Social Media Posts Twist Meaning of CDC, FDA Disclosure on Bivalent Booster


SciCheck Digest

Government health agencies disclosed a potential safety concern for strokes in those 65 and older with one of the COVID-19 vaccines, but the agencies haven’t found any causal relationship and the concern was flagged by just one of several monitoring systems. Anti-vaccine campaigners, however, have wrongly claimed the agencies have found a link between the boosters and strokes.

Full Story

Most of the systems set up to detect early signs of potential problems with the COVID-19 vaccines haven’t given any indication that the bivalent booster from Pfizer/BioNTech may correspond with a certain type of stroke in those 65 and older.

But one system did. So the Centers for Disease Control and Prevention and the Food and Drug Administration issued an explainer on Jan. 13 reporting that the Vaccine Safety Datalink — a real-time monitoring system that the CDC has used since 1990 — had “met the statistical criteria to prompt additional investigation into whether there was a safety concern for ischemic stroke in people ages 65 and older who received the Pfizer-BioNTech COVID-19 Vaccine, Bivalent.”

The CDC and FDA also explained:

  • The Vaccine Adverse Event Reporting System had not flagged it.
  • Pfizer/BioNTech’s own global safety database had not flagged it.
  • Other countries haven’t noted an increased risk for this type of stroke with bivalent vaccines.
  • A large study of bivalent vaccines from both Pfizer/BioNTech and Moderna drawing from the Centers for Medicare and Medicaid Services database showed no increased risk of ischemic stroke.
  • A preliminary study using the Veterans Affairs database didn’t show an increased risk of ischemic stroke following a bivalent shot.

“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,” the CDC and FDA wrote in the explainer.

Despite all the context offered by the CDC and FDA and the fact that the similarly formulated COVID-19 vaccine from Moderna wasn’t flagged, anti-vaccine campaigners took the announcement and used it to wrongly suggest that the agencies had found a causal link between the booster and strokes.

Dr. Simone Gold, who has frequently spread misinformation about COVID-19 treatments, wrote in a Twitter thread about the announcement: “We’ve know the truth for a while. We’ve warned the public about the potential dangers associated with experimental biological agents. The public health officials who marketed these vaccines as ‘completely safe’ while censoring dissenting voices have blood on their hands.”

Robert F. Kennedy Jr., who leads the anti-vaccination organization Children’s Health Defense, took a similar approach, referencing the accumulation of claims from anti-vaccination campaigners since the shots became available as though they were correct. “Alarming new information from the CDC + FDA that everyone else has known for almost two years,” he wrote on Twitter.

And conspiracy theorist Stew Peters took it even further, claiming on Rumble that the “CDC ADMITS Vaxx Causes Strokes.”

But, as is clear from the explainer that publicly disclosed the potential problem, the CDC has not identified any kind of causal link.

The extent of its finding is that data collected through the VSD indicated that researchers should investigate whether or not those who are 65 or older may be at risk of having an ischemic stroke in the first few weeks following a booster dose with the Pfizer bivalent vaccine. The system’s flag was based on comparing the risk for stroke in the 21 days following vaccination with days 22-42 following vaccination.

An ischemic stroke happens when a vessel that brings blood to the brain is obstructed, usually by fatty deposits on vessel walls and blood clots, according to the American Stroke Association. This type of stroke accounts for more than 80% of all strokes, according to the Cleveland Clinic.

The VSD found that out of about 550,000 people who were 65 or older, 130 had strokes within three weeks of getting the shot, according to reporting in the Washington Post. None of those who had a stroke died.

“Often these safety systems detect signals that could be due to factors other than the vaccine itself,” the statement disclosing the potential issue said.

Safety monitoring systems like VSD are designed to catch a wide array of potential signals so that researchers can assess whether health agencies should change their recommendations. We’ve written about examples similar to this one, where systems detected a potential issue and researchers found little or no evidence that the vaccines were causing the problem.

We’ve also written about examples where safety monitoring systems detected a problem that researchers found was likely related to a vaccine, as in the case of the Johnson & Johnson vaccine, which can cause a particular kind of clotting that also involves low levels of blood platelets. The condition is very rare, and the CDC has recommended the two mRNA vaccines over J&J’s, which has accounted for only a small percentage of the vaccine doses administered in the U.S.

As the Washington Post article noted, officials are skeptical that there’s a safety risk here because there’s no reason that this kind of problem would arise now, considering the large number of vaccines administered globally over the last two years, and there’s also no reason that it would be present in only the Pfizer-BioNTech shot and not in the similar one from Moderna.

The fact that public health officials could detect a potential issue associated with the vaccines, though, shows that the system is working.

But, in this case, it didn’t warrant a change in recommendations for the vaccines, which have been effective in preventing serious illness from COVID-19 and, studies have shown, can actually reduce the risk of stroke after infection with the virus that causes COVID-19.

Editor’s note: SciCheck’s articles correcting health misinformation are made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over’s editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation.


Centers for Disease Control and Prevention. “CDC & FDA Identify Preliminary COVID-19 Vaccine Safety Signal for Persons Aged 65 Years and Older.” 13 Jan 2023.

Hale Spencer, Saranac. “Widespread Claims Misrepresent Effectiveness of COVID-19 Vaccines.” 26 Aug 2022.

American Stroke Association. Ischemic Stroke (Clots). Accessed 20 Jan 2023.

Cleveland Clinic. Ischemic Stroke. Accessed 20 Jan 2023.

McGinley, Laurie and Lena H. Sun. “Extensive review affirms covid booster is safe after system flagged risk.” Washington Post. 13 Jan 2023.

Hale Spencer, Saranac. “Social Media Posts Misrepresent FDA’s COVID-19 Vaccine Safety Research.” 23 Dec 2022.

Robertson, Lori and Eugene Kiely. “Q&A on the Rare Clotting Events That Caused the J&J Pause.” Updated 6 May 2022.

Kim, Young-Eun, et al. “Association Between Vaccination and Acute Myocardial Infarction and Ischemic Stroke After COVID-19 Infection.” JAMA – Journal of the American Medical Association. 22 Jul 2022.

Leave a Reply

Your email address will not be published. Required fields are marked *