Face Masks Considerably Reduce Covid-19 Cases in Germany [pdf]

If I understand correctly, this is also not a randomized trial. I am not sure how they constructed the "control" group here to which they are comparing. It is a synthetic group where they extrapolate cases from n-7 days where n is the day of introduction of mandatory use of masks?

It is hard to not be tempted to think about several confounding variables that could have played a role in the drop that is seen, like people being more vigilant, following distance guidelines, working from home more often, the list goes on. Nevertheless, it was an interesting read.

We use the synthetic control method to analyze the effect of face masks on the spread of Covid-19 in Germany. Our identification approach exploits regional variation in the point in time when face masks became compulsory. Depending on the region we analyse, we find that face masks reduced the cumulative number of registered Covid-19 cases between 2.3% and 13% over a period of 10 days after they became compulsory. Assessing the credibility of the various estimates, we conclude that face masks reduce the daily growth rate of reported infections by around 40%