The CDC is still recommending masking in areas with "high" transmission levels - fewer than 4% of U.S. counties - as well as indoor masking to protect high-risk contacts in "medium" counties, including New York, New Jersey, Massachusetts, Pennsylvania, Washington and California.
Researchers for the Cochrane study are affiliated with a geographically disparate range of institutions in the U.K., Canada, Australia, Italy and Saudi Arabia.
- The team added 11 new randomized controlled trials and "cluster-RCTs," which randomize groups of subjects rather than individuals, to its prior review from November 2020, for a total of 78 studies.
The updated Cochrane review included 10 cluster RCTs in the "community" and two with healthcare workers totalling about 290,000 participants, comparing the effect of surgical versus no masks.
- In the community, mask-wearing "probably makes little to no difference" in either influenza-like or COVID-like illness (0.95) or "laboratory-confirmed" infections of either virus (1.01).
- Researchers had more trouble determining the effects of respirators such as N95s versus surgical masks.
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