Following a flag of a possible link between the Pfizer-BioNTech bivalent COVID-19 vaccine and an increased risk of ischemic stroke among people ages 65 and older, FDA and CDC on Friday said it's "very unlikely" a "true clinical risk" exists.

FDA and CDC flag potential stroke risk

Late last year, concerns about a possible link between the bivalent COVID-19 vaccine and ischemic strokes emerged after data from the Vaccine Safety Datalink (VSD), a federal safety surveillance system, suggested Americans ages 65 and older may be at an increased risk of ischemic stroke in the 21 days after receiving the shot.

An ischemic stroke occurs when a blood clot blocks one of the blood vessels in the brain, which stops blood flow and could lead to brain cells dying.

VSD found that, among around 550,000 people ages 65 and older who received the Pfizer-BioNTech bivalent booster, 130 had ischemic strokes in the 21 days following their shot.

FDA and CDC announced the potential link on Friday, saying that the preliminary COVID-19 vaccine "safety signal" is being investigated. The agencies are still recommending the shots.

Further analysis finds stroke link unlikely

After further analysis of the Vaccine Adverse Event Reporting System, CMS' database, and a preliminary study of the Veterans Affairs database, FDA and CDC said "it is very unlikely" that the link exists.

"Often these safety systems detect signals that could be due to factors other than the vaccine itself," the agencies said. "All signals require further investigation and confirmation from formal epidemiologic studies. When one system detects a signal, the other safety monitoring systems are checked to validate whether the signal represents an actual concern with the vaccine or if it can be determined to be of no clinical relevance."

FDA and CDC noted that other factors may have contributed to the increased number of ischemic strokes found by the safety system.

"Although the totality of the data currently suggests that it is very unlikely that the signal in VSD represents a true clinical risk, we believe it is important to share this information with the public, as we have in the past, when one of our safety monitoring systems detects a signal," the agencies said.

Federal officials intend to discuss their findings during a meeting of scientific advisors to FDA on future COVID-19 vaccines on Jan. 26.

In a statement, Pfizer and BioNTech said that "there is no evidence to conclude that ischemic stroke is associated with the use of the companies' COVID-19 vaccines."

"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 bivalent shot, the companies added.

Rep. Cathy McMorris Rodgers (R-Wash.), chair of the House Energy and Commerce Committee, said CDC and FDA will testify about the findings before her committee.

"These agencies must rapidly investigate, in an open and transparent manner, whether or not the vaccine may have contributed to the reported strokes," she said in a statement. "If there's one lesson that the CDC and FDA should have taken away from the pandemic, it's the importance of providing honest, clear, precise, and timely information to the American people about the potential risks and benefits of COVID-19 interventions, including vaccination." (Ingram, MedPage Today, 1/13; Mandavilli, New York Times, 1/13; Choi, The Hill, 1/13; Leonard/Gardner, Politico, 1/13; Sabes, FOX News, 1/13)