"No confirmed cases of Ebola virus disease were reported in the week to 4 October," WHO said in its latest situation report on the west African Ebola outbreak.
"This is the first time that a complete epidemiological week has elapsed with zero confirmed cases since March 2014," it said.
The deadliest-ever Ebola outbreak since the virus was identified in central Africa in 1976 has killed 11,312 of the 28,457 people infected since December of 2013, according to the latest WHO figures.
Liberia was declared free of Ebola transmission in early September, and Sierra Leone in late September began a 42-day countdown to becoming Ebola free.
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WHO said Wednesday that the epidemic had clearly entered a third phase and that the focus was on driving "case incidence to zero, and ensure a sustained end to (Ebola) transmission."
The UN health agency said that all contacts had been completely followed up in Sierra Leone, which has seen no new cases for the past three weeks.
But the danger is still not over.