WHO representative Dr. Anders Nordstrom said Wednesday that while about 70 people are in quarantine in northern districts, more than 48 contacts remain missing from the northern Kambia district.
Nordstrom called for strengthened surveillance efforts in communities and hospitals.
A young woman's body tested positive for Ebola after she died and was given a traditional burial. Her aunt later tested positive for Ebola.
Nearly 4,000 people died before Sierra Leone was first declared free from transmission November 7. Ebola has killed more than 11,300 people, mostly in Sierra Leone, Liberia and Guinea, since late 2013.