Study finds CDC mask research relied on low-quality studies and overstated claims

A new analysis of the CDC’s Morbidity and Mortality Weekly Report found that the agency’s published studies on masks relied on low-quality evidence and frequently overstated their findings.

Three researchers examined every MMWR study on masks from 1978 through 2023. They identified 77 studies that met their criteria. All were published after 2019, and 75 came from the United States.

Twenty-two of the studies, or 28.6 percent, were observational and had no comparison group. None of the 77 studies were randomized trials. Thirty-five studies, or 45.5 percent, examined mask use in community settings.

Only 23 studies, or 30 percent, attempted to measure whether masks reduced infection. Eleven studies, or 14.3 percent, reported a statistically significant result. Despite these limits, 58 studies, or 75.3 percent, stated that masks were effective.

Fifty-five studies, or 71 percent, used causal language that linked mask wearing to lower transmission, even though the data did not support that conclusion. Only one study cited conflicting evidence.

The researchers noted that the MMWR reports were used to justify federal and state mask policies, including requirements for children as young as two and mandates on public transportation. They concluded that the journal’s mask coverage produced low-quality evidence and unsupported claims.

The review also traced the CDC’s shifting guidance. In March 2020 the agency did not recommend masks for the general public. By April 3 it advised cloth coverings for people age 2 and older. In July 2020 the CDC director cited an MMWR study of two Missouri hairstylists to urge universal masking. Later recommendations extended to schools and daycares.