Thread by @doctorow

Writing in the Journal of Evaluation in Clinical Practice, Oxford Primary Care Prof @trishgreenhalgh presents a peer-reviewed "Comprehensive Review" of the efficacy of masks in the context of the covid-19 pandemic.

tldr: Masks work.

onlinelibrary.wiley.com/doi/10.1111/je…

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Greenhalgh invited colleagues who disputed this to make their best arguments, then she rebutted each of those arguments, citing both public health practice and studies.

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She starts by noting that "face covering" (she dislikes the term mask, but I'm gonna use it here for brevity) skeptics "completely ignore various types of evidence... basic science, mathematical modelling and real‐world case examples of asymptomatic transmission..."

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The most important point for me in her work is that the majority of debate centers on research about whether masks protect wearers, but the point of asking the public to mask up is to prevent spreading - that is, to protect people other than the wearer.

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The evidence that even non-medical masks do this is pretty strong, especially masks that use two layers of "different fabrics with different physical and electrostatic properties." These appear to filter 90% of droplets.

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The way to control the pandemic is to reduce the number of people each newly infected person spreads the disease to until it is below one. With "R0 < 1," the number of infections will steadily decline.

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And it really looks like masks - even bad masks worn badly by many people - can knock covid-19's R0 below 1. That is, cloth masks can reverse the curve and end the pandemic.

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What's more, the evidence against masks is really weak, including studies purporting to show that mask-wearing may encourage riskier behavior, or that masks will cause anxiety, or facial rashes, or other negative outcomes.

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Bottom line: "Mathematical modelling suggests that a face covering that is 60% effective at blocking viral transmission and is worn by 60% of the population will reduce R0 to below 1.0."

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"This leaves plenty of room for error as people make their own imperfect coverings from old clothing and as some people either cannot or will not wear a face covering."

eof/