Johnson & Johnson on Friday said its single-dose vaccine was 72% effective in preventing COVID-19 in the United States but a lower rate of 66% was observed globally in the large trial conducted across three continents and against multiple variants.In the trial of nearly 44,000 volunteers, the level of protection against moderate and severe COVID-19 was 66% in Latin America and just 57% in South Africa, where a particularly worrying variant of the novel coronavirus is circulating.Those results compare to the high bar set by two authorized vaccines from Pfizer Inc/BioNTech SE and Moderna Inc, which were around 95% effective in preventing symptomatic illness in pivotal trials when given in two doses.