GENEVA -- The World Health Organization (WHO) on Thursday declared Sierra Leone free of a recent flare-up of the Ebola virus, marking the second time western Africa rids itself of Ebola.

As of Thursday, two incubation cycles of the virus, 42 days, have passed since the last person confirmed to have Ebola virus disease in the country was tested negative for a second time.

WHO said this marks another milestone in the country's effort to defeat Ebola, hailing its government, partners and people on the effective, swift response to this latest outbreak.

However, WHO continues to stress that Sierra Leone, as well as Liberia and Guinea, are still at risk of Ebola flare-ups, largely due to virus persistence in some survivors, and must remain on high alert and ready to respond.