BUNIA, Congo (AP) — Congo on Saturday began vaccinating healthcare staff against Ebola in Bunia, the epicenter of the fastest-growing outbreak of the disease in history as they are “particularly exposed and deeply involved in the response,” the provincial governor said.

Health workers and other front-line workers in the Ebola response are being prioritized to receive the Ervebo vaccine that was effective in past Ebola outbreaks caused by a different, more common type of virus. Clinical trials are ongoing to find a licensed vaccine for the current outbreak, caused by the Bundibugyo virus, which has no licensed vaccine or treatment.

The rollout is targeting the Bunia and Mongbwalu zones in Ituri province, health authorities said Saturday. Both areas are among the hardest-hit by the virus.

“Our collective duty is to protect life. Launching this vaccination drive for front-line staff in Ituri is not an arbitrary choice, as these workers are particularly exposed and deeply involved in the response,” the military governor of Ituri, Maj. Gen. Gaby Kasongo Mulumba, said Saturday.

The campaign in Ituri and North Kivu is expected to cover 20,000 front-line workers over a period of six to nine months.

As of Saturday, the country had recorded 7,541 confirmed cases, including 3,639 deaths, according to government figures, while 1,823 people have recovered. The outbreak is concentrated in Ituri province and is known to be the deadliest of all past 17 Ebola outbreaks in the country.

The outbreak is spreading under extremely difficult conditions, fueled by insecurity, displacement, a health workers’ strike and intense population movements. The World Health Organization has said it remains out of control and is on track to surpass the 2014-2016 West Africa Ebola outbreak, the deadliest on record, which killed more than 11,000 people, primarily in Guinea, Liberia and Sierra Leone.

Dr. Jeannot Elua, whom The Associated Press met at a health facility in Bunia, said the vaccine was “beneficial.”

“We healthcare workers don’t really have a choice: we are going to receive it,” he said. “Some people here had already received the previous vaccine. I received it myself, because it was mandatory.”

Another front-line health worker, Dr. Jean Paul Uzele, who was vaccinated with Ervebo during the 10th Ebola outbreak, said it was unclear to him whether he should get vaccinated again.

“If it arrives, we will receive it too," he said. “But the question is whether we need to be vaccinated a second time."

Last month, the Ervebo vaccine was rolled out in Kisangani in the Tshopo province. The campaign was expected to cover 14 health zones in Tshopo, Bas-Uele and Haut-Uele provinces.

The use of Ervebo is taking place in a specific scientific context. The vaccine is licensed for use against Ebola virus disease caused by the Zaire strain — a different, more common type of Ebola virus.

The World Health Organization said last month that 70,000 doses of Ervebo were approved for use in Congo.

About 50,000 doses are intended for front-line and health workers, while 20,000 doses will be used in a clinical trial to study whether the vaccine protects against Bundibugyo virus, which is causing the current outbreak and has no licensed vaccine or treatment.

The vaccine is being used under a compassionate-use program, which allows a medical product to be used in a serious disease situation even though it has not been specifically approved for that particular use.

Health authorities said Ervebo could offer some protection against the Bundibugyo virus because the two viruses are related. But whether the vaccine can actually prevent illness in people infected with Bundibugyo Ebola is still being studied.