Scientists at Oxford University asserted their experimental coronavirus vaccine has been shown in an early trial to prompt a protective immune response in hundreds of people who got the shot. British researchers first began testing the vaccine in April in about 1,000 people, half of whom got the experimental vaccine. Such early trials are usually designed only to evaluate safety, but in this case experts were also looking to see what kind of immune response was provoked.In research published Monday in the journal Lancet, scientists said that they found their experimental Covid-19 vaccine produced a dual immune response in people aged 18 to 55.