The Ebola outbreak in the Democratic Republic of the Congo appears to be spreading southward from its current epicenter at Beni, raising concerns it will take root in some larger population centers, including a major regional hub, warned a senior World Health Organization official.

Should the outbreak spread more widely in cities like Butembo — where there have already been a number of cases — or Goma, a major center further south, the scale of the outbreak could tax the available supplies of an experimental vaccine being used to help contain spread, said Dr. Peter Salama, who heads the WHO’s emergencies program.

As of Sunday more than 42,000 doses of the vaccine had been used in this outbreak and one earlier in the year in the western part of the DRC. Merck, which is developing the vaccine, has committed itself to maintaining a stockpile of 300,000 doses.

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“We are extremely concerned about the size of the vaccine stockpile,” Salama told STAT in an interview. “And we’re regularly in touch with Merck to discuss what can be done about that.”

This outbreak, centered in the province of North Kivu in northeastern DRC, is now the second largest in known history, with 440 cases and 255 deaths as of Sunday. That is massive in traditional Ebola outbreak terms, but still pales in comparison to the West African outbreak of 2014-2016, where more than 28,000 people were infected and more than 11,000 died.

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“We are extremely concerned about the size of the vaccine stockpile.” Dr. Peter Salama, WHO

The vaccine is being used in a ring vaccination approach. People who have been in close contact with cases — and the contacts of those contacts — are offered vaccine. The idea is to hem in the virus and prevent onward transmission.

Frontline workers such as healthcare providers, ambulance drivers, and people who conduct safe burials of the Ebola dead are also offered the chance to be vaccinated. On a few occasions when vaccination teams knew they would have limited access to a village or neighborhood because of the threat of violence, they offered vaccine to all comers.

There have been calls from some outside observers to use that geographic approach more broadly to try to extinguish this outbreak, which likely began in late April and is expected to continue well into next year. Salama has resisted these calls.

“When you consider that the global … stockpile is 300,000 doses and Beni itself has a population of 400,000 and Butembo a million, you can imagine that when people are proposing large scale geographical vaccination that becomes extremely impractical,” he said.

Merck has told STAT it takes about a year, start to finish, to produce a batch of the vaccine, which it has provisionally called V920. Some of the vaccine in the stockpile is already in vials, but much of it is stored in bulk. Converting the bulk product into vaccine in vials can take between four and five months, a Merck spokeswoman explained.

Salama suggested the size of the Ebola vaccine stockpile will need to be reconsidered in future, saying he doesn’t think 300,000 doses is sufficient. “I don’t believe that’s adequate for urban outbreaks. And … I think urban outbreaks are increasingly going to be the norm, including urban outbreaks in conflict-affected areas,” he said.

Salama expects this epidemic — occurring in a densely populated region with massive political instability — will take at least another six months to contain. “And that’s if security holds,” he said.

There have been sporadic rebel attacks on Beni, where the outbreak response efforts are headquartered. Recently a number of response team staff were evacuated to Goma for some counseling and rest after their hotel was caught in the crossfire during an attack on the nearby U.N. peacekeepers’s encampment. They have all since returned to Beni, Salama said.

Transmission in Beni appears to be subsiding, he said, with containment efforts making inroads. While that is good news, the southward spread of cases is dampening moods.

“None of us would ever like to see a scenario where we have an outbreak in Goma,” said Salama. “It’s a very big city and it’s also a crossroads in Eastern Africa in terms of transportation. Air. Road. Truck. Everything.”