Mary Murtaugh is a recently retired nurse who says she has never been shy about promoting good health. She wasn’t shy the day she asked her favorite grocery store cashier if she’d been vaccinated. The cashier was emphatic. No. And she wasn’t going to be. After listening to the cashier explain that she’d had a bad reaction to a flu shot, Murtaugh persisted. “I told her that I would be worried if she contracted COVID,” Murtaugh says, and the clerk promised to think about it. Promising to think about something is often no more than a polite deflection. But the...