Ebola experts are in Sierra Leone's Kambia district investigating a case that emerged less than a week after the country's last known patient was discharged from a hospital, a World Health Organization spokeswoman said Monday.

Once the source of transmission is found and contacts are traced, a vaccination trial will also begin in the northern Sierra Leone area, WHO spokeswoman Dr. Margaret Harris said.

"It's a step back and a disappointment, but it wasn't a surprise as it's near the border with Guinea," where cases remain, said Harris, adding that further transmission can be stopped.

Samples from a 67-year-old woman's corpse tested positive for Ebola, WHO technical coordinator Margarette Lamunu said. The woman, who died and was safely buried Aug. 29, was treated at home in Kafta village, so more Ebola cases are expected, Lamunu said.

Liberia had a similar situation. A sample from a corpse tested positive in late June after the country had been declared Ebola-free in May. It was quickly contained and Liberia may again declare itself free from Ebola transmissions this week, Harris said.

Authorities decided weeks ago to extend a vaccine trial from Guinea to Sierra Leone, Harris said.

The trial for the VSV-EBOV - developed by Canada and licensed to Merck - is done where there is Ebola, she said. Experts target contacts of the infected, and contacts of those contacts to create a buffer zone around a case to prevent its spread. Now that there is a case, and once contacts are traced, they will seek participants and the trial will begin in the area, Harris said.

The worst Ebola outbreak in history has killed nearly 4,000 people in Sierra Leone and more than 11,300 overall.