Department of Research & Evaluation News

An RSV vaccine continued to protect adults aged 60 and older against RSV-related hospitalization and emergency department visits for at least 3 seasons after vaccination, according to a new study by Kaiser Permanente in Southern California. RSV, or respiratory syncytial virus, is a common respiratory virus that can cause serious illness, hospitalization, and even death among older adults.

The study published on September 18, 2026, in Infectious Diseases and Therapy.

The study found that the vaccine’s protection gradually declined over time but remained substantial through a third RSV season. During the first RSV season after vaccination, vaccinated older adults were 81% less likely to require hospital or emergency care because of RSV than similar people who were not vaccinated. Protection remained strong in the second and third seasons, when vaccinated individuals were 64% and 60% less likely, respectively, to need hospital or emergency care because of RSV.

“This is a relatively new vaccine, and it targets an infection that causes a substantial amount of illness and even death, particularly among older adults,” said lead author Sara Y. Tartof, PhD, MPH, an infectious disease epidemiologist with the Kaiser Permanente Southern California Department of Research & Evaluation. “One of the biggest unanswered questions has been how long protection lasts after vaccination. Our findings show that meaningful protection persists into a third RSV season, giving clinicians, patients, and policymakers important evidence that this vaccine continues to reduce the risk of serious RSV illness over time.”

Researchers analyzed 3,131 lower respiratory tract disease events and 4,797 acute respiratory illness events among Kaiser Permanente Southern California members aged 60 and older who sought hospital or emergency care between November 2023 and April 2026. The study compared patients who tested positive for RSV with similar patients who had respiratory illness but tested negative for RSV. The study demonstrated protection in a medically complex population in which more than 90% of participants had high-risk chronic medical conditions and about 22% were immunocompromised.

The study is the first large observational study of RSV vaccine effectiveness through a third season after vaccination, extending previous evidence that had been limited to two seasons. The findings may help inform future RSV vaccination policy decisions.

In addition to Dr. Tartof, other authors from the Department of Research & Evaluation include Gabriella Goodwin, MS, Jeff Slezak, MS, Vennis Hong, MPH, Sally Shaw, DrPH, MPH, Banshri Kapadia, MS, Brigitte C. Spence, DrPH, MPH, Gregg S. Davis, PhD, MPH, and Hina Chowdhry, MPH. Authors from Pfizer Inc include Evan J. Zasowski, PharmD, MPH, Negar Aliabadi, MD, MS, Qing Liu, MS, Sabrina Welsh, MPH, Michael Dutro, PharmD, Erica Chilson, PharmD, Alejandro Cane, MD, MS, Kyla Hayford, PhD, and senior author Elizabeth Begier, MD, MPH. Additional authors include Timothy B. Frankland, MA, of the Kaiser Permanente Hawaii Center for Integrated Health Care Research; Bradley Ackerson, MD, of the Southern California Permanente Medical Group; and Joseph A. Lewnard, PhD, of the University of California, Berkeley.