Germany has had strict requirements for people to use N95-style masks and… a raging coronaplague to go with those masks (but the failure of two-way N95 masking should not shake our faith in one-way N95 masking! Don’t stay home to avoid COVID-19, get on a plane with an N95 mask, eat in restaurants, stay in hotels, etc.). There was a natural experiment done recently in which Hamburg maintained its mask order while the rest of Germany dropped it. Here are the curves: