Bivalent (two-strain) vaccines better protect against COVID-19–related thromboembolic events, including strokes, embolisms, and heart attacks, than monovalent (one-strain) vaccines, according to a a new study.
The study, published in Morbidity and Mortality Weekly Report, was based on outcomes seen among Medicare enrollees ages 65 and older and adults ages 18 years or older with end-stage renal disease (ESRD) receiving dialysis.
Outcomes among those who had received a bivalent mRNA COVID-19 booster were compared to patients who had only the monovalent COVID-19 primary vaccine series.
The Medicare beneficiaries, who did not have compromised immune systems entered the study on 4 September, 2022. Researchers updated vaccination status daily, with follow-ups for six months, or a COVID-19–related thromboembolic event.
The authors defined a thromboembolic event as an ischemic stroke, venous thromboembolism, or myocardial infarction from seven days before through 30 days after COVID-19 diagnosis.
For both those 65 and older and those with ESRD, getting a bivalent booster was associated with getting a seasonal flu vaccine, and an original monovalent booster.
During the study period, 22,001 Medicare beneficiaries had a COVID-19–related thromboembolic event, as did 1,040 ERSD beneficiaries. The researchers calculated vaccine effectiveness (VE) against COVID-19–related thromboembolic events among immunocompetent beneficiaries aged 65 years and older of 47 per cent, with lower VE estimates more than 60 days after bivalent vaccine receipt (42 per cent) compared with VE estimates 7 to 59 days after bivalent vaccine receipt (54 per cent).
For those with ESRD, adjusted VE against COVID-19–related thromboembolic events was 51 per cent, with lower VE estimates more than 60 days after bivalent vaccine receipt (45 per cent) than 7 to 59 days after bivalent vaccine receipt (56 per cent).
“These findings can be interpreted as the incremental benefit of a recent bivalent dose compared with earlier receipt of original monovalent doses and are consistent with reported lower rates of COVID-19–related thromboembolic events among vaccinated than among unvaccinated persons,” the authors said.
These findings can be interpreted as the incremental benefit of a recent bivalent dose compared with earlier receipt of original monovalent doses.
A vaccine safety signal detected in January 2023 by the Centers for Disease Control and Prevention, suggested a possible link between receipt of a Pfizer-BioNTech bivalent COVID-19 mRNA vaccine and an increased the risk for an ischemic stroke event in the 21 days following vaccination in people age 65 and older.
That signal has not been proven, and a review of additional studies has not provided clear and consistent evidence of a safety problem with ischemic stroke and bivalent mRNA COVID-19 vaccines, the authors said.
They conclude: “To prevent COVID-19–related complications, including thromboembolic events, adults should stay up to date with recommended COVID-19 vaccination.”
