A pivotal trial of vaccines against the strain of Ebola that is driving a deadly outbreak across parts of Africa has begun in the Democratic Republic of Congo, where the epidemic originated and the vast majority of cases and deaths have occurred. The first participant was vaccinated on Friday in Bunia, eastern Congo, with Merck & Co.’s Ervebo shot, which has been approved to guard against a different Ebola species. An experimental alternative developed specifically against Bundibugyo by Moderna...