(1/4) Case-control study of 1,050 contacts of #COVID19 cases in 3 clusters in Thailand, showing that wearing a mask was associated with reduced risk of becoming infected.

The type of mask worn did not matter, but it did have to be worn consistently.
https://wwwnc.cdc.gov/eid/article/26/11/20-3003_article

(2/4) Physical distancing, shorter contact duration, hand washing, and wearing a mask always when in contact with a case (but not just sometimes) was associated with reduced risk of becoming infected.

Sharing dishes, cups, and cigarettes was associated with increased risk.

(3/4) Wearing a mask was not associated with a false sense of security.

People who wore a mask were more likely to maintain physical distance and to practice hand hygiene than those who did not wear a mask.

(4/4) The authors noted that "no single protective measure was associated with complete protection from COVID-19. All measures, including mask-wearing, handwashing, and social distancing, can increase protection against COVID-19 in public settings."