Health workers and other frontline workers in the Ebola response are being prioritised 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.
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.
The rollout is targeting the Bunia and Mongbwalu zones in Ituri province, health authorities said on 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, Major General Gaby Kasongo Mulumba, said.
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 7541 confirmed cases, including 3639 deaths, according to government figures, while 1823 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, fuelled by insecurity, displacement, a health workers' strike and intense population movements.
The World Health Organisation 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.
The WHO said last month that 70,000 doses of Ervebo were approved for use in Congo.
About 50,000 doses are intended for frontline 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.
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