When the Centers for Disease Control and Prevention and the U.S. Food and Drug Administration announced an investigation into whether Pfizer’s bivalent COVID-19 booster caused strokes in certain age groups, it caused concern— and misinformation. 

The CDC and FDA announced their investigation in a Jan. 13 press release. The agencies said a "preliminary safety signal" was identified for people 65 and older, but their investigation found "it is very unlikely that the signal … represents a true clinical risk." 

But some people on social media claimed otherwise. 

Dr. Joseph Ladapo, Florida’s surgeon general, wrote on Twitter, "What better time than a Friday afternoon for (the CDC) and (U.S. Food and Drug Administration) to let Americans know that the mRNAshots they’ve been pushing may be causing strokes?"

"Glad I’m not vaxxed," posted an Instagram user. 

In their announcement, the agencies wrote that a database called the Vaccine Safety Datalink identified a safety signal. A CDC spokesperson told us that’s akin to a "warning or indication" that there may be a safety concern that needs attention. 

The Vaccine Safety Datalink is a collaborative project between the CDC and health care organizations that provide data on vaccines to monitor their safety. 

"Using VSD data that are updated each week, the rates of adverse events that occur in people who have received a particular vaccine are compared to the rate of adverse events that occurs in a similar group of people who have not received that vaccine," according to the CDC. "If the rate of adverse events among vaccinated people is higher than among the comparison group, the vaccine may be associated with an adverse event." 

The CDC and FDA were notified in November 2022 about a safety signal with the Pfizer bivalent booster, with data showing it could possibly be linked to a risk of ischemic strokes in older people. Ischemic strokes occur when blood clots or other particles block the blood vessels to the brain. 

The system found that 130 people ages 65 and older had a stroke within 21 days of receiving the Pfizer booster, out of 550,000 seniors who received the shot, a CDC spokesperson told CNBC. 

The Mayo Clinic estimates that 87% of all strokes are ischemic, and they affect about 800,000 people in the United States each year. Ischemic strokes are most common in elderly people but can occur at any age. 

A safety signal is not confirmation that the vaccine was the cause — only an alert to the possibility, the CDC spokesperson told us. Confirming a causal link between the vaccine and stroke risk requires an investigation and "confirmation from formal epidemiologic studies," according to the Jan. 13 news release.

"When one system detects a signal, the other safety monitoring systems are checked to validate whether the signal represents an actual concern with the vaccine or if it can be determined to be of no clinical relevance," the news release said.

The agencies wrote that they evaluated additional databases to see whether they provided data similar to what Vaccine Safety Datalink reported. Those included the Centers of Medicare and Medicaid Services database, the Veterans Affairs database, the Vaccine Adverse Event Reporting System managed by the CDC and FDA and Pfizer-BioNtech’s global safety database. The agencies also examined information from other countries, which have not seen an increased risk for ischemic stroke. 

"It is important to note that, to date, no other safety systems have shown a similar signal and multiple subsequent analyses have not validated this signal," the CDC spokesperson said. 

Daniel Salmon, director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health, told PolitiFact that when it comes to safety signals about vaccines, false positives are common. 

For example, with the H1N1 vaccine, Salmon said, there were three safety signals for idiopathic thrombocytopenic purpura, Bell's palsy and Guillain-Barré syndrome. But the first two illnesses were not real associations to the vaccine, and Guillain-Barré syndrome cases was very rare, Salmon said. 

A Pfizer spokesperson said the totality of evidence does not show that the boosters cause strokes.

The CDC recommends that people 6 months and older get vaccinated against COVID-19, and that all people who are eligible get boosters. The safety signal did not change that guidance.

According to CDC statistics, updated COVID-19 boosters make risk of hospitalization almost three times less likely and make the risk of death 19 times less likely.

"Staying up to date with vaccines is the most effective tool we have for reducing death, hospitalization, and severe disease from COVID-19, as has now been demonstrated in multiple studies conducted in the United States and other countries," the CDC spokesperson said.