But a forthcoming study in the New England Journal of Medicine, based on roughly 200 survivors, found that around half still had traces of the virus in their semen after six months, a clinician familiar with the study told Reuters.

“The old advice of three months is no longer good,” the clinician said. “The number of people with persistent virus in their semen is much greater than expected.”

The clinician, who was not authorized to speak about the study, added that the risk might not only be from sex but also from masturbation.

“It's not the sex that is dangerous, it's the semen that is dangerous,” said Aylward, who mentioned the study during a news conference but did not give details. “How people actually get exposed, in soiled linens or whatever, is not clear.”

Transmission through semen may explain why a few cases continue to occur even though the outbreak has been almost completely eradicated by an intense international effort, recently bolstered by the deployment of a trial vaccine in Guinea and Sierra Leone.

“This virus and this outbreak in particular has a nasty sting in the tail,” Aylward said. “It's not finished, by a long shot.”

The latest flare-up, in a village on the northern border of Sierra Leone, followed the death of a 67-year-old woman late last month, 50 days after the previous confirmed case in the region. Transmission chains are considered to have been broken after 42 days with no new infections.

However, Aylward said that sexual transmission was “obviously not a huge risk, because if it were we would have seen a lot more in the areas that were hardest hit at the beginning of this outbreak.”

Reuters