Quantifying the preventive effect of wearing face masks

An important task in combating the ongoing covid-19 effect lies in estimating the effect of different preventive measures. Here we focus on the preventive effect of enforcing the use of face masks. Several publications study this effect, however often using different measures such as: the relative attack rate in case-control studies, the effect on incidence growth/decline in a specific time-frame, the effect on the number of infected in a given time-frame. These measures all depend on community-specific features and are hence not easily transferred to other community settings. We argue that a more universal measure is the relative reduction in the reproduction number, which we call the face mask effect, EFM. It is shown how to convert the other measures to EFM. We also apply the methodology on three publications using different other measures (two of them resulting in two different estimates each, and all five estimates of EFM lie between 20-40%, suggesting that mandatory face masks reduce the reproduction number by 20-40% as compared to no individuals wearing face masks.