Johnson & Johnson asked U.S. regulators Thursday to clear the world’s first single-dose COVID-19 vaccine, an easier-to-use option that could boost scarce supplies. J&J’s vaccine was safe and offered strong protection against moderate to severe COVID-19, according to preliminary results from a massive international study. It didn’t appear quite as strong as two-dose competitors made by Pfizer and Moderna -- a finding that may be more perception than reality, given differences in how each was tested.