ABUJA, Nigeria — The World Health Organization declared Monday that Nigeria is free of Ebola, a rare victory in the months-long battle against the fatal disease.

Nigeria’s containment of the lethal disease is a “spectacular success story,” WHO country director Rui Gama Vaz told a news conference in Abuja, Nigeria’s capital. Nigeria reported 20 cases of Ebola, including eight deaths.

The WHO announcement came after 42 days passed — twice the disease’s maximum incubation period — since the last case in Nigeria tested negative.

“The outbreak in Nigeria has been contained,” Vaz said. “But we must be clear that we only won a battle. The war will only end when West Africa is also declared free of Ebola.”

WHO said Nigeria had traced nearly every contact of Ebola patients in the country, all of whom were linked to the country’s first patient, a Liberian airline passenger who arrived with symptoms in Lagos and died soon after.

For an outbreak to be declared officially over, WHO convenes a committee on surveillance, epidemiology and lab testing to determine that all conditions have been met.

Vaz warned that Nigeria’s geographical position and extensive borders make the country, Africa’s most populous, vulnerable to additional imported cases of Ebola.

“Therefore there is need to continue to work together with states to ensure adequate preparedness to rapidly respond, in case of any potential re-importation,” he said.

The disease continues to spread rapidly in Liberia, Sierra Leone and Guinea and has claimed more than 4,500 lives.