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In truth, the CDC's, U.K.'s, and WHO's earlier guidance was much more consistent with the best medical research on masks' effectiveness in preventing the spread of viruses. That research suggests that Americans' many months of mask-wearing has likely provided little to no health benefit and might even have been counterproductive in preventing the spread of the novel coronavirus.

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RCTs are uniformly regarded as the gold standard in medical research, yet the CDC basically ignores them apart from disparaging certain ones that particularly contradict the agency's position. In a "Science Brief" highlighting studies that "demonstrate that mask wearing reduces new infections" and serving as the main public justification for its mask guidance, the CDC provides a helpful matrix of 15 studiesnone RCTs. The CDC instead focuses strictly on observational studies completed after Covid-19 began.

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In sum, of the 14 RCTs that have tested the effectiveness of masks in preventing the transmission of respiratory viruses, three suggest, but do not provide any statistically significant evidence in intention-to-treat analysis, that masks might be useful. The other eleven suggest that masks are either uselesswhether compared with no masks or because they appear not to add to good hand hygiene aloneor actually counterproductive. Of the three studies that provided statistically significant evidence in intention-to-treat analysis that was not contradicted within the same study, one found that the combination of surgical masks and hand hygiene was less effective than hand hygiene alone, one found that the combination of surgical masks and hand hygiene was less effective than nothing, and one found that cloth masks were less effective than surgical masks.

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