The first clinical trial of an Ebola vaccine is expected to begin in the Democratic Republic of Congo by the end of October, World Health Organization experts said Tuesday.

The DR Congo's worst-ever Ebola outbreak has passed the 9,000-case mark, with the number of deaths exceeding 4,000.

Among the many challenges facing the response is the lack of any approved vaccines or treatments for the Bundibugyo species of Ebola behind the outbreak.

Ervebo, a vaccine approved for use against the more common Zaire strain of Ebola, will go through a full-scale human trial in the DRC to see if it offers any cross-protection against Bundibugyo.

‘Strong reassurances’

"We have got strong reassurances that... it should be starting before the end of this month," said Anthony Scott, chair of the WHO's Strategic Advisory Group of Experts on Immunisation (SAGE).

The group said insights that Ervebo might offer cross-protection against Bundibugyo were no match for evidence from standardised vaccine trials.

The so-called Tumaini trial in the DRC -- a single trial framework that can test the efficiency of multiple vaccines -- should also eventually incorporate two Bundibugyo-specific candidate vaccines, which are moving through earlier-stage, smaller-scale clinical trials.

One is a ChAdOx1 adenoviral vector jab being developed by the University of Oxford; the other, by the US-based Moderna group, uses mRNA technology.