Africa’s first participation in a COVID-19 vaccine trial started Wednesday as volunteers received injections developed at the University of Oxford in Britain, while officials said the continent of 1.3 billion people cannot be left behind. The large-scale trial is being conducted in South Africa, Britain and Brazil. South Africa has nearly one-third of Africa’s confirmed cases with more than 106,000, including more than 2,100 deaths. The African continent now has nearly 325,000 cases as countries loosen restrictions under economic pressure from citizens who say they have to feed their families. Shortages of testing materials and medical supplies remain a problem as Africa could become the world’s next hot spot.