading to tighter confidence intervals. Methodological differences, including how outcomes were defined and verified, and the fact that the Pittsburgh study was a regional retrospective analysis, account for the variation. The critical conclusion is that all real-world studies confirm a substantial, clinically meaningful protective effect .

What’s Next: Tracking Durability into Future Seasons

Several ongoing research projects are now monitoring the long-term performance of maternal RSV vaccination.

The CASSATT trial (ClinicalTrials.gov ID NCT06813872) is a collaboration between Pfizer and the University of Pittsburgh that is actively enrolling and will prospectively track the real-world effectiveness of Abrysvo in infants in Western Pennsylvania. The study is designed to continue surveillance through the 2026–27 RSV season to monitor the durability of protection, any waning of immunity, and the vaccine’s effectiveness against emerging RSV strains .

Similar long-term tracking is underway in the UK. The BORLAND project (Benefits Of RSV Maternal vaccination Using A Scottish National Dataset) will use national health data to monitor outcomes across multiple future RSV seasons .

These ongoing studies will provide the longitudinal data needed to refine vaccination guidelines, understand the optimal timing of doses during pregnancy, and maintain vaccine confidence as the virus evolves.