Epidemiologists remain stuck:

Second, we did not find any evidence of decreased risk of transmission in individuals who reported mask use. Although this finding collides with the evidence reported elsewhere,8 we did not have fine-grained data on type of mask (surgical vs FFP2) or use of other measures of personal protective equipment (PPE) or other infection control practices, thus limiting our ability to make clear inferences about the effect of PPE on transmission risk. Mask use is probably correlated with type of exposure, which might further confound associations, but we did not note any association between mask use and risk either in our unadjusted analysis (table 3) or in a multivariable model excluding type of exposure (data not shown)

Source: Transmission of COVID-19 in 282 clusters in Catalonia, Spain: a cohort study – The Lancet Infectious Diseases

By EdwardM