Pfizer's Bivalent Booster Doesn't Increase Stroke Risk. So Why Are We Talking About It?

Last week, a Centers for Disease Control and Prevention (CDC) vaccine monitoring database signaled a possible increased risk of ischemic strokes from Pfizer’s bivalent COVID-19 booster in adults 65 and older.

In a joint statement, the CDC and the Food and Drug Administration (FDA) said the signal has not been identified with the Moderna COVID-19 vaccine. The agencies are currently investigating whether the signal represents an actual concern with the Pfizer booster.

“Although the totality of the data currently suggests that it is very unlikely that the signal in [Vaccine Safety Datalink] represents a true clinical risk, we believe it is important to share this information with the public,” the agencies wrote in the statement.

No other database has flagged stroke as a side effect of Pfizer’s bivalent booster. Health experts say that the booster should help protect people from stroke—one of the complications of a COVID-19 infection.

“Getting sick with COVID-19 infection is a risk factor for blood clotting and stroke. So getting vaccinated is beneficial and can protect you from these conditions,” said Sophia Newcomer, PhD, MPH, BSPH, an associate professor in the School of Public and Community Health Sciences at the University of Montana.

Newcomer added that this report is a preliminary finding that “needs to be interpreted alongside findings from multiple other surveillance systems, both in the U.S. and other countries that have not found links between the new booster vaccines and stroke risk.”

In an emailed statement, Pfizer maintained that its bivalent booster is safe and effective.

“There is no evidence to conclude that ischemic stroke is associated with the use of the companies’ COVID-19 vaccines,” the company wrote to Verywell. “Compared to published incidence rates of ischemic stroke in this older population, the companies to date have observed a lower number of reported ischemic strokes following the vaccination with the Omicron BA.4/BA.5-adapted bivalent vaccine.”

The CDC and FDA alert the public when they begin investigations into vaccine side effects, even if these side effects are unlikely or the connection has not yet been proven. Mostly, the current disclosure demonstrates transparency in vaccine monitoring and a commitment to safety, Newcomer said.

Pfizer’s bivalent booster is an updated version of its original vaccine, which was not associated with an increased risk of stroke. Both the original vaccine and the booster are closely monitored through multiple databases.

“We have really high standards for vaccine safety in the U.S.,” Newcomer said. “This finding is just a signal that there might be a potential safety issue and now more in-depth studies will be conducted.”

Other vaccines, such as the influenza vaccine, have been shown to reduce stroke risks. Since the flu is associated with an increased risk of inflammation, blood clotting, and stroke, getting vaccinated against the flu may help prevent these severe outcomes.

“We say that vaccines are safe because they’ve been studied—and we know that the benefits of vaccinations far outweigh the risk,” Newcomer said. “I believe that this will continue to be the case with the COVID-19 vaccines.”

The CDC and FDA plan to discuss their investigation at a committee meeting on January 26.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.