DAKAR, Senegal — The World Health Organization declared on Thursday the end to the deadliest Ebola outbreak on record, which killed and sickened tens of thousands of people in West Africa, even as it cautioned that more flare-ups of the disease were likely.
The announcement in Geneva came after a recent chain of cases in Liberia was snuffed out, marking the first time since the start of the epidemic two years ago that Guinea, Liberia and Sierra Leone — the three countries that were hardest hit by the virus — had reported zero cases for at least 42 days, or two incubation periods of the virus.
Margaret Chan, the director general of the WHO, hailed the "monumental achievement" in curbing the outbreak, which, the United Nations said, killed more than 11,300 people and infected more than 28,500. At the height of the outbreak, the bodies of victims piled up in the streets of towns and cities that were overwhelmed and ill equipped to cope with the scale and speed of transmission.
But in a statement released in Geneva, Chan added that "our work is not done, and vigilance is needed to prevent new outbreaks."
The immediate threat stems from persistence of the virus in body fluids, notably in the semen of male survivors, up to a year after they are free of the disease and show no symptoms, said Rick Brennan, the WHO's director of emergency risk management, in Geneva.
Ten flare-ups had been reported across the three countries in the past nine months, four of them in Liberia and three each in Guinea and Sierra Leone, "and we are anticipating more," Brennan said.
The risk, although significant, was low, he said. The new cases had occurred on average 27 days apart, but there have been none since mid-November. Any risk diminishes over time, as survivors' immune systems clear out the virus.
WHO officials said that the health authorities in the affected countries had put in surveillance and rapid response mechanisms for managing the risk, and that those measures had proved effective in containing the flare-ups.
"People of course want to return to a normal, but it's a new normal," said Peter Graaff, a WHO director who is in charge of Ebola response. "Ebola has been added to a number of their diseases that affect the population."
The three West African countries now have the world's biggest pool of expertise in handling Ebola and greater professionalism, confidence and resources for dealing with it, he said.
Their approach, Graaf said, is that "it's a problem, a big problem, it's going to affect us again, but we know how to handle it."
Sierra Leone was declared free of Ebola transmission in November and Guinea at the end of December; Liberia was declared Ebola free in May but then reported a few new cases.
"Today is a monumental day of achievement — the end of human-to-human transmission of the Ebola virus in the region and Liberia. It is a collective effort for the international community and the people of Liberia for being once more Ebola-free," Tolbert Nyenswah, Liberia's deputy minister of public health, said in a telephone interview. "It does not mean that we have put our tools down and lay down our guys. We keep moving on with our surveillance system."
The U.N. secretary general, Ban Ki-moon, said Wednesday that the Ebola outbreak in West Africa had been "a fundamental test" of the world's ability to come together to stanch the pandemic.
Beating back Ebola is among the few tangible achievements that senior U.N. officials cite as an example of global cooperation, though the WHO has been widely criticized for failing to act quickly enough.
Ban indirectly acknowledged the criticism by saying that he looked forward to hearing the recommendations of independent panels about how to better prepare the global public health system for emergencies. He also said that he would ask the WHO to issue reports on health security over the next two years.
Lingering effects of the disease are evident throughout the region. UNICEF estimates that nearly 23,000 children who lost one or both parents or caregivers to Ebola still need support, and that 1,260 children who survived the disease must endure medical problems even as they struggle to be accepted back into their communities.
"To have contained this epidemic is an achievement," said Manuel Fontaine, a regional director for UNICEF, "but we can't forget the terrible toll Ebola has taken on these countries."
Dionne Searcey reported from Dakar, Nick Cumming-Bruce from Geneva, and Clair MacDougall from Monrovia, Liberia. Somini Sengupta contributed reporting from the United Nations.