Bivalent COVID-19 Vaccines Cut COVID-Related Strokes and Clots by About Half

COVID-19 raises the risk of blood clots and the events they cause — ischemic strokes, venous thromboembolism and heart attacks — especially in older adults and people with end-stage kidney disease on dialysis. A CDC study of Medicare enrollees found that a bivalent COVID-19 vaccine dose cut those COVID-related events roughly in half, compared with having had only the original vaccines.

CDC's summary of the findings. Credit: CDC, Morbidity and Mortality Weekly Report.

The study

Two cohorts of Medicare fee-for-service enrollees were followed from September 4, 2022, to March 4, 2023: people 65 and older, and adults 18 and older with end-stage renal disease (ESRD) on dialysis. Everyone had already had original (monovalent) COVID-19 vaccines. Those who got a bivalent dose were compared with those who did not, counting COVID-related strokes, clots and heart attacks from 7 days before to 30 days after a COVID-19 diagnosis. The result is the extra protection from a bivalent dose.

Among dialysis patients, those who got a bivalent dose were more likely to have had flu shots and an original booster. Effectiveness was higher in the first two months after the dose than later — for dialysis patients, 56 percent at 7–59 days and 45 percent after 60 days, a difference that was not statistically significant. An analysis of all thromboembolic events, whether or not related to COVID-19, also showed protection.

Why it matters

In January 2023, CDC and the Food and Drug Administration announced a preliminary safety signal: a question of whether the Pfizer-BioNTech bivalent vaccine raised the risk of ischemic stroke in the 21 days after vaccination in people 65 and older. As more data came in, the signal weakened and was no longer statistically significant, and other studies found no clear, consistent evidence of a problem. These findings add context to that: the vaccine protected against the very strokes and clots that COVID-19 causes. Adults 65 and older and adults with ESRD should get all recommended COVID-19 vaccine doses.

Limits

The study measures added benefit over earlier original vaccination, not vaccination against none, because early vaccinations were often missing from claims. More than 70 percent of the population had been infected by the third quarter of 2022, and infection-induced immunity may affect results. Bivalent doses were recent while original doses were older, so they could not be compared at the same time since vaccination. Some confounding, such as undocumented infections or Paxlovid use, may remain.

Sources

Based on "Effectiveness of Bivalent mRNA COVID-19 Vaccines in Preventing COVID-19–Related Thromboembolic Events Among Medicare Enrollees Aged ≥65 Years and Those with End Stage Renal Disease — United States, September 2022–March 2023," by Amanda B. Payne and colleagues (CDC; Centers for Medicare & Medicaid Services; Acumen LLC), Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.