These folks used a synthetic control to do the exact quasi experiment you described: https://www.iza.org/publications/dp/13319/face-masks-considerably-reduce-co

These folks used a synthetic control to do the exact quasi experiment you described: https://www.iza.org/publications/dp/13319/face-masks-considerably-reduce-covid-19-cases-in-germany-a-synthetic-control-method-approach https://www.iza.org/publications/dp/13319/face-masks-conside...