Unmasking the mask studies: why the effectiveness of surgical masks in preventing respiratory infections has been underestimated

Abstract

Background: Pre-pandemic empirical studies have produced mixed statistical results on the effectiveness of masks against respiratory viruses, leading to confusion that may have contributed to organizations such as the WHO and CDC initially not recommending that the general public wear masks during the COVID-19 pandemic. Methods: A threshold-based dose–response curve framework is used to analyse the effects of interventions on infection probabilities for both single and repeated exposure events. Empirical studies on mask effectiveness are evaluated with a statistical power analysis that includes the effect of adherence to mask usage protocols. Results: When the adherence to mask-usage guidelines is taken into account, the empirical evidence indicates that masks prevent disease transmission: all studies we analysed that did not find surgical masks to be effective were under-powered to such an extent that even if masks were 100% effective, the studies in question would still have been unlikely to find a statistically significant effect. We also provide a framework for understanding the effect of masks on the probability of infection for single and repeated exposures. The framework demonstrates that masks can have a disproportionately large protective effect and that more frequently wearing a mask provides super-linearly compounding protection. Conclusions: This work shows (1) that both theoretical and empirical evidence is consistent with masks protecting against respiratory infections and (2) that nonlinear effects and statistical considerations regarding the percentage of exposures for which masks are worn must be taken into account when designing empirical studies and interpreting their results.

Quantitative errors in the Cochrane review on "Physical interventions to interrupt or reduce the spread of respiratory viruses"

“…Finally, we show that existing data indicate with high probability-a 15X odds ratio-that effective masking leads to improved health outcomes. Our methodology (detailed in Appendices) and results confirm previous analysis that when the assumptions of studies are corrected, the available empirical evidence is consistent with masks being effective [18].…”

“…The complex question of mask effectiveness, including its dependence on specific conditions of studies, can be framed in terms of the extent to which mask wearing reduces the inhaled and exhaled viral load (i.e. the inhalation and exhalation efficacies) [18,24]. 5 RCTs always require theory to interpret their results (e.g., in our analysis above, we use the theoretical assumption that any effect masks have will occur only while they are worn, see also Appendix D).…”

“…Following the analysis of ref. [18] we can define f v) . Now suppose we wish to consider a bunch of exposure events with viral loads v 1 , v 2 , .... that are each separated enough in time that the probability of being infected by any of them is independent.…”

Surgical masks and filtering facepiece class 2 respirators (FFP2) have no major physiological effects at rest and during moderate exercise at 3000-m altitude: a randomised controlled trial

“… 38 Incidentally, perceived dyspnoea and discomfort were higher when exercise was performed first, suggesting a sensitising effect of exercise on mask-related unpleasant feelings, a desensitising effect of the cognitive tests, or both. These subjective factors should not be underestimated because they affect adherence to mask use, which is a key variable of mask effectiveness, 4 and which can be even more challenging in remote high-altitude environments.…”

“…Wearing face masks mitigates the transmission of SARS-CoV-2 and other airborne pathogens 4 but can also have side effects. At sea level, they were mainly related to mild hypercapnia and, particularly during exercise, slight oxygen desaturation.…”