Johnson & Johnson has kicked off the final trial of a single-shot Covid-19 vaccine. The company said that the vaccine will be tested on 60,000 people and it expects results of the Phase III trials by year-end or early next year. If successful, the drug would simplify distribution of millions of doses compared with leading rivals using two doses. Rival vaccines from Moderna, Pfizer and AstraZeneca all require two shots separated by several weeks, which make them much more difficult to administer. The company plans to manufacture as many as 1 billion doses in 2021