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Surgical masks in fact reduce the spread of the COVID virus

Brian Leiter

A large, randomized study done in Bangladesh offers strong evidence; surgical masks were also more effective than cloth masks.  An informative news account here; an excerpt:

Among the roughly 178,000 individuals who were encouraged to wear them, the scientists found that mask-wearing increased by almost 30 percent and that the change in behavior persisted for 10 weeks or more. After the program was instituted, the researchers reported an 11.9 percent decrease in symptomatic Covid symptoms and a 9.3 percent reduction in symptomatic seroprevalence, which indicates that the virus was detected in blood tests….

"A 30-percent increase in mask-wearing led to a 10 percent drop in Covid, so imagine if there was a 100-percent increase — if everybody wore a mask and we saw a 100-percent change," he said.

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2 responses to “Surgical masks in fact reduce the spread of the COVID virus”

  1. Anonymous

    Yes, the cluster RCT testing community masking policies in Bangladesh is fascinating; the sort of trial that should have been performed in the US long ago. This thread, https://threadreaderapp.com/thread/1433906821022957572.html, offers a more informative and nuanced summary of the trial than does the NBC article, including potential limitations to the US.

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  2. John Turri

    Interesting study. After reading it just once, and in the interest in seeing what discussion of these points here brings, I'd like to call attention to a few details included in the version of the full paper available here (accessed Sep 6, 2021): https://www.poverty-action.org/sites/default/files/publications/Mask_RCT____Symptomatic_Seropositivity_083121.pdf

    – Based on the abstract, Results section (pp. 22-3), and Figure 1 (p. 23), comparing surgical-mask villages to control villages, the relative reduction of symptomatic seroprevalence was over 11% (.8% and .71%, although the figure reports a slightly different spread for "symptomatic seroprevalence" of .76% and .67%). Thus the absolute reduction was .09%.
    – I'm happy to be corrected, but I believe that, practically speaking, this implies that in a population of 100,000 people over 12 weeks, 90 fewer people would report symptoms and test positive on an IgG antibody test, if they wore surgical masks rather than no face covering (100,000 * .008 – 100,000 * .0071 = 800 – 710 = 90).
    – The reported p-value for the surgical-mask/control comparison is .043 (p. 23, Figure 1) ("adjusting for baseline covariates," p. 2). For a sample size in the tens or hundreds of thousands, I am suspicious of a p-value of .043. (However, there is almost certainly an error in at least one of the reported p-values in that figure.)
    – "Neither participants nor field staff were blinded to intervention assignment," p.2. This could affect self-report measures, by either influencing assessment of one's own case, or response bias (the authors recognize this on p. 22). Seemingly relevant to this, from the Methods section in the abstract (p. 2): "At 5 and 9 weeks follow-up, we surveyed all *reachable* participants about COVID-related symptoms. Blood samples collected at 10-12 weeks of follow-up *for symptomatic individuals* were analyzed for SARS-CoV-2 IgG antibodies," and from the note for Table 2 (p. 21), where they define "baseline symptom rate … as the rate of surveyed individuals in a village who report symptoms coinciding with The WHO definition of a probable" infection.

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