In the American Journal of Medicine, a clinical research study named An analysis of studies pertaining to masks in Morbidity and Mortality Weekly Report: Characteristics and quality of studies from 1978 to 2023 analyzed 77 studies assessing mask effectiveness.
77 studies published after 2019 were analyzed. Zero were randomized studies. 58 stated masks were effective. Less than 15% showed statistically significant results. The level of evidence generated was low and the conclusions were most often unsupported by the data.