But as the crisis has played out around the world and intensified in parts of the U.S., reasons have emerged to doubt the wisdom of this guidance, which ranks among the most forceful warnings against mask use by national health authorities anywhere and does not differentiate between medical-grade masks and simple cloth coverings. A number of societies where mask use is more widespread, and where mask shortages have been less severe, seem to have had more success containing the virus. Now, some health experts, who say there is no evidence for the claim that masks increase users’ risk of catching the virus, are calling for more widespread use of face coverings in the U.S.

“Guidance needs to change and needs to be clear that these nonmedical, nonsurgical masks are beneficial to the general public and should be worn when outside of the home,” said Robert Hecht, a professor of epidemiology at the Yale School of Public Health.

The increasing calls for more use of masks raise the question of whether authorities’ recommendations were based on genuine concerns about spreading Covid-19 or instead motivated by a desire to prevent a run on limited supplies of masks: “Seriously people — STOP BUYING MASKS!” tweeted Adams in late February.