Johnson & Johnson has asked U.S. regulators to clear the world’s first single-dose COVID-19 vaccine, an easier-to-use option that could boost scarce supplies. Preliminary results from a massive study showed J&J’s vaccine was safe and offered strong protection against moderate to severe COVID-19. It didn’t appear quite as strong as two-dose competitors made by Pfizer and Moderna. Johnson & Johnson filed an application with the Food and Drug Administration on Thursday. An FDA panel will meet later this month and pore over the results. The company also is studying a two-dose version of its vaccine, but results won’t be available for several more months.