The U.S. Centers for Disease Control and Prevention said on Friday that a possible security issue with the bivalent Covid-19 vaccine made by Pfizer and BioNTech, but unlikely to pose a real risk. The agency said it continues to recommend 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 type of stroke in people age 65 and older who recently received one of the updated booster shots. received from Pfizer.
A rapid response analysis of that signal revealed that seniors who received a bivalent booster were more likely to have ischemic strokes within the first three weeks after their injections, compared to weeks four through six.
Ischemic strokes, the most common form, are blockages of blood to the brain. They are usually caused by clots.
The Vaccine Safety Datalink, or VSD, is one network of major health systems across the country that provides data on the safety and efficacy of vaccines through patients’ electronic health records. The CDC said it had identified potential confounding factors in the data coming from the FO that could skew the data and require further investigation.
Of the approximately 550,000 seniors who received bivalent boosters from Pfizer and were followed by the VSD, 130 suffered a stroke in the three weeks following the injection, according to a CDC official who spoke to CNN on condition of anonymity because they were not authorized to administer the share 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 side effect at this time,” he said. “It’s like a radar system. You get a blip on the radar and you have to do further research to find out if that plane is friend or foe.”
The same safety signal has not been detected with the bivalent Moderna booster, the CDC said in its post.
The agency noted that it has looked for the same increase in strokes and has been unable to find the same increase in strokes in other major collections of medical records, including that of Medicare, the U.S. Department of Veterans Affairs, as well as its Vaccine Adverse Event Reporting System, known as VAERS.
Neither Pfizer nor other countries using the vaccine have seen an increase in these types of strokes, the agency said, and the signal was not detected in other databases.
The CDC says it is not recommending any change in vaccination practices at this time and that the risks of Covid-19 for older adults still outweigh any safety concerns with the vaccine.
“While the totality of the data currently suggests that the signal in FO is highly unlikely to represent any true clinical risk, we feel it is important to share this information with the public, as we have done in the pastwhen one of our safety monitoring systems detects a signal,” the statement read.
“CDC and FDA will continue to evaluate additional data from this and other vaccine safety systems. These data and additional analyzes will be discussed in due course January 26 meeting of the FDA’s Advisory Committee on Vaccines and Related Biological Products.”
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 in numerous other monitoring systems in the U.S. and globally, and there is no evidence to conclude that ischemic stroke is related to the use of the companies’ COVID-19 vaccines.
“Compared to published ischemic stroke incidence rates in this older population, the companies have so far observed a lower rate of reported ischemic strokes following vaccination with the Omicron BA.4/BA.5-adapted bivalent vaccine.”
The bivalent boosters from Pfizer/BioNTech and Moderna protect both against the original strain of the coronavirus and against the subvariants Omicron BA.4 and BA.5. According to the CDC, only about 50 million Americans ages 5 and older have received them since they were approved last fall data.
Schaffner said he was part of a briefing with members of the Covid-19 Vaccine Work Group on Thursday. He could not share specific details about the briefing, but said the safety signal had been discussed.
His biggest takeaway was that the security monitoring system works.
It’s very likely this is a false signal, he said, but it’s being investigated, which is important.
“You want a surveillance system that occasionally sends false signals. If you don’t get signals, you’re afraid you’re missing things.”
Schaffner said he would absolutely tell people to get their Covid-19 booster if they haven’t already — even those 65 and older.
“No doubt the risk of a whole host of side effects, including hospitalization, is much, much higher with Covid-19 than with the vaccine,” he said.
He also said the signal — if genuine — may be more a factor of numbers than an indication that one manufacturer’s vaccine is riskier than another.
Nearly two-thirds of the people in the US who got an updated booster — 32 million — got Pfizer, compared to about 18 million Moderna shots.