FDA study doesn’t prove Pfizer COVID vaccine causes blood clots

CLAIM: A study by the U.S. Food and Drug Administration proves that Pfizer’s COVID-19 vaccine causes blood clots.

AP’S ASSESSMENT: False. The study, conducted in part by researchers from the FDA, is being misrepresented. The research showed an association between elderly recipients of Pfizer’s COVID-19 vaccine and instances of pulmonary embolisms. However, the authors note that the findings do not prove a link to the vaccine. The FDA confirmed that the agency has not found any new causal relationships between the Pfizer vaccine and the potential adverse event.

THE FACTS: Social media users are misinterpreting a study published this month in the journal “Vaccine,” to falsely claim it proves that the FDA has admitted that Pfizer’s COVID-19 shot causes blood clots.

“So the FDA finally came out and said that Pfizer’s Covid shot causes blood clots? Only 2 years late!” wrote one Twitter user who received more than 37,000 shares and 168,000 likes on the post.

Other social media users shared a link to a post from a conservative publication that has spread vaccine misinformation in the past. While the article more accurately conveys the results of the study, the contents are behind a paywall and its headline left out key context, stating: “Pfizer’s COVID-19 Vaccine Linked to Blood Clotting: FDA.”

But the FDA made no such an announcement, and the claims take the study’s findings out of context, according to experts.

The study, “Surveillance of COVID-19 vaccine safety among elderly persons aged 65 years and older,” was conducted by researchers affiliated with the FDA, the health policy firm Acumen LLC, the Department of Economics at Stanford University and the U.S. Centers for Medicare & Medicaid Services.

The researchers evaluated more than a dozen health-related events of interest following COVID-19 vaccination among recipients 65 and older, using Centers for Medicare & Medicaid Services data from December 2020 through January 2022. The rates of each event were compared to historical rates prior to COVID-19 vaccination.

Specifically, researchers found that rates of pulmonary embolism — blood clots in the lungs — met the threshold to be considered a statistical signal, which means an association between the vaccine and the health event was detected.

However, the researchers concluded in the paper that the FDA is currently not taking any regulatory actions based on the detection, because “the signals are still under investigation and require more robust study.” The FDA made a similar statement in July about such events.

Abby Capobianco, a press officer for the FDA, wrote in an email statement to The Associated Press on Wednesday, “The FDA has not found any new causal relationships between the Pfizer-BioNTech COVID-19 Vaccine and potential adverse events of special interest identified in 2021.”

“The FDA continues to find that the Pfizer-BioNTech COVID-19 Vaccine meets the FDA’s rigorous standards for safety and effectiveness,” she added.

The study also notes that the new findings “should be interpreted cautiously” because they do not prove that the vaccine caused the safety outcomes.

“An early warning system does not prove that the vaccines cause these outcomes,” the paper states, adding: “FDA strongly believes the potential benefits of COVID-19 vaccination outweigh the potential risks of COVID-19 infection.”

The study authors also outlined several important limitations of the research. For example, the analysis did not adjust for underlying risk factors such as comorbidities among some recipients, which, along with other aspects of the data used for the research, may have led to false positive or negative signals.

They also reported that the data may underestimate or overestimate certain conditions and that the results of the study, conducted among elderly patients, “may not be generalizable to those younger than 65 years.”

Several study authors did respond to requests for additional comment.

Dr. Jeffrey Olin, a cardiologist and professor at Mount Sinai’s Icahn School of Medicine in New York, reiterated that the paper found a “signal,” but emphasized that this is not the same as causation.

“There was a slightly increased signal in that vaccine compared to the other ones they tested,” said Olin, who is also the director of vascular medicine at Mount Sinai’s center for cardiovascular health. “So you can’t assess causation. All you can say is there’s an association. And then if you want to assess causation you need to have a much more sophisticated study.”

Olin noted that the group who received the Pfizer vaccine were older, more likely to be in nursing homes and had more comorbidities that would put them in the hospital — all things that are associated with blood clotting.

“It’s impossible to sort out those things as the cause versus the vaccine,” he said.

Dr. Eric Adler, a cardiologist and professor at the University of California, San Diego, said that claims about the study circulating on social media are a misinterpretation of the actual findings.

“They’re saying that they have a sign in their studies that there was statistical significance which means that there were more pulmonary embolisms than could have been expected, but this doesn’t rule out play of chance,” said Adler, who is also the medical director of the Heart Transplant Program at UCSD.

Pfizer declined to comment on the study specifically but wrote in a statement to the AP that adverse events reported after vaccination can’t immediately be attributed to the vaccine.

“AEs reported may not have any causal relationship to the vaccine,” the statement read, referring to adverse events using initials. “Rather, the event may be due to an underlying disease or some other factor such as past medical history or concomitant medication or the AEs may be coincidental.”

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Associated Press writer Josh Kelety in Phoenix contributed to this report.

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