A vaccine safety monitoring system in late November picked up a signal that the updated Pfizer coronavirus vaccine booster was possibly linked to an increased risk of strokes in people 65 and older.
But a deep dive into several large databases failed to confirm the preliminary information, leading federal health officials to conclude the risk is extremely low -- and probably nonexistent, those officials said Friday.
The Centers for Disease Control and Prevention, whose system detected the early signal, and the Food and Drug Administration, which also assesses vaccine safety, have decided there is no need to change the recommendation that everyone 6 months and older should stay up to date with their coronavirus vaccinations, including those 5 and older who are eligible for the updated booster, according to officials speaking on the condition of anonymity under ground rules set by the agencies.
The early signal involved the bivalent booster -- which targets the original virus and omicron subvariants and became available in September -- made by Pfizer and its German partner, BioNTech. It did not apply to Moderna's updated booster, officials said.
"We have looked at the totality of the evidence and there are no concerns at this time that this represents a true safety signal," an FDA official said.
Sometimes, when listening, "like with a radio, you are going to hear some static in the background," the official said, adding it does not mean someone is talking. Chances are "this is just static."
The CDC and FDA described the early signal in a statement updating their websites Friday afternoon.
The signal was detected in the Vaccine Safety Datalink, a collaboration involving the CDC and about a dozen health care organizations with electronic health records on 12 million people. As part of routine monitoring for possible adverse events, officials noticed late last year that they were picking up indications of higher-than-expected stroke risk, officials said.
Among about 550,000 people 65 and older who had already been vaccinated and received a booster dose of the Pfizer bivalent vaccine, 130 people had strokes in the first three weeks after getting the shot. No deaths have been reported.
That finding raised a question because it suggested that people who received the bivalent were more likely to have an ischemic stroke in the 21 days following vaccination compared with Days 22-44 following vaccination.
The findings prompted officials to look for similar findings.
CDC officials conducted a different analysis in the Vaccine Safety Datalink system, using the same data but different methodology, and were not able to replicate the finding.
Officials also searched other systems, including those of Medicare, the Department of Veterans Affairs and Pfizer's global surveillance network. Regulators in other countries, including Israel, also were consulted, but no evidence of similar findings emerged, the officials said.
The statement from the two agencies said the government uses multiple systems to detect potential safety problems and "often these safety systems detect signals that could be due to factors other than the vaccine itself."
Kit Longley, a spokesman for Pfizer, said the company and its partner, BioNTech, have been made aware of the "limited reports" of stroke.
"There is no evidence to conclude that ischemic stroke is associated with the use of the companies' covid-19 vaccines," Longley said.
Safety experts say that safety signals occur frequently; if they don't, the system might not be sensitive enough. But there is a difference between signals and genuine safety risks.
The agencies said they will continue to evaluate data from the systems and will discuss the data and additional analyses at the FDA's Jan. 26 meeting with its vaccine advisers.