The systematic review and meta-analysis by Derek Chu and colleagues1 has several problems. First, the investigators combine data on SARS-CoV-2, SARS-CoV, and MERS-CoV. The characteristics of the diseases caused by these viruses are different.2, 3 The basic reproduction number of MERS-CoV is close to 1,2 mild illness was infrequent for SARS-CoV,3 and relevant presymptomatic, paucisymptomatic, or asymptomatic transmission occurs commonly only with SARS-CoV-2,3, 4 which will affect performance of control measures. Therefore, findings of the meta-regression on physical distancing shown in figure 3 of the Article1 and the meta-analysis of mask use shown in figure 4 of the Article1 cannot be interpreted.
Second, even if combining data from different diseases were valid, the assumed linear association between distance and the log risk ratio of disease in the meta-regression of physical distancing appears inappropriate: visual inspection of figure 3A in the Article suggests that the relationship is non-linear. Modelled absolute risk estimates of figure 3B are therefore problematic.
Third, only three studies on SARS-CoV-2 contributed to the meta-analysis of masks versus respirators. As detailed in the appendix (pp 1–2), one study was erroneously included, another was incorrectly extracted. When doing a random-effects meta-analysis of the two eligible studies on SARS-CoV-2 using corrected data, we found a pooled unadjusted risk ratio of SARS-CoV-2 infection comparing masks versus respirators with control of 0·22 (95% CI 0·01–8·96; appendix). The third study5 was appropriately included, but crude and adjusted risk ratios for SARS-CoV-2 infection comparing masks versus respirators with control shown in figure 5 of the Article1 are confounded because mask use was fully correlated with intensive hand hygiene (appendix).
In view of the observed errors, we did an audit of a random sample of 14 studies included in the analysis. For ten out of 14 studies, we found errors (appendix pp 3–20).
We declare no competing interests.
Supplementary Material
References
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