CDC identifies possible safety issue with Pfizer’s updated Covid-19 vaccine, but says people should still be boosted


The US Centers for Disease Control and Prevention said on Friday that there is a possible safety concern with the bivalent Covid-19 vaccine made by Pfizer and BioNTech, but that it is unlikely to pose any 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 — has detected a possible increase in a certain type of stroke in people ages 4 and older. 65-year-old who recently received one of Pfizer’s updated booster shots.

A quick response analysis of that signal revealed that older adults who received a bivalent booster may be more likely to have ischemic strokes within the first three weeks after their strokes, 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 a network of large health care systems nationwide that provides data on vaccine safety and efficacy through electronic patient records. The CDC said it has identified possible confounding factors in the data from the VSD that could influence the data and warrant further investigation.

Of the approximately 550,000 seniors who received Pfizer bivalent boosters and were monitored by the VSD, 130 had strokes 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 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 working group.

“These strokes are not a confirmed adverse event at this time,” he said. “It’s like a radar system. You are getting a radar blip and need to do further investigation to find out if that aircraft is friend or foe.

The same safety signal was not detected with the Moderna bivalent booster, the CDC said in its advisory.

The agency noted that it searched and failed to find the same increase in strokes in other large medical record collections, including those maintained by 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 any increase in this type of stroke, the agency said, and the signal has not been detected in any other database.

The CDC says it is not recommending any changes to vaccination practices at this time and that the risks of Covid-19 for older adults continue to outweigh any possible safety concerns with the vaccine.

“While the totality of the data currently suggests that the signal in the VSD is very unlikely to pose a true clinical risk, we believe it is important to share this information with the public, as we have done in the past when one of our safety monitoring detect a signal,” the alert states.

“CDC and FDA will continue to evaluate additional data from these and other vaccine safety systems. These data and further analyzes will be discussed at the upcoming 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 United States and around the world, and there is no evidence to conclude that ischemic stroke is associated with the use of companies’ COVID-19 vaccines.

“Compared with published incidence rates of ischemic stroke in this elderly population, companies to date have observed fewer reported ischemic strokes following vaccination with Omicron BA.4/BA.5-adapted bivalent vaccine.”

The bivalent boosters from Pfizer/BioNTech and Moderna protect against the original strain of the coronavirus and the Omicron subvariants BA.4 and BA.5. Only about 50 million Americans ages 5 and older have gotten them since they were allowed last fall, according to data from the CDC.

Schaffner said he attended a briefing on Thursday with members of the Covid-19 Vaccine Work Group. She couldn’t share specific details about the briefing, but said the safety signal was discussed.

Its biggest advantage was that the security surveillance system works.

It’s very likely a false signal, he said, but it’s being investigated, which is important.

“You want a surveillance system that sends false signals from time to time. If you’re not getting any signal, you’re worried about missing something.

Schaffner said he would absolutely tell people to get their Covid-19 booster if they haven’t yet, even those aged 65 and older.

“Undoubtedly, the risk of a whole range of adverse events, including hospitalization, is much, much greater with Covid-19 than it is with the vaccine,” he said.

He also said the signal, if real, could be more of a numerical factor than an indication that one manufacturer’s vaccine is riskier than the other.

Nearly two-thirds of people in the United States who received an upgraded booster — 32 million — received Pfizer, compared with about 18 million injections from Moderna.

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