On Jan. 24, 2025, researchers from the Veterans Affairs Portland Health Care System in Oregon reported results from a target trial emulation study that showed the estimated effectiveness of the respiratory syncytial virus (RSV) among older U.S. veterans in the 2023-2024 respiratory virus season against infection, emergency department/urgent care (ED/UC) visits, and hospitalization was 78%, 79%, and 80%, respectively.

The observational study was designed to fill in knowledge gaps remaining after clinical trials (eg, vaccine effectiveness (VE) in people with weakened immune systems, effects on healthcare use for infection).

The researchers used electronic health records at the Veterans Health Administration (VHA) to emulate a target trial comparing a single dose of RSV vaccine (RSVPreF3 [Arexvy] or RSVpreF [Abrysvo]) with no vaccination in veterans aged 60 years and older from September through December 2023, with follow-up until March 2024. 

The 146,852 vaccine recipients were matched with as many as four unvaccinated controls (582,936 total) in four monthly nested sequential trials. Of all participants, 94.0% were men, and the median age was 75.9 years. A total of 41.5% of participants had cardiovascular disease, 30.2% had chronic lung disease, and 5.2% had weakened immune systems. Of all vaccinees, 69.2% received Abrysvo, and 29.9% received Arexvy.

“In June, 2023, the Advisory Committee on Immunization Practices recommended a single dose of RSV vaccine in adults aged 60 years and older, using shared clinical decision making,” the study authors noted. “By the end of the 2023–24 respiratory illness season, only 24% of eligible adults in the USA had received an RSV vaccine.”

In total, there were 66 ED/UC visits (1.3 per 1,000 person-years) in the vaccinated group and 289.3 (5.7 per 1,000 person-years) in the unvaccinated group. There were 15 hospitalizations (0.3 per 1,000 person-years) and 80.3 (1.6 per 1,000 person-years) in the vaccinated and unvaccinated groups, respectively.

“These findings support current recommendations for RSV vaccination among individuals aged 60 years and older,” the researchers wrote. “Extended follow-up over subsequent respiratory illness seasons is needed to assess the duration of the protective effect of vaccination.”

More research is needed to determine the optimal RSV vaccination schedule, VE in women in a real-world setting, and VE against post-RSV ICU admissions, complications, and death. The study is published in The Lancet Infectious Diseases.