Jan. 14, 2016 | CNN

The World Health Organization declared an end Thursday to the current Ebola outbreak in West Africa. But the global health organization cautioned that Liberia, Guinea and Sierra Leone remain at high risk for additional small outbreaks of the disease and must remain vigilant.

For now, the WHO said in a statement, "all known chains of transmission have been stopped in West Africa."

The organization said its job was not over. More flare-ups were expected, and strong surveillance and response systems would be critical in the months to come, it said.

Liberia was first declared free of Ebola transmission in May 2015, but the virus has been reintroduced twice since then, with the latest flare-up occurring in November. Thursday's announcement came 42 days -- two 21-day incubation cycles of the virus -- after the last confirmed patient in Liberia.

Three hardest-hit countries now have zero cases

"WHO commends Liberia's government and people on their effective response to this recent re-emergence of Ebola," said Dr. Alex Gasasira, the WHO representative in Liberia. "The rapid cessation of the flare-up is a concrete demonstration of the government's strengthened capacity to manage disease outbreaks. WHO will continue to support Liberia in its effort to prevent, detect and respond to suspected cases."

This date marks the first time since the start of the epidemic two years ago that all three of the hardest-hit countries -- Guinea, Liberia and Sierra Leone -- have reported zero cases for at least 42 days. Sierra Leone was declared free of Ebola transmission on November 7 and Guinea on December 29.

"Detecting and breaking every chain of transmission has been a monumental achievement," said Dr. Margaret Chan, WHO director-general. "So much was needed and so much was accomplished by national authorities, heroic health workers, civil society, local and international organizations and generous partners. But our work is not done and vigilance is necessary to prevent new outbreaks."

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