An Ebola vaccine trial has begun in the city of Bunia in the Democratic Republic of Congo, with healthcare workers among those being prioritized for vaccination.

The campaign is taking place in Ituri and North Kivu provinces, the areas worst affected by the outbreak, and is expected to reach about 20,000 people over the next six to nine months.

Health officials say the vaccine being tested was effective against an earlier Ebola variant, but its effectiveness against the current outbreak remains uncertain because the outbreak is caused by the Bundibugyo virus.

The World Health Organisation says the outbreak is the worst Ebola outbreak ever recorded in the Democratic Republic of Congo.

The trial, known as the BRAVO study, was launched on September 19 in Bunia, the capital of Ituri Province, which is now considered the epicentre of the outbreak.

It is being conducted by Medecins Sans Frontieres (MSF) and its research arm Epicentre, in collaboration with Congolese health authorities, the Africa Centres for Disease Control and Prevention (Africa CDC) and other scientific partners.

The study will target about 20,000 frontline workers in Ituri and North Kivu who are particularly exposed and deeply involved in the response.

Military Governor of Ituri, Major General Gaby Kasongo Mulumba, said launching the vaccination for frontline staff is not an arbitrary choice because of their high risk.

The vaccine in use is Ervebo, which was developed and licensed against the Zaire ebolavirus rather than the Bundibugyo virus driving the current outbreak.

The World Health Organisation says there is currently no licensed vaccine or specific treatment for Bundibugyo virus disease.

WHO said early laboratory and animal evidence suggested Ervebo might provide some protection against Bundibugyo, but whether it protects humans has not been established.

The BRAVO study is expected to run for nine to 12 months, including three months of vaccination and at least six months of participant follow-up.

Researchers will follow participants over time to assess the potential effect of Ervebo against the Bundibugyo virus.

A separate Phase 3 trial based on a randomized ring-vaccination strategy is also planned to more rigorously assess Ervebo’s effectiveness.

The trial comes amid an expanding outbreak. The International Coordinating Group on Vaccine Provision approved an initial allocation of 70,000 doses of Ervebo to the DRC in August.

Of that total, 50,000 doses were allocated for health and frontline workers, while 20,000 were set aside for the clinical trial.

According to the Africa CDC, 16,520 doses had been received in the DRC as of mid-September, and vaccination under a protocol-governed programme began on August 27.

As of September 17, a total of 3,771 people had been vaccinated, including 3,063 in Tshopo Province and 708 in Bas-Uele Province, according to the DRC government.

As of September 20, the country has reported 7,541 confirmed cases, including 3,639 deaths, with 1,823 recoveries, making it the largest Ebola outbreak ever in the DRC and the second-largest globally after the 2014 to 2016 West Africa epidemic.