Sierra Leone is to vaccinate around 200 people who came into direct or indirect contact with a woman who died of Ebola on Saturday, a spokeswoman for the U.N. World Health Organization said on Wednesday.

The death of the woman, a trader from Kambia District near the border with Guinea, sets back efforts to end an 18-month epidemic that has infected more than 28,000 people in Guinea, Sierra Leone and Liberia and killed more than a third of them.

The 67-year-old woman died five days after Sierra Leone started a 42-day countdown to being declared free of Ebola. The previous new case of the disease was reported on Aug 8.

"We will vaccinate those in the (Tonko Limba) chiefdom who came into direct contact with the deceased and those contacts they also came into close contact with," said WHO spokeswoman Margaret Harris.

"We have to ensure that everybody is fully informed and consents," she said, adding that voluntary vaccinations with a drug that arrived from Guinea and has proved effective there would start on Thursday or Friday.

The fresh case is discouraging and a reminder of the difficulty of battling Ebola, said Pallo Conteh, head of the National Ebola Response Centre in Sierra Leone, adding that more cases may be recorded as a result of this victim.

He appealed to a woman he named as the niece of the victim to come forward, saying she was at high risk.

The outbreak has ebbed only to flare back up since it was first declared in March 2014. Liberia was declared Ebola-free in May, but a fresh cluster of cases appeared nearly two months later. Liberia's last case was discharged on July 23.

Scientists say sexual transmission is the most likely explanation for the resurgence in Liberia since the virus can live on in semen beyond the usual 21-day incubation period.