What the Cochrane “masks don’t work” review does (and does not) tell us

You may have seen some commentary and debate on a recently updated Cochrane review on physical interventions to interrupt the spread of respiratory viruses. I’m stepping into the debate only to clarify a few small points – and to provide an overview of what the review does (and does not) tell us. However, my firm advice is to read the review for yourself and come to your own conclusions.

Here’s a few key points / take-aways:

  • This Cochrane review is an update of a previously published review (first published in 2007). The whole premise of the Cochrane review family is to provide confidence to the reader that the review was done in a highly systematic way. So in that sense, whilst we can and should debate the inclusion and the exclusion criteria, the review methodology itself seems sound to me.
  • The review is about more than face masks, and includes RCTs on hand hygiene, gargling, and combination interventions.
  • The review is about more than pandemic SARS-CoV-2. In fact, most of the studies included in the review predate the pandemic and concern endemic influenza. This is an important point when it comes to questions about mask efficacy in community settings because you’d expect the real-world efficacy of masks in a community setting to be affected by prevalence (and more effective at higher levels of prevalence).
  • The main findings of the review are that:
    • There is not good evidence that wearing a surgical/medical masks vs. no mask in the community reduces respiratory virus transmission.
    • The impact of wearing FFP3/N95 respirators vs. medical/surgical masks in reducing respiratory virus transmission is uncertain.
    • Hand hygiene interventions are probably beneficial in reducing respiratory virus transmission.
  • The review includes only RCTs and Cluster RCTs. As we know, this is not the only type of evidence that is available to us, and for physical interventions to prevent the spread of the respiratory viruses, many other types of evidence are “admissible” in the debate – from laboratory scale studies, through to cohort studies and everything else in between. As a brief illustration of how important this point is, take a look at this Lancet systematic review and meta-analysis with a very similar goal in mind: to test whether physical interventions prevent the spread of SARS-CoV-2. This study found a large number of articles (172) that met the inclusion criteria, not a single one of which was an RCT! Unlike the Cochrane review, this study concluded that physical distancing, face masks, and eye protection were all associated with a significantly reduced risk of SARS-CoV-2 acquisition.

So, I don’t see this review as a disingenuous attempt to skew the available evidence but rather an illustration of the limited RCT evidence base that we have for masks use in the context of the SARS-CoV-2 respiratory virus pandemic. This is not the same thing as concluding that “masks don’t work” – and there’s a huge library of data to suggest that masks do reduce the risk of respiratory virus transmission.


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