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Since the beginning of the COVID-19 pandemic, considerable debate (some political, some scientific) has centered on the protective effectiveness of face masks. Laboratory trials have shown that masks do filter virus-sized particles from the air. Now, researchers in California have compared SARS-CoV-2 test positivity rates among those who did or did not consistently wear masks in indoor public spaces; they also evaluated the relative effectiveness of the three mask types.
Mask use was assessed from February to December 2021 among 652 participants who had tested positive for SARS-CoV-2 (cases) and 1176 matched participants who tested negative (controls). Overall, consistent wearing of any mask type was associated with an adjusted odds ratio of 0.44 for a positive test (95% confidence interval, 0.24–0.82). Among participants who reported the type of mask, odds of positivity were lowest with respirators, highest with cloth masks, and intermediate with surgical masks. Stepwise increased protection was proportional to frequency of mask wearing. Furthermore, compared with controls, a higher proportion of cases were unvaccinated.
Andrejko KL et al. Effectiveness of face mask or respirator use in indoor public settings for prevention of SARS-CoV-2 infection — California, February–December 2021. MMWR Morb Mortal Wkly Rep 2022 Feb 11; 71:212. (https://doi.org/10.15585/mmwr.mm7106e1)
Comment
It's somewhat ironic that these effectiveness data are published at a time when mask wearing is on the decline (associated meritoriously with falling COVID-19 cases and hospital admissions). However, one thing is certain: This is not the last epidemic or pandemic to be caused by an aerosol-borne respiratory pathogen, and it will be beneficial to know from the very start that mask-wearing helps limit contagion.