Investigation reignites questions about what steps were taken to safeguard public health from any unintended consequences of the vaccination rollout
US 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 US 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 US Centers for Disease Control and Prevention (CDC) assured healthcare professionals and the public that its Vaccine Adverse Event Reporting System (VAERS) could quickly spot any potentially harmful reactions following vaccination.
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 CDC and FDA “chose to rely” entirely on 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 owing to a flaw in the detection methodology.
The problem arose because over 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 rollout of the vaccines.
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.
When the CDC finally ran PRR analyses in 2022, CDC director Walensky said 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 – as the vaccines had rolled out more than two years earlier – of the detection deficiency, 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 switch 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.”
16/09/2026
Notes for editors
Investigation: US officials knew covid vaccine safety system was flawed—but suppressed the FDA doctor who alerted them doi: 10.1136/bmj-2026-100806
Funding: BMJ Investigations Unit
Link to Academy of Medical Sciences press release labelling system: http://press.psprings.co.uk/
Externally peer reviewed? Yes
Evidence type: Investigation
Subject: Adverse event reporting
First published in 1840, The BMJ currently ranks among the world’s top five general medical research journals.
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