Bivalent (two-strain) COVID-19 vaccines help protect against COVID-19–related thromboembolic events, including strokes, embolisms, and heart attacks, more so than monovalent (one-strain) vaccines, according to a study today in Morbidity and Mortality Weekly Report.
The study 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 (ie, were immunocompetent), entered the study on September 4, 2022. Researchers updated vaccination status daily. Follow-up continued until March 4, 2023, or a COVID-19–related thromboembolic event.
The authors defined a thromboembolic event as an ischemic stroke, venous thromboembolism, or myocardial infarction from 7 days before through 30 days after COVID-19 diagnosis.
VE of 47% to 51% for thrombotic events
Among 12,706,176 Medicare beneficiaries aged 65 years and older who had previously received an original COVID–19 vaccine, 5,683,208 (44.7%) received a bivalent dose. Among 78,618 Medicare beneficiaries aged 18 and older years with ESRD receiving dialysis, 23,229 (29.5%) received a bivalent dose, including 7,239 (31.2%) aged 18 to 64 years and 15,990 (68.8%) aged 65 years or older.
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 an adjusted vaccine effectiveness (VE) against COVID-19–related thromboembolic events among immunocompetent beneficiaries aged 65 years and older of 47%, with lower VE estimates more than 60 days after bivalent vaccine receipt (42%) compared with VE estimates 7 to 59 days after bivalent vaccine receipt (54%).
For those with ESRD, adjusted VE against COVID-19–related thromboembolic events was 51%, with lower VE estimates more than 60 days after bivalent vaccine receipt (45%) than 7 to 59 days after bivalent vaccine receipt (56%).
'Incremental benefit'
"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.
Moreover, the findings are notable in light of a vaccine safety signal detected in January 2023 by the Centers for Disease Control and Prevention, which 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.
In conclusion, the investigators said their study suggests adults over the age of 65 and those with ESRD should remain up to date on boosters.
