Congo’s Ebola outbreak shows signs of slowing at the epicentre of the flare-up, but is widening as the infection spreads further afield, modelling suggests.

The daily tally of new cases in Democratic Republic of Congo (DRC) was falling at the end of August, but more cases were being found outside Ituri province, where the infection burst out.

Latest official figures say the outbreak has now killed more than 3,000 people out of over 6,100 confirmed cases, with the infection spreading faster than any previous epidemic of the viral haemorrhagic fever.

Modelling by Airfinity, a health analytics firm, suggests the true toll of cases could be more than four times higher than reported.

The outbreak remains on track to eclipse the 2014 West Africa outbreak later this month, to become the largest ever recorded.

Airfinity said that data from late August showed the seven-day averages of reported cases and reported deaths had fallen by 16 per cent and 18 per cent respectively.

The firm said it was “too early to confirm the decline will hold, and it may mask subnational variation: The outbreak has fluctuated before and a week of falling case and death counts does not rule out another surge.

“Even if the decline continues at this current pace, low case levels remain weeks away.”

The growth in cases had started to ease in Ituri, but had only plateaued in North Kivu province and was accelerating in Haut-Uélé.

Overall, the “outbreak’s footprint keeps growing, threatening [to] spread along the Congo river, even as weekly activity seems to ease”.

The firm’s forecasts give a 50 per cent chance of the outbreak hitting 50,000 cases – double the size of the 2014 West Africa outbreak.

The US Centres for Disease Control said on Tuesday that efforts to contain the ebolavirus “remained below established response targets” and the fast expansion of the disease across many health zones indicated “uncontrolled expansion of the outbreak.”

Epidemiologists are particularly worried about the proportion of deaths among people who are not in hospital and who have not already been traced as vulnerable contacts to other cases.

The number of such cases and deaths said to be “occurring outside known transmission chains” shows that the relief effort has yet to trace and get a grip of how the virus is spreading.

“Until every chain is found and broken, the epidemic will continue and will continue to pose a threat to DRC, its neighbours and the region as a whole,” the WHO’s director-general, Tedros Adhanom Ghebreyesus, said this week in Geneva.

Health officials have meanwhile begun using anonymised mobile phone data to map population movements in disease hotspots, giving an indication of where the infection may spread next.

Population movement has been an important driver of the spread so far. Ituri’s mining industry and its busy towns have been blamed for fanning the outbreak.

Officials have also begun checking traffic along the major Congo river transport corridor after cases reached the river port of Kisangani.

“Population movements have always been a factor in outbreaks, but they are particularly important in the current epidemic,” Olivier Le Polain, head of epidemiology and analytics for response at the WHO health emergencies programme, told Reuters.

The outbreak is caused by the rare Bundibugyo strain of the virus, rather than the more common Zaire strain, meaning there are no approved vaccines to use to slow the spread.

Trials for three potential vaccines should begin by October or November, Dr Tedros said.

“Two new vaccines against Bundibugyo virus are now in safety trials in the UK and Canada,” he said.

One is being developed by the US-based Moderna group using mRNA technology and another by the University of Oxford.

The WHO has also recommended a full-scale human trial of Ervebo, a vaccine approved for use against the Zaire strain, to see if it offers any cross-protection against Bundibugyo.