January 17, 2023

2 min read

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Health officials said a safety signal detected by a national surveillance system is “very unlikely” to represent a “true clinical risk” for stroke among older people who received the Pfizer-BioNTech bivalent COVID-19 booster.

The CDC and FDA said they came to that conclusion after multiple analyses of other vaccine surveillance databases failed to validate the signal from the CDC’s Vaccine Safety Datalink (VSD), a near real-time surveillance system.


Information from the VSD met the statistical criteria to trigger an investigation into whether people aged 65 years or older were more likely to experience ischemic stroke within 21 days of receiving the updated booster compared with 22 to 42 days after vaccination.

“Often these safety systems detect signals that could be due to factors other than the vaccine itself,” the CDC and FDA said in a statement. “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.”

Analyses of five other safety databases — including the Vaccine Adverse Event Reporting System managed by the CDC and FDA, and databases kept by CMS and the U.S. Department of Veterans Affairs — did not show any increase in reporting of ischemic stroke related to bivalent COVID-19 booster vaccines.

“No change in vaccination practice is recommended,” the CDC and FDA said. The CDC still recommends that eligible people aged 6 months or older stay up to date on vaccinations and boosters, which have been shown to reduce the risk for severe disease from COVID-19.

“There is no evidence to conclude that ischemic stroke is associated with [Pfizer-BioNTech’s] COVID-19 vaccines,” Pfizer told Healio in a statement. “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 vaccination with the omicron BA.4/BA.5-adapted bivalent vaccine.”

The CDC and FDA said they will continue to evaluate data from the safety systems, with the most recent data and analyses expected to be discussed by the FDA’s Vaccines and Related Biological Products Advisory Committee at its Jan. 26 meeting.

“Although the totality of the data currently suggests that it is very unlikely that the signal VSD represents a true clinical risk, we believe it is important to share this information with the public,” the CDC said.


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