GENEVA, Switzerland (AFP)—WHO vaccine experts on Friday recommended a full-scale human trial of the only existing approved Ebola vaccine to see if it offers cross-protection against a deadly strain spreading in the DR Congo.

The Ervebo vaccine has proved safe and efficacious against the more common Zaire strain of Ebola virus, and World Health Organization experts said early data, notably from animal trials, showed it may also give some protection against the rare Bundibugyo strain exploding in the DRC.

The recommendation came as the number of confirmed cases in the Democratic Republic of Congo went past the 4,000 mark, now standing at 4,053, including 1,850 fatalities.

WHO chief Tedros Adhanom Ghebreyesus warned again Friday that the virus was spreading faster than response efforts can catch up.

The UN health agency’s technical advisory group on candidate vaccine prioritisation (TAG-CVP) for the Bundibugyo outbreak response met last week to review data that has emerged since their meeting in May, in the days following the outbreak declaration.

“The members recommended Ervebo, the only licensed Ebola vaccine, be prioritised for inclusion in a randomised clinical trial in the context of the ongoing outbreak,” a WHO statement said.

A spokesman told AFP it would be “a ring trial in the DRC”, involving the circle of contacts around known cases.

The experts looked at several studies, including one which showed that three out of four non-human primates vaccinated with Ervebo survived Bundibugyo, compared with one out of four control animals.

They also looked at an unpublished study on ferrets which found that the research-grade version of Ervebo provided 100-percent protection against Bundibugyo, whereas all control animals died within 10 days.

Tedros called the findings “good news”.

– Managing expectations –

The TAG-CVP experts unanimously agreed that there were no safety concerns about using Ervebo in a clinical research study vaccinating case contacts, noting that it is already licensed by regulatory agencies and has been given to hundreds of thousands of people in Africa.

“Expectations need to be managed about its likely protective effect if used in a Phase III ring vaccination trial as efficacy against symptomatic disease and transmission may not be high, whereas protection against a fatal outcome may be achieved,” their report said.

Final-stage Phase III trials of potential drugs of vaccines typically involve hundreds to thousands of people.

Nonetheless, a specific Bundibugyo vaccine remains the preferred option for fighting the current outbreak, they stressed.

Two potential Bundibugyo vaccines are in the early stages of clinical trials on humans, while a third is being worked on to bring it to that stage.

Ebola is spread by contact with bodily fluids and causes a haemorrhagic fever.

Around 87 percent of cases in the DRC outbreak are in the northeastern Ituri province, with 11 percent in neighbouring North Kivu.

– Stockpile of doses –

The Gavi global vaccine alliance maintains a stockpile of 500,000 Ervebo doses.

It said doses from the stockpile were to be made available for the trials, with some already in the DRC.

“This is already the largest Ebola outbreak in DRC’s history, and could well become the largest outbreak ever,” said Gavi chief executive Sania Nishtar.

“Given the terrible toll on communities and frontline workers, we are encouraged that the approved Ebola vaccine Ervebo… could help reduce severe illness and deaths.”

Gavi said that around 55,000 frontline workers in Ituri and North Kivu have received Ervebo through previous preventive vaccination campaigns.