More than 250 people have been enrolled in a trial in eastern Congo of an experimental pill that could stop them developing Ebola after exposure, in what experts describe as a critical step against the deadliest outbreak of the disease in the country's history.

The trial, known as EBO-PEP, is under way at a health centre in Rwampara, near Bunia, the main city of Ituri province and the epicentre of the outbreak. It is testing obeldesivir, an antiviral pill made by Gilead, given soon after suspected contact with the virus. This approach is called post-exposure prophylaxis, or PEP. The outbreak is caused by Bundibugyo virus, a rarer strain of Ebola for which there is no approved vaccine or treatment. The trial began in July and is one of several studies launched to find protection or at least slow the spread.

Coordinated by the French medical nonprofit ALIMA, with Congo's National Institute of Biomedical Research and the French institute ANRS Emerging Infectious Diseases, the study aims to recruit nearly 1,000 adults and children over 12 who had contact with a confirmed case in the previous five days and have no symptoms. Half will receive a placebo. Participants are monitored in person daily for 21 days, with a final phone call or visit after 42 days. Anyone who develops symptoms receives supportive treatment immediately, which has been shown to help when illness is caught early.

Participants include health workers and relatives of patients. One woman who joined in August after her husband tested positive said no one in her family has since fallen ill; another enrolled after an uncle died. Because Ebola spreads only once symptoms begin, people exposed must otherwise wait and see. ALIMA's research coordinator noted that PEP differs from a vaccine, which can take days or weeks to protect, and from a cure, since it targets people not yet sick. Investigators say most participants are glad to take part, though recruiting people who live far away is hard, and the study is sometimes disrupted when participants learn relatives have died.

The stakes are high. At least 8,300 confirmed cases and 4,000 deaths have been recorded since the outbreak was declared in mid-May, and it has spread to seven provinces. The World Health Organization says it is outpacing efforts to track it and is set to exceed West Africa's 2014–2016 epidemic, which killed 11,000 people. Officials cite late detection, with patients typically identified about seven days after symptoms begin, often mistaken for malaria. Shortages of ambulances and fuel, and the fact that nearly 6,000 of 31,000 contacts were not seen on a recent day, hamper the response. Congo has also begun vaccinating health workers with Ervebo, developed against the more common Ebola strain; whether it protects against Bundibugyo is still being studied.

International support is growing. French Foreign Minister Jean-Noël Barrot visited the Rwampara centre and Bunia's biomedical laboratory on 3 October, pledging to increase France's humanitarian funding, though he gave no figure.