LONDON (AP) — The World Health Organization says there were no Ebola cases reported last week — the first time an entire week has passed without any new confirmed patients since the devastating outbreak began last March.

The U.N. health agency said in a report issued Wednesday that all contacts of Ebola cases in Sierra Leone have now been followed for 21 days without falling sick, suggesting the country might soon be free of the disease.

Still, more than 500 people are being tracked in Guinea and WHO said there is "considerable risk" of further spread. Scientists have also lost track of where the virus was recently spreading.

Since it first broke out in Guinea's forest region last year, Ebola has killed more than 11,200 people in West Africa.