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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 starting in September — made by Pfizer and its German partner, BioNTech. It did not apply to Moderna's updated booster, officials said.

Government vaccine safety experts have combed through databases containing millions of records in the United States and consulted with regulators in other countries but have not found any indication that the statistical signal represents a clinical risk to patients. They said they would continue to analyze the data.

"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.

The FDA official said the government's sensitive vaccine safety systems are like a radio with the volume turned up high. 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 and not someone real talking."

The review is happening as the pandemic grinds on, with the number of daily deaths rising and an increasingly frustrated White House urging Americans to get the booster to bolster their protection. Uptake remains low: Only 16 percent of people 5 and older and only 39 percent of those 65 and older, the most vulnerable group, have received the booster, according to the CDC.

The CDC and FDA described the early signal in a statement updating their websites Friday afternoon. The signal set off a debate among officials about whether and how to release the information. CDC officials argued for releasing it, while some other officials, including at the FDA, were concerned that putting out unconfirmed data would fuel anti-vaccine sentiment and scare older Americans into avoiding the boosters. Ultimately, the agencies decided to release the information in the hopes that transparency "will build confidence," said a CDC official.

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 through 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 that the government uses multiple systems to detect potential safety problems and that "often these safety systems detect signals that could be due to factors other than the vaccine itself."

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

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. He added that about 550 million doses of the companies' omicron-targeted bivalent shot have been delivered globally and about 30 million in the United States.

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. One of the reasons officials believe strongly there is not a safety risk is that they have not been able to come up with a plausible reason that such a problem would occur now — given the huge number of vaccines that have been administered worldwide since the end of 2020 — or why a problem would affect the Pfizer-BioNTech booster and not Moderna's. Both shots use mRNA technology.

Public health experts say it's important for the health agencies to be as transparent as possible about potential adverse events. If the information is not presented, public health agencies could be accused of hiding data. But such reports can be misinterpreted as causal when they are not.

The agencies said they will continue to evaluate data from the systems and will discuss the data and additional analyses at the FDA's previously scheduled Jan. 26 meeting with its vaccine advisers.

The Vaccine Safety Datalink has electronic health data that includes the kind of vaccine given to patients, the dates of vaccination and other vaccinations administered on the same day. It uses information on illnesses diagnosed at doctors' offices, in emergency rooms and in hospitals.

The system set up to monitor coronavirus vaccine safety is the most extensive in U.S. history.

In 2021, the system detected a link between the Johnson & Johnson vaccine and a rare but potentially deadly blood clotting and bleeding syndrome called thrombosis with thrombocytopenia, or TTS. The data prompted the FDA to impose new restrictions in May, saying only people who were unable or refusing to get other vaccines should receive the Johnson & Johnson shot.

There have also been concerns about the mRNA vaccines and the possible risk of a rare condition known as myocarditis, inflammation of the heart muscle, in adolescent and young men. But federal health officials have said data shows that the known risks of COVID-19 illness and its related, possibly severe complications far outweigh the potential risks.

In previous investigations of the Johnson & Johnson vaccine and links to blood clots, and possible risks of myocarditis, officials found stronger signals as they searched additional safety databases. But that was not the case in this instance.

"Here, as we continue to dig … it's disappearing rather than becoming stronger," the FDA official said.

previous coverage

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• New variant XBB.1.5 is ‘most transmissible’ yet, could fuel COVID wave

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