US officials sidelined warnings about flawed COVID vaccine safety algorithm, investigation finds
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U.S. health officials knowingly relied on a compromised algorithm to detect signals of harm from mRNA COVID-19 vaccines and silenced efforts to fix it, finds an investigation published by The BMJ today.
The findings are based on government emails released under the Freedom of Information Act and in response to U.S. Senate investigators, as well as exclusive interviews with public health officials and other scientists by investigative journalist David Willman.
Before rolling out COVID-19 vaccines in December 2020, the U.S. Centers for Disease Control and Prevention (CDC) assured health care professionals and the public that its Vaccine Adverse Event Reporting System (VAERS) could quickly spot any potentially harmful reactions following vaccination.
Surveillance plans went unfulfilled
The CDC planned to use two "data mining" techniques: "proportional reporting ratios" (PRRs) and an "empirical Bayesian" method provided by the Food and Drug Administration (FDA), which operated VAERS jointly with the CDC. The two agencies planned to share and discuss results.
But according to a letter by CDC Director Rochelle Walensky, the agency did not perform PRR analyses until 2022, and both the CDC and FDA "chose to rely" entirely on the FDA's Bayesian method.
The BMJ can reveal that the FDA's algorithm failed to signal a potential relationship between mRNA COVID vaccination and myocarditis (inflammation of the heart muscle), pericarditis (inflammation of the fluid-filled sac around the heart), Bell's palsy, tinnitus and other reported disorders because of a flaw in the detection methodology.
A known detection flaw
The problem arose because more than 90% of initial VAERS reports were for the new mRNA COVID vaccines made by Pfizer and Moderna. If both vaccines elevated the risk of an adverse event like myocarditis in roughly equal amounts, however, the "observed" frequency and "expected" frequency would be similar, resulting in no automated alert.
Internal records show that FDA officials were aware of this limitation before and during the pandemic. Government documents also show that CDC officials were informed during the vaccine rollout.
In early 2021, FDA medical officer Dr. Ana Szarfman, working with statistician William DuMouchel, who had developed the Bayesian algorithm, warned top officials about the flaw and proposed an updated algorithm that flagged signals. But Szarfman was asked to "cease and desist." Meanwhile, officials continued to cite the lack of system alerts while reassuring clinicians and the public of the vaccines' safety.
Alerts that went unreported
When the CDC finally ran PRR analyses in 2022, Walensky said the results revealed "no additional unexpected safety signals." Yet the analyses—examined by The BMJ—show hundreds of adverse events that met the agency's alert criteria, including myocarditis, pericarditis, Bell's palsy and tinnitus, which the FDA's Bayesian method had not triggered.
In October 2023, the FDA's pharmacovigilance chief acknowledged in an email to colleagues that the agency knew of the detection deficiency as the vaccines rolled out more than two years earlier, but did not respond to requests for comment.
Approached by The BMJ, Szarfman insisted that she had not sought to undermine public support for the COVID vaccines. Expressing frustration, she noted, "Very few people understand the statistics. That's the problem."
DuMouchel said he could not explain officials' resistance to switching to the updated method, stating, "I think that they were wrong." He also regretted that FDA officials rejected Szarfman's proposed fixes for VAERS, adding, "If they had paid attention to Ana, they would have done better."
Publication details
US officials knew covid vaccine safety system was flawed—but suppressed the FDA doctor who alerted them, The BMJ (2026). DOI: 10.1136/bmj-2026-100806
Journal information: British Medical Journal (BMJ)
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Clinical pharmacologyPreventive medicine Provided by British Medical Journal Who's behind this story?
Swati Mestri
Swati Mestri holds a bachelor's degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space. Full profile →
Robert Egan
Bachelor's in mathematical biology, Master's in creative writing. Well-traveled with unique perspectives on science and language. Full profile →
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