(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.
"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."
The WHO cautioned that the three countries remain at high risk of additional small outbreaks of Ebola, like the most recent one in Liberia. To date, 10 such flare-ups have been identified that were not part of the original outbreak and are likely the result of the virus persisting in survivors even after recovery.
Evidence shows that the virus disappears relatively quickly from survivors, but can remain in the semen of a small number of male survivors for as long as a year, and in rare instances, be transmitted to sexual partners.
The Ebola epidemic claimed the lives of more than 11,300 people and infected more than 28,600. It devastated families, communities and the economic systems of all three countries.
While the WHO statement was full of praise to mark the occasion, there were dissenting voices.
The international medical humanitarian organization Doctors Without Borders, also known as Medecins Sans Frontieres, said the response to the outbreak had been slow, and the global health community had some lessons to learn for the future.
"Today is a day of celebration and relief that this outbreak is finally over," said Joanne Liu, MSF's international president. "We must all learn from this experience to improve how we respond to future epidemics and to neglected diseases. This Ebola response was not limited by lack of international means but by a lack of political will to rapidly deploy assistance to help communities. The needs of patients and affected communities must remain at the heart of any response and outweigh political interests."