Sierra Leone celebrated last month when it discharged the last remaining ebola patient from its treatment centres.
But since then a new spate of cases has erupted, leaving two dead and five people in treatment.
“Over 680 people in the village of Robureh are now under a 21-day quarantine,” Amadu Thullah, a spokesman for the local ebola response centre said.
Those locked down included her parents, close relatives and classmates.
“They are classified as high risk although they have not exhibited any signs or symptoms of the disease,” added health ministry spokesman Seray Turay.
“The surveillance team of the ebola response centre have intensified their investigations and is working to nip the issue in the bud.”
The worst outbreak of Ebola on record has killed more than 11,000 people in Sierra Leone, Guinea and neighbouring Liberia since it began in December 2013.
Liberia was declared ebola-free this month but growing evidence that the virus may survive longer than previously thought in sperm has raised fears of fresh outbreaks.
Hundreds Quarantined as Ebola Returns to Sierra Leone: 16 yr-old dies of apparent sexually transmitted infection, http://t.co/DIgMnM9b0Z— Liz Szabo (@LizSzabo) September 15, 2015
The teenage girl, Kadiatu Thullah, died on Sunday at the International Medical Corps ebola treatment unit
Emmanuel Conteh, head of the ebola response centre for the district of Bombali in northern Sierra Leone, said that some 690 people in the village of Robuya where Kadiatu lived would be isolated for three weeks.
“Seven of her primary contacts have been taken to the ebola treatment unit,” he told Reuters.
Three patients who came into contact with the girl at another health facility have also been taken to the treatment unit.
Health workers were investigating how the teenager got infected, since she had not travelled outside the village in years.
Initial suspicions are that she had sex with an ebola survivor.
“We are baffled by that possibility because the survivor in question was discharged in March, way beyond the 90-day period within which sexual transmission is said to be possible,” Conteh said.
The head of Sierra Leone’s ebola response, Pallo Conteh, has warned of a possible new surge of the virus after a woman died in the Kambia district, on the border with Guinea.