WASHINGTON, D.C.: The United States is getting a third vaccine to prevent Covid-19, as the Food and Drug Administration on Saturday cleared a Johnson & Johnson shot that works with just one dose instead of two. Health experts are anxiously awaiting a one-and-done option to help speed vaccinations, as they race against a virus that already has killed more than 510,000 people in the US 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 percent protective against the most severe Covid-19 illness, in a massive study that spanned three continents — protection that remained strong even in countries such as South Africa, where the variants of most concern are spreading. “This is really good news,” Dr. Francis Collins, director of the National Institutes of Health, told The Associated Press Saturday. “The most important thing we can do right ... » Learn More about Johnson & Johnson 1-dose shot cleared, giving US 3rd Covid-19 vaccine