That changed in mid-May after the Centers for Disease Control and Prevention advised vaccinated Americans that they could go maskless in most indoor settings. The next week, the store told employees that they could no longer ask customers to cover their faces. So mask use plummeted, and the anxiety of Ms. Wainwright and other workers shot up. “We just feel like we’re sitting ducks,” said Ms. Wainwright, who estimated that far fewer than half of patrons wore masks on a recent Sunday. “Now it’s just a free-for-all.” More than a dozen retail, hospitality and fast-food workers across the country interviewed by The New York Times expressed alarm that their employers had used the C.D.C. guidance to make masks optional for vaccinated customers. Some, like Ms. Wainwright, said they had been vaccinated but worried they could still get sick or infect family members who were not or could not get vaccinated. Others said they had yet to be vaccinated.