Johnson & Johnson said Friday that its single-dose Covid-19 vaccine reduced rates of moderate and severe disease, but the shot appeared less effective in South Africa, where a new coronavirus variant has become common. Overall, the vaccine was 66% effective at preventing moderate to severe disease 28 days after vaccination. But efficacy differed depending on geography. The shot was 72% effective among clinical trial volunteers in the U.S, but 66% among those in Latin America, and just 57% among those in South Africa. Though markedly below the levels seen with the first two authorized Covid-19 vaccines, those rates are above the thresholds originally set by the FDA for a vaccine to be considered useful. The vaccine reduced severe disease alone by 85%, and prevented Covid-related hospitalization or death, Johnson & Johnson said.