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During the first season of use, respiratory syncytial virus (RSV) vaccination among older adults was associated with a substantially reduced risk of hospitalization, a test-negative, case-control study indicated.

In adults 60 and over, vaccine effectiveness from October 2023 to March 2024 against RSV-associated hospitalization reached 75% (95% CI 50-87), according to researchers led by Diya Surie, MD, of the CDC in Atlanta.

As reported in , effectiveness remained similar when estimated with inverse probability of vaccination weighting to balance for potential confounders (79%, 95% CI 56-90), and when analyzed across age groups: at 75% (95% CI 31-91) for adults ages 60 to 74 years and 76% (95% CI 40-91) for those age 75 and older.

The CDC recommends a single dose of RSV vaccine for all adults age 75 and older, as well as for those ages 60 to 74 years at increased risk for severe RSV disease, with three vaccines now approved for older adults.

The agency first recommended RSV vaccination for older adults in 2023 based on their high efficacy against RSV demonstrated in prelicensure randomized trials. However, the trials "were not powered to assess efficacy against RSV-associated hospitalization," the authors noted, and the studies excluded immunocompromised patients and underrepresented certain high-risk groups, such as those age 75 and older.

Indeed, at the time the CDC made its initial recommendation, members of the Advisory Committee on Immunization Practices expressed concerns over trial data and whether enrolled participants represented those most at risk for serious outcomes.

The current findings expand upon the prelicensure trial data in two ways, Surie and colleagues said. "First, they provide evidence of vaccine protection against RSV-associated hospitalization, of which an estimated 60,000 to 160,000 occur annually among U.S. adults aged 65 years and older. Second, they demonstrate protection in a population that more closely represents those at high risk of severe RSV disease, including adults aged 75 years and older and those with immunocompromising conditions."

For the study, 2,978 adults ages 60 and over were included: with 367 representing the case patients (12.3%) and the rest acting as controls. Among the 288 RSV cases with a known subtype, 73% were RSV B. Overall, the individuals had a median age of 72, a median Charlson Comorbidity Index score of 5, and 24.2% of them were immunocompromised.

Participants were hospitalized with acute respiratory illness at one of 24 hospitals in 19 states participating in a surveillance network from Oct. 1, 2023 to March 31, 2024. They had clinical respiratory virus testing within 10 days of illness onset. Nasal swabs were tested by reverse transcription polymerase chain reaction for RSV, SARS-CoV-2, and influenza. Case patients tested positive for RSV only, and control patients tested negative for all of the viruses.

RSV vaccination was defined as receipt of the vaccine 14 days or more before illness onset and status was determined via electronic medical records, immunization registries, and self-reporting; 2.5% of the case patients and 9.8% of control patients were vaccinated, with a median interval between vaccination and illness onset of 84 days.

Vaccinated patients were more likely to be older (median 75 vs 72 years for unvaccinated patients), white (83% vs 61%), immunocompromised (32% vs 23%), and to have an outpatient visit in the past year (96% vs 90%). They were also more likely to live in communities with a lower Social Vulnerability Index score (median 0.37 vs 0.58, with a score of 1 representing the highest vulnerability; P<0.01 for all).

Limitations of the study included that disparities in vaccine uptake and low uptake in the first season of use "might influence generalizability and the potential for residual confounding from unmeasured factors," Surie and colleagues wrote. Additionally, future studies are "important to confirm these findings and further examine vaccination effectiveness among subgroups and by time since vaccination."

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