Face masks considerably reduce COVID-19 cases in Germany.
Proc Natl Acad Sci U S A. 2020 Dec 22;117(51):32293-32301. doi: 10.1073/pnas.2015954117. Epub 2020 Dec 3.
Proc Natl Acad Sci U S A. 2020.
PMID: 33273115
Free PMC article.