AstraZeneca Plc found that 16 participants who received a placebo in its clinical trial contracted severe Covid-19, a sign that the shot could block the worst cases of disease.The British drugmaker and its partner, the University of Oxford, said earlier Monday that none of the trial participants who received the vaccine had become severely ill, and that none of the patients in that group were hospitalized.“That’s very important," said Moncef Slaoui, chief scientific adviser to Operation Warp Speed, in an interview with Bloomberg News. “It’s exciting."Slaoui revealed the placebo-arm data after Astra and Oxford said their vaccine prevented 70% of people from getting Covid-19 in large trials in the UK and Brazil.