Before revision:  Here , we show that studies 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.
After revision:  Here , we show that when the adherence to mask-usage guidelines is taken into account, the empirical evidence indicates that masks prevent disease transmission: all studies we analyzed 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.