Double-zero-event studies matter: A re-evaluation of physical distancing, face masks, and eye protection for preventing person-to-person transmission of COVID-19 and its policy impact

Abstract

In a recent timely systematic review, Chu et al. [1] assessed the effectiveness of face masks, eye protection, and physical distancing for preventing COVID-19. Because the sample sizes are not large, especially in some studies of COVID-19, this review contains a considerable number of studies with zero counts of infection events, creating challenges in estimating effect sizes. If zero counts appear in both groups, this double-zero-event study (DZS) is omitted from the analyses, as implied in the forest plots in Chu et al. [1] Specifically, at least 9 out of 44 studies in this review are DZS with 1784 subjects. An omission of information about the rare outcome in DZS or artificial correction of the zero counts could impact the conclusions. [2], [3], [4], [5].