The US Centers for Disease Control and Prevention said Friday that there is a possible safety issue with the bivalent Covid-19 vaccine made by Pfizer and BioNTech but that it is unlikely to represent a true risk. The agency said it continues to recommend that people stay up-to-date with Covid-19 vaccines.
The CDC said one of its vaccine safety monitoring systems – a “near real-time surveillance system” called the Vaccine Safety Datalink – detected a possible increase in a certain kind of stroke in people 65 and older who recently got one of Pfizer’s updated booster shots.
A rapid response analysis of that signal revealed that seniors who got an bivalent booster might be more likely to have ischemic strokes within the first three weeks after their shots, compared with weeks four through six.
Ischemic strokes, the most common form, are blockages of blood to the brain. They’re usually caused by clots.
The Vaccine Safety Datalink, or VSD, is a network of large health systems across the nation that provides data about the safety and efficacy of vaccines through patients’ electronic health records. The CDC said it had identified possible confounding factors in the data coming from the VSD that may be biasing the data and need further investigation.
Of about 550,000 seniors who got Pfizer bivalent boosters and were tracked by the VSD, 130 had strokes in the three weeks after the shot, according to a CDC official who spoke to CNN on condition of anonymity because they weren’t authorized to share the data. None of the 130 people died.
The number of strokes detected is relatively small, said Dr. William Schaffner, an infectious disease expert at Vanderbilt University and a member of the CDC Advisory Committee on Immunization Practices’ Covid-19 Vaccine Work Group.
“These strokes are not a confirmed adverse event at the moment,” he said. “It’s like a radar system. You’re getting a blip on the radar, and you have to do further investigation to discover whether that airplane is friend or foe.”
The same safety signal has not been detected with the bivalent Moderna booster, the CDC said in its notice.
The agency noted that it has looked for and failed to find the same increase in strokes in other large collections of medical records, including those maintained by Medicare, the US Department of Veterans Affairs, as well as its Vaccine Adverse Event Reporting System, known as VAERS.
Neither Pfizer nor other countries that are using the vaccine have seen any increase in this kind of stroke, the agency said, and the signal was not detected in any other databases.
The CDC says that it does not recommend any change to vaccination practices at this time and that the risks of Covid-19 for older adults continue to outweigh any possible safety issues with the vaccine.
“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 notice says.
“CDC and FDA will continue to evaluate additional data from these and other vaccine safety systems. These data and additional analyses will be discussed at the upcoming January 26 meeting of the FDA’s Vaccines and Related Biological Products Advisory Committee.”
Pfizer said in a statement Friday, “Neither Pfizer and BioNTech nor the CDC or the U.S. Food and Drug Administration (FDA) have observed similar findings across numerous other monitoring systems in the U.S. and globally and 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 vaccination with the Omicron BA.4/BA.5-adapted bivalent vaccine.”
The bivalent boosters from Pfizer/BioNTech and Moderna protect against the original strain of the coronavirus as well as the Omicron BA.4 and BA.5 subvariants. Only about 50 million Americans ages 5 and up have gotten them since they were authorized last fall, according to CDC data.
Schaffner said he was part of a briefing Thursday with members of the Covid-19 Vaccine Work Group. He couldn’t share specific details about the briefing but said the safety signal was discussed.
His biggest takeaway was that the safety surveillance system is working.
It’s very likely that this is a false signal, he said, but it’s being investigated, which is important.
“You want a surveillance system that occasionally sends up false signals. If you don’t get any signals, you’re worried that you’re missing stuff.”
Schaffner said he would absolutely tell people to get their Covid-19 booster if they haven’t done so yet – even those 65 and older.
Get CNN Health's weekly newsletter
Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.
“Undoubtedly, the risk of a whole series of adverse events, including hospitalization, is much, much greater with Covid-19 than it is from the vaccine,” he said.
He also said the signal – if real – may be more a factor of numbers than an indication that one manufacturer’s vaccine is riskier than the other.
Nearly two-thirds of the people in the US who’ve gotten an updated booster – 32 million – have gotten Pfizer, compared with about 18 million Moderna shots.