Johnson & Johnson’s single dose of Covid-19 vaccine works safely, the US Food and Drug Administration said, paving the way for it to authorize a third vaccine in the US as early as this weekend.Vaccine was 66.1% effective in preventing moderate to severe disease and appeared safe, the FDA said on Wednesday, and the shot also showed signs of slowing the spread of the virus.J&J’s shot would be the third cleared for use in the US, as health authorities pick up the pace of vaccinations and try to lock in gains reducing daily cases, hospitalizations and deaths while remaining ahead of any new variants that threaten to evade treatment.Health authorities have been enthusiastic over adding a new coronavirus vaccine to their arsenal. J&J’s shot wasn’t quite as effective in its late-stage trial as the pair already in use, but it worked safely by a number of measures.