An experimental vaccine against the coronavirus showed encouraging results in very early testing, triggering hoped-for immune responses in eight healthy, middle-aged volunteers. Study volunteers given either a low or medium dose of the vaccine by Cambridge, Massachusetts-based Moderna Inc. had antibodies similar to those seen in people who have recovered from COVID-19. In all, 45 people have received one or two shots of the vaccine, which was being tested at three different doses. The kind of detailed antibody results needed to assess responses are only available on eight volunteers so far. The vaccine seems safe, the company said, but much more extensive testing is needed to see if it remains so. A high dose version is being dropped after spurring some short-term side effects.