Samples taken from the body of a 22-year-old woman who died this week in the northern part of the country tested positive for the virus, local media reported.
The U.N. health agency had warned that more outbreaks were likely in the three countries that have been hardest hit by the two-year epidemic: Liberia, Sierra Leone and Guinea.
The WHO declares a country Ebola-free only after it has gone 42 days without a case, twice the incubation period for the disease.
But research has shown that the virus can persist in survivors for months after recovery, leading some scientists to question the usefulness of the 42-day target.
Sierra Leone’s neighbor, Liberia, experienced two flare-ups after being given the all-clear in May. It was again declared Ebola-free Thursday.
Sierra Leone first achieved the milestone Nov. 7. The new case emerged during a 90-day period of heightened surveillance, and the government responded quickly, the WHO said.
Local authorities and their international partners were investigating the origin of the case, identifying those who came into contact with the woman and taking measures to prevent further spread of the disease.
The epidemic that began in Guinea in December 2013 has infected more than 28,500 people and killed 11,300, according to the latest figures from the WHO.
“We are now at a critical period in the Ebola epidemic as we move from managing cases and patients to managing the residual risk of new infections,” Dr. Bruce Aylward, the WHO’s special representative for the Ebola response, said Thursday. “We still anticipate more flare-ups and must be prepared for them.”
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