Respiratory syncytial virus (RSV) vaccines were an estimated 79% effective against dangerous virus-related blood clots among Medicare recipients in the 2023-24 respiratory virus season, a US Centers for Disease Control and Prevention (CDC)–led research team reports in Emerging Infectious Diseases.
Of 15.6 million Medicare fee-for-service beneficiaries aged 65 years and older who were studied, 58% were women, and 13% had weakened immune systems.
“Respiratory virus infections, including respiratory syncytial virus (RSV) infections, have been associated with increased risk for myocardial infarction, ischemic stroke, and venous thromboembolism,” the study authors wrote. “In 1 US-based surveillance network, ≈22% of adults >50 years of age who were hospitalized with RSV experienced an acute cardiac event.” They cited a 2024 JAMA Internal Medicinestudy.
In June 2023, the CDC’s Advisory Committee on Immunization Practices (ACIP) recommended that adults aged 60 years and older receive a single RSV vaccine dose. The researchers aimed to estimate the vaccine effectiveness (VE) of a single dose against virus-related thromboembolic events in this age-group in the same season that participants received the vaccine.
Little to no waning in first 4 months
VE against RSV-related thromboembolic events (heart attack, ischemic stroke, and venous thromboembolism) was 79% (95% confidence interval [CI], 74% to 83%) among all participants. Estimates didn’t differ substantially between immune-compromised (VE, 69% [95% CI, 56% to 78%]) and not–immune-compromised (VE 82% [95% CI, 77% to 86%]) beneficiaries. The study was underpowered to estimate VE against each thromboembolic event.
As expected, RSV vaccines provided higher protection against RSV-associated thromboembolic events compared with all-cause thromboembolic events.
Estimated VE against thromboembolic events was 75% (95% CI, 63% to 83%) among participants aged 65 to 74 years and 80% (95% CI 74%–84%) among those aged 75 and older. VE point estimates by time since vaccination were all within 4 percentage points (14 to 59 days, 80% [95% CI, 72% to 86%]; 60 to 119 days, 79% [95% CI, 72% to 84%]; 120 days or later, 75% [95% CI, 60% to 84%]).
VE estimates differed only slightly by brand (Arexvy, 76% [95% CI, 70% to 81%]; Abrysvo, 85% [95% CI, 77% to 90%]).
Extending the follow-up period for thromboembolic events produced an estimated VE of 78%, and restricting analyses to periods of high RSV circulation generated a VE estimate of 79%.
Estimates of VE against all-cause thromboembolic events, regardless of previous RSV diagnosis, were lower (21% [95% CI, 19% to 22%]) than for primary analyses. VE against RSV-related thromboembolic events based on models with inverse probability of treatment weights (IPTW) was 71% (95% CI, 62% to 77%), which was comparable to estimates obtained in non-IPTW models.
“As expected, RSV vaccines provided higher protection against RSV-associated thromboembolic events compared with all-cause thromboembolic events,” the investigators wrote. “Time since vaccination results suggest minimal to no waning over the first 4 months postvaccination. Other analyses of RSV-associated hospitalization demonstrated more noticeable waning over a shorter period.”
