Fears that the COVID booster shots made by Pfizer-BioNTech may increase the risk of strokes in people ages 65 and older were not borne out by an intensive scientific investigation, federal officials said Friday.

“It is very unlikely” that the risk is real, the officials said. They urged Americans 6 months and older to continue getting booster shots. Federal officials decided to disclose the concern and the results of their investigation despite worries that the revelation might fuel anti-vaccine sentiment.

“We believe it is important to share this information with the public,” a joint statement from the Centers for Disease Control and Prevention and the Food and Drug Administration said.

Officials declined requests to discuss details of their investigation.

The bivalent vaccine is designed to thwart the original version of the coronavirus as well as versions of the omicron variant that were circulating in the United States in the summer. The bivalent vaccines made by Pfizer-BioNTech and Moderna are the only boosters available in the United States now, and scientists are engaged in a heated debate over their usefulness compared with the original vaccines.

It is unclear whether the new reassurance will prompt Americans to receive a bivalent shot if they have not already done so. Only 39% of adults ages 65 and older, and only 16% of those ages 5 and older, have so far received a bivalent booster shot.

The concerns about a possible link to ischemic strokes — which can interrupt blood supply to the brain — first emerged late last year. Data from the Vaccine Safety Datalink, a federal safety surveillance system, hinted that Americans ages 65 and older might be at increased risk of an ischemic stroke in the 21 days after receiving a Pfizer-BioNTech bivalent shot.

That signal was specific to the bivalent vaccine made by Pfizer-BioNTech. No similar concerns were linked to the original COVID vaccines or to Moderna’s bivalent boosters.

The data prompted federal officials to comb through other vaccine safety databases, as well as observations from the United States and other countries. The investigators did not find evidence of an increased risk of strokes in any of these sources, federal officials said in their statement.

Federal officials plan to discuss the findings Jan. 26 at a meeting of scientific advisers to the FDA on future COVID vaccines.

The Vaccine Safety Datalink is a real-time surveillance system, a collaboration between the CDC and integrated health care organizations and networks across the United States. The system uses electronic health data from about a dozen sites in the nation to monitor vaccine safety.

Among about 550,000 people ages 65 and older who had received the Pfizer-BioNTech bivalent booster, 130 had ischemic strokes in the 21 days after receiving the shot, raising fears that the two events were related. But analysis of the data using a different method did not reveal an increased risk of ischemic stroke.

Another database, the Vaccine Adverse Event Reporting System, managed by the CDC and the FDA, also did not pick up a signal for ischemic strokes. Neither did a large study of the bivalent vaccines relying on data from the Centers for Medicare and Medicaid Services, a preliminary study using the Veterans Affairs database, nor Pfizer-BioNTech’s global safety database.

Other countries have not observed an increased risk for ischemic stroke with the bivalent vaccines, the agencies said. About 795,000 strokes are reported in the United States each year, and about 87% of those are ischemic strokes, according to the CDC.

“There is no evidence to conclude that ischemic stroke is associated with the use of the companies’ COVID-19 vaccines,” Pfizer and BioNTech said in a statement. More than 30 million doses of the companies’ bivalent vaccine have been administered in the United States so far, according to a spokesperson for Pfizer.

“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” after the bivalent shot, the companies said.

This article originally appeared in The New York Times.

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