The latest Ebola outbreak in the Democratic Republic of Congo will again turn to Merck & Co’s experimental rVSV-ZEBOV that was so successful in attacking the last outbreak. But this time around, the challenge of fighting the disease lies in the fact that the latest outbreak is in a war zone.

An Ebola outbreak in Congo was declared on Aug. 1 when samples collected from hospitalized patients tested positive for the flavivirus, just days after health officials declared the end of the previous one. As of Aug. 3, a total of 43 cases—13 confirmed and 30 probable—have been reported, including 33 deaths.

A local senior health official told Reuters that vaccination using the Merck shot is expected to begin on Wednesday. The WHO has said the exact virus strain causing the current outbreak has not been confirmed, but "it’s extremely likely that it’s Ebola virus Zaire strain," Peter Salama, the World Health Organization’s deputy director-general for emergencies told reporters at a Friday briefing.

Deployment of rVSV-ZEBOV, which targets the same type of virus, faces many difficulties, partly because it requires cold chain storage and transportation, which can be difficult in resource-constrained regions. And this time around, security is a new challenge.

"Here we are responding to an outbreak of a high-threat pathogen with one of the highest mortality rates of any known diseases, but in the context of a war zone," said Salama. "We are at the top of the degree of difficulty scale."

Tarik Jašarevi?, a WHO spokesperson, told FierceVaccines on Monday that final sequencing result is expected in the coming days. If it is indeed the Zaire strain, then Merck’s rVSV-ZEBOV could come into play. The vaccine is deemed highly effective and safe but has not yet been approved. WHO’s Strategic Advisory Group of Experts on Immunization (SAGE) has recommended its use under a compassionate use pathway during Ebola outbreaks of the Zaire strain.

"We are in the process of assembling the critical components required for licensure with regulatory agencies and are looking into options to accelerate the registration process," Merck spokeswoman Pamela Eisele told FierceVaccines. Merck now expects filing with the FDA and the EMA in 2019, said Eisele. 

In a previous ring study in Guinea designed to stop the virus from spreading by vaccinating people close to Ebola cases, no one who got the Merck shot immediately after exposure to the virus was infected. The vaccine was used in the same manner on about 3,500 people in Congo’s previous outbreak, which lasted from early May to late July.

Tarik Jašarevi?, a WHO spokesperson, told FierceVaccines on Monday that final sequencing result is expected in the coming days. If it is indeed the Zaire strain, then Merck’s rVSV-ZEBOV could come into play. The vaccine is deemed highly effective and safe but has not yet been approved. WHO’s Strategic Advisory Group of Experts on Immunization (SAGE) has recommended its use under a compassionate use pathway during Ebola outbreaks of the Zaire strain.

"We are in the process of assembling the critical components required for licensure with regulatory agencies and are looking into options to accelerate the registration process," Merck spokeswoman Pamela Eisele told FierceVaccines. Merck now expects filing with the FDA and the EMA in 2019, said Eisele. 

In a previous ring study in Guinea designed to stop the virus from spreading by vaccinating people close to Ebola cases, no one who got the Merck shot immediately after exposure to the virus was infected. The vaccine was used in the same manner on about 3,500 people in Congo’s previous outbreak, which lasted from early May to late July.

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