Department of Research & Evaluation News

A respiratory syncytial virus vaccine given to older adults remained highly effective at preventing severe illness over 2 RSV seasons, according to a Kaiser Permanente Southern California study.

The study offers real-world evidence that the bivalent RSVpreF vaccine continued to protect adults 60 and older from RSV-related hospitalizations and emergency department visits well beyond the first season after vaccination. The study was published ahead of its August 2026 print date in The Lancet Regional Health – Americas.

“These findings provide reassuring evidence that RSV vaccination continues to offer strong protection against serious illness beyond the first season after vaccination,” said the study’s lead author, Sara Y. Tartof, PhD, an infectious disease epidemiologist at the Kaiser Permanente Southern California Department of Research & Evaluation. “Older adults, particularly those with underlying medical conditions, face a higher risk of hospitalization from RSV.

“Our results show that the vaccine delivers durable protection across multiple seasons, which is important as clinicians and policymakers consider future vaccination strategies.”

RSV is a leading cause of acute raspatory disease

Although RSV has been best known for affecting infants, it can also cause serious illness in adults, especially older adults. The virus is a leading cause of severe acute respiratory disease in older adults and adults with underlying medical conditions. It is responsible for an estimated 160,000 hospitalizations and 120,000 emergency department visits each year among U.S. adults ages 65 and older.

This study evaluated RSVpreF vaccine effectiveness among Kaiser Permanente Southern California members 60 and older who were hospitalized or treated in emergency departments for acute respiratory illness or lower respiratory tract disease between November 2023 and April 2025. The vaccine was administered at 7 study hospitals and 82 associated medical offices. Researchers used a retrospective test-negative case-control design, a commonly used approach for evaluating vaccine performance in real-world care settings.

The study included 10,997 hospital or emergency department visits for lower respiratory tract disease in the broad analysis. Researchers also did a stricter analysis that included 2,317 such visits.

Vaccine remained strong across 2 seasons

Researchers found that vaccine protection remained strong across 2 RSV seasons. Effectiveness against RSV-associated hospitalization or emergency department care was 81% in the first season after vaccination and 76% in the second season. Across both seasons combined, effectiveness was 80% against RSV-related hospitalization or emergency department visits and 82% against severe disease requiring oxygen.

The findings build on an earlier Kaiser Permanente Southern California analysis from the first months after RSV vaccine availability. That study found RSVpreF was 84% effective against RSV-related lower respiratory tract disease requiring hospitalization or emergency department care among adults 60 and older during the 2023-24 RSV season.

Study built on findings from first season of protection

The larger, 2-season analysis also provided more information about how protection held up in groups at higher risk for severe RSV illness. Researchers found that protection declined slightly over time, from 87% in the first 6 months after vaccination to 77% between 6 and 18 months after vaccination. The decline appeared more pronounced among immunocompromised adults, with effectiveness decreasing from 74% in the first season to 54% in the second season.

The researchers noted that longer-term, multi-season data and targeted studies in high-risk populations will be needed to better understand durability, optimize revaccination strategies, and inform future RSV vaccination policy. Dr. Tartof and her team are currently analyzing the third season durability, which will be published at a future date.

Authors included Sara Y. Tartof, PhD; Gabriella Goodwin, MS; Jeff M. Slezak, MS; Vennis Hong, MPH; Sally Shaw, DrPH; Banshri Kapadia, MS; Brigitte C. Spence, MPH; Gregg S. Davis, PhD; and Hina Chowdhry, MPH, of the Kaiser Permanente Southern California Department of Research & Evaluation; Timothy B. Frankland, MA, of the Kaiser Permanente Hawaii Center for Integrated Health Care Research; Bradley Ackerson, MD, of Southern California Permanente Medical Group; Joseph A. Lewnard, PhD, of the University of California, Berkeley School of Public Health and College of Engineering; and Negar Aliabadi, MD; Evan Zasowski, PharmD; Qing Liu, MS; Sabrina Welsh, MPH; Michael Dutro, PharmD; Erica Chilson, PharmD; Luis Jodar, PhD; Bradford D. Gessner, MD; and Elizabeth Begier, MD, of Pfizer Inc.