The Health Ministry said vaccinations would start at the weekend, the first time the vaccine would come into use since it was developed two years ago.

The vaccine, developed by Merck and sent from Europe by the World Health Organization, is still not licensed but proved effective during limited trials in West Africa in the biggest ever outbreak of Ebola, which killed 11,300 people in Guinea, Liberia and Sierra Leone from 2014-2016.

Health officials hope they can use it to contain the latest outbreak in northwest Democratic Republic of Congo.

Peter Salama, WHO’s deputy director-general for emergency preparedness and response, said the current number of cases stood at 42, with 23 deaths attributed to the outbreak.

“Our current estimate is we need to vaccinate around 8,000 people, so we are sending 8,000 doses in two lots,” he told Reuters in Geneva.

“Over the next few days we will be reassessing the projected numbers of cases that we might have and then if we need to bring in more vaccine we will do so in a very short notice.”

Health workers have recorded confirmed, probable and suspected cases of Ebola in three health zones of Congo’s Equateur province, and have identified 432 people who may have had contact with the disease.

WHO spokesman Tarik Jasarevic said the supplies sent to Congo included more than 300 body bags for safe burials in affected communities. The vaccine will be reserved for people suspected of coming into contact with the disease, as well as health workers.

“In our experience, for each confirmed case of Ebola there are about 100-150 contacts and contacts of contacts eligible for vaccination,” Jasarevic said. “So it means this first shipment would be probably enough for around 25-26 rings - each around one confirmed case.”

The vaccine is complicated to use, requiring storage at a temperature between -60 and -80 degrees Celsius.

“It is extremely difficult to do that as you can imagine in a country with very poor infrastructures,” Salama said.

“The other issue is, we are now tracing more than 4,000 contacts of patients and they have spread out all over the region of northwest Congo, so they have to be followed up and the only way to reach them is motorcycles.”

The outbreak was first spotted in the Bikoro zone, which has 31 of the cases and 274 contacts. There have also been eight cases and 115 contacts in Iboko health zone.

The WHO is worried about the disease reaching the city of Mbandaka with a population of about 1 million people, which would make the outbreak far harder to tackle. Two brothers in Mbandaka who recently stayed in Bikoro for funerals are probable cases, with samples awaiting laboratory confirmation.

The WHO report said 1,500 sets of personal protective equipment and an emergency sanitary kit sufficient for 10,000 people for three months were being put in place.