Here's how the CDC tried to use bad science to convince people to wear masks during COVID
CDC Mask Studies Found Lacking in Quality, Researchers Say
A recent analysis of decades of publications in the Centers for Disease Control and Prevention's flagship weekly journal has raised questions about the scientific rigor behind the agency's mask recommendations during the COVID-19 pandemic.
The study, published as a preprint on the medical research repository medrxiv in mid-2023, was authored by researchers Vinay Prasad and Tracey Beth Hoeg, along with a third co-author. Titled "An analysis of studies pertaining to masks in Morbidity and Mortality Weekly Report: Characteristics and quality of all studies from 1978 to 2023," the paper examined every MMWR article related to masks over a 45-year span.
The researchers found that of 77 studies meeting their inclusion criteria, all were published after 2019 — none appeared prior to the pandemic. Nearly all, 75 out of 77, were conducted in the United States alone.
The study identified several methodological concerns. Of the 77 studies reviewed, 22, or roughly 29 percent, were observational and lacked a comparison group. None of the studies were randomized trials. The most common setting for the research was the community, accounting for about 45 percent of the total.
Only about 30 percent of the studies — 23 out of 77 — actually attempted to measure mask effectiveness. Of those, just 14.3 percent, or 11 studies, reported a statistically significant result. Despite this, approximately 75 percent of all studies reviewed concluded that masks were effective. The authors noted that more than 70 percent of the studies used causal language, implying that masks directly reduced COVID-19 transmission, even though the study designs did not support such a claim. Only one study cited conflicting evidence, and a single mannequin-based study acknowledged the absence of a proven causal relationship.
In their summary, the authors wrote: "MMWR publications pertaining to masks drew positive conclusions about mask effectiveness over 75 percent of the time despite only 30 percent testing masks and fewer than 15 percent having statistically significant results. No studies were randomized, yet over half drew causal conclusions. The level of evidence generated was low and the conclusions drawn were most often unsupported by the data. Our findings raise concern about the reliability of the journal for informing health policy."
The researchers suggested the pattern may reflect pressure to align with the CDC's prevailing policy positions, though they did not assert motive outright.
CDC's Masking Timeline
The study also documented the CDC's evolving masking guidance and the role MMWR studies played in shaping it.
In March 2020, the CDC did not recommend mask-wearing for healthy individuals, consistent with guidance from the then-U.S. Surgeon General. Over the following weeks, a coordinated campaign to recommend masks took hold. On April 3, 2020, the CDC advised people aged 2 and older to wear cloth face coverings in public.
On July 15, 2020, the CDC director recommended universal masking, citing an MMWR study involving two hairstylists in Missouri who continued working while symptomatic and wearing masks. The study was later cited as evidence that masks could prevent viral spread.
By fall 2020, the CDC recommended universal masking in schools and daycares, and state, district, and county mandates followed for children as young as 2. A federal mandate requiring masks on public transportation took effect in January 2021.
Criticism of Specific Claims
The study authors highlighted instances where CDC leadership cited research they considered inadequate. Former CDC Director Rochelle Walensky, for example, told the public that masks could help reduce the chance of COVID-19 infection by more than 80 percent. According to the analysis, this claim was based on an observational phone survey that found an association between respondents' recollection of mask-wearing and self-reported COVID-19 tests — a finding that was not statistically significant for cloth masks.
The authors noted that the CDC's own MMWR had, in some cases, published research that overstated its findings, used causal language unsupported by the data, and lacked comparison groups or randomization.
Independent reviews have reached different conclusions on mask effectiveness. A systematic review published by the Cochrane Library found insufficient evidence that surgical or N95 masks prevent the spread of respiratory viruses, a point the study authors referenced in their discussion.
Broader Implications
The researchers concluded that the quality of evidence published in MMWR warranted scrutiny, particularly given the role those publications played in shaping public health mandates affecting millions of Americans. They called into question whether the journal's findings should serve as the basis for health policy.
The broader debate over masking during the pandemic has remained politically and socially contentious. The new analysis adds a data-driven dimension to longstanding questions about the standards of evidence used to justify public health measures, even as public health agencies continue to weigh the balance between precaution and scientific certainty in managing respiratory disease outbreaks.
