Johnson & Johnson has shared results from an 8 country trial of its COVID-19 vaccine candidate, showing that a single dose is 66% effective at preventing moderate to severe illness and 85% effective at guarding against more severe disease. One important caveat from the trial is a variance by country – the vaccine was 72% effective in the United States, but only 57% effective in South Africa where a more contagious local strain of the disease is dominant.