WASHINGTON (AP) — The U.S. now has a third vaccine to prevent COVID-19. The Food and Drug Administration on Saturday cleared a Johnson & Johnson shot that works with just one dose instead of two. Health experts have anxiously awaited a one-and-done option to help speed vaccinations. The virus has already killed more than 510,000 people in the U.S. and is mutating in increasingly worrisome ways. The FDA said J&J’s vaccine offers strong protection against what matters most: serious illness, hospitalizations and death. One dose was 85% protective against the most severe COVID-19 illness, in a massive study that spanned three continents.