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More than a year after masks began to blossom on the streets of many cities in response to the Covid-19 crisis, guidance over how best to use them continues to change. In January some European governments required medical-grade coverings in public settings in response to new variants of the coronavirus that causes Covid-19. One of U.S. President Joe Biden’s first acts in office was to break with his predecessor by issuing a nationwide, if limited, mask mandate. In April and then again in May, however, U.S. health authorities relaxed their recommendations for fully vaccinated people. Here’s a look at some of the issues involved.