The World Health Organization confirmed this weekend 13 Ebola cases and noted an additional 30 likely cases, including 33 deaths, in eastern DRC near the border with Uganda.

Just last month, the DRC declared victory against an Ebola outbreak elsewhere in the country that began this spring, in which it used the experimental Merck vaccine rVSV-ZEBOV. The vaccine is one of several developed in response to the 2014-2016 Ebola epidemic that spanned several West African countries and killed over 11,000 people.

But scientists still lack enough evidence to give it a ringing endorsement, because it would be unethical to conduct the usual sort of clinical trials with a control group necessary to approve most drugs. The Ebola virus has a fatality rate of around 50%.

DRC authorities still have on hand thousands of doses of the Merck vaccine, Reuters reports.

“Since we are coming out of another Ebola outbreak, we have kept staff and equipment in place,” said Dr Matshidiso Moeti, WHO Regional Director for Africa in a WHO statement: “This allows us to have a head start in response to this cluster.”

Health workers will need to ensure that the vaccines remain refrigerated throughout the supply chain: a tall order in under-developed regions. It will be even harder to reach the target population this time, because this outbreak took place in a region where active civil conflicts have displaced over 1 million civilians.