KINSHASA, Sept. 24 -- For the health worker who received the injection on Saturday in Bunia, in the fractured northeastern corner of the Democratic Republic of Congo, the substance entering the arm carries a question medicine cannot yet answer: Will it protect against the virus spreading across six provinces and killing roughly one in every two people it infects?

The vaccine is Ervebo, engineered to fight a different strain of Ebola. The outbreak consuming northeastern Congo is caused by Bundibugyo virus, a related pathogen, but one for which no licensed vaccine or approved treatment exists. The World Health Organization, backed by Doctors Without Borders, began administering 50,000 doses to frontline health workers on September 19 under...