A possible cohort study of states that did and states that didn't mask, comparing the base rates of infection, rates of transmission, should be possible. Hospital admissions were covid screened, supposedly, exclude non-respiratory admissions (ie the motorcyclist who was admitted to trauma after a crash who tested positive before admission) who were positive. The key would be finding institutions who screened prior to admission (negatives) and prior to discharge (positive conversions), and compare those rates between the states with different policies.

I suspect that data won't be available or reliable, though, since it appears we cured the common cold as most of them became covid cases.

(See Naquin, A. et al, Morbidity and Mortality Weekly Report Surv.Summ. 2024 Oct 31; 73(6):1-18).

pubmed.ncbi.nlm.nih.gov…)

But it looks like the cold and flu are bouncing back, so maybe we can get a prospective study in comparing states like MD to other states with similar climates, and population densities that don't do this.