DR Congo’s worst ever Ebola outbreak has passed 9,000 cases and more than 4,000 deaths, health authorities said on Tuesday.

The outbreak was declared in mid-May but experts believe the virus emerged weeks or even months earlier.

The country’s 17th Ebola outbreak started in Ituri province in northeastern DRC.

It’s since spread to seven provinces in the north and east, where limited healthcare resources and the presence of armed groups have complicated the response.

Medical workers and humanitarian staff also face distrust from local communities.

The current outbreak is caused by the Bundibugyo strain of the virus, for which there’s no approved vaccine or treatment.

A clinical trial for a vaccine is scheduled to launch in the DRC this month, the World Health Organization said on Tuesday.

For now, frontline volunteers have received doses of the Ervebo vaccine, which is effective against the Zaire strain of the virus.