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The respiratory syncytial virus (RSV) vaccine (Abrysvo) was linked with fewer hospitalizations and emergency department (ED) visits in older patients during the first season of the vaccine's rollout, according to a retrospective analysis.

From November 2023-April 2024, adjusted RSV vaccine effectiveness against hospitalizations or ED visits was 90% (95% CI 24-99) among adults ages 60 and older during the first 5 months of vaccine availability, reported Sara Tartof, PhD, MPH, of Kaiser Permanente in Pasadena, California, at the IDWeek annual meeting in Los Angeles.

Also, in a secondary analysis, adjusted vaccine effectiveness against RSV-related acute respiratory illness (ARI) ED visits was 93% (95% CI 47-99) and 91% against RSV-related ARI ED visits or hospitalizations (95% CI 61-89).

Among test-positive cases, 99.7% were unvaccinated (n=371) and 0.3% (n=1) were vaccinated. Among the test-negative controls, 96.8% (n=480) were unvaccinated and 3.2% (n=16) were vaccinated.

The FDA approved Pfizer's recombinant RSVPreF vaccine in June 2023 for use in people ages 60 and older, shortly after the approval of GSK's recombinant RSVPreF3 adjuvanted vaccine (Arexvy) for the same age group. Approval of the Pfizer vaccine was based on results of the , which showed that it prevented RSV-associated acute lower respiratory tract illness in older adults.

However, there were a limited number of hospitalization and ED outcome visits in the RENOIR trial, "which has hampered vaccine efficacy estimation against more severe clinical outcomes," Tartof told attendees. Furthermore, RENOIR excluded immunocompromised persons and those with the most severe comorbidities. Also, vaccine effectiveness estimates were not available for the oldest age groups, she pointed out.

Of note, over 76% of the total patient population in the current study scored 3 or more on the Charlson Comorbidity Index, indicating some burden from comorbidities. Also, over 16% were immunocompromised.

A recent analysis of CDC data also demonstrated an association of RSV vaccination with lower rates of hospitalizations among people ages 60 and older. That analysis also included high-risk, older individuals.

"A big question about these vaccines is going to be durable immunity in subsequent seasons because there's no recommendation at this point to give a booster," commented session moderator Taison Bell, MD, MBA, of the University of Virginia in Charlottesville.

"I'm pleased that we have evidence now on immunocompromised individuals. That's very remarkable," he added.

Tartof said her group will be looking at durability of immunity of the Pfizer vaccine over successive respiratory seasons in follow-up analyses.

In June, the CDC Advisory Committee on Immunization Practices (ACIP) backed off its initial recommendation that all adults ages 60 and older were eligible to receive an RSV vaccine in the context of shared decision-making. The ACIP now recommends a one-time shot for all adults ages 75 or older and adults ages 60-74 who are at increased risk of severe RSV disease.

The observational retrospective study, which used a test-negative design, included older adults who were hospitalized or in the ED with severe ARI and with RSV test results available. By definition, severe ARI required supplemental oxygen.

Tartof and colleagues conducted the analysis using electronic health records data from Kaiser Permanente Southern California. The study population of 5,120 patients was divided into RSV-positive cases and RSV-negative cases and then each of those groups was further divided into unvaccinated individuals and those vaccinated for RSV using the Pfizer vaccine.

Test negative controls for the "strict" control definition used in the primary analysis required ARI events negative for RSV, human metapneumovirus, and other vaccine preventable pathogens, but positivity for a non-vaccine preventable pathogen. Test negative controls for the "broad" control definition used in the sensitivity analysis included any ARI event negative for RSV.

Of the total study population, about 61% were ages 75 or older. Among the positive test cases, about 68% were in this older age group versus about 58% of the strict control group.

Only 2.7% of the total study population received the RSV vaccine. Most of the study population (81%) was also hospitalized (vs ED visits), including 74.5% in the RSV-positive cases population.

One problem researchers encountered was that rates of testing for RSV in the study population was low. To mitigate the problem of low clinical testing rates for RSV, researchers tested salvaged respiratory swabs collected for influenza/SARS-CoV-2 testing from ED and inpatient encounters. Swabs were then tested for RSV using an 18-pathogen multiplex panel.

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