US CDC still looking at potential stroke risk from Pfizer bivalent Covid shot

New data from one US Centers for Disease Control and Prevention (CDC) database shows a possible stroke risk link for older adults who received an updated Pfizer/BioNTech Covid-19 booster shot, but the signal is weaker than what the agency had flagged earlier in January, health officials said on Thursday.

US Food and Drug Administration officials said they had not detected a link between the shots and strokes in two other safety monitoring databases.

The new data was presented at a meeting of outside experts that advise the FDA on vaccine policy.

Earlier this month, US health officials said they had detected the possible link to ischemic strokes in people over age 65 who received the newer booster shots in its Vaccine Safety Datalink (VSD) database. They said at the time it was very unlikely to represent a true clinical risk.

Dr. Nicola Klein of healthcare company Kaiser Permanente, which maintains VSD data for the CDC, said the rate of strokes observed in the database had slowed in recent weeks, but the signal was still statistically significant, meaning likely not by chance.

Most of the confirmed cases had also received a flu vaccine at the same time, which might be a factor, she said.

FDA scientist Richard Forshee said the agency plans to study whether there is any increased risk of stroke from receiving the two shots at the same time.

Both agencies still recommend older adults receive the booster shots, now tailored to target Omicron variants as well as the original coronavirus.

Dr. Walid Gellad, professor of medicine at University of Pittsburgh, said the issue required further investigation.

"Sometimes signals are not clear," Gellad said in an email. "It makes sense to look into it more, and it doesn't make sense to change practice given the known benefits (of getting the booster) in this age group."