GENEVA -- The World Health Organization (WHO) on Thursday declared the end of the most recent flare-up of Ebola virus disease in Liberia, saying all known chains of transmission have been stopped in West Africa.

The WHO announcement came 42 days (two 21-day incubation cycles of the virus) after the last confirmed patient in Liberia tested negative twice for the disease.

It is 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 7 November 2015 and Guinea on 29 December. Liberia was first declared free of Ebola transmission in May 2015, but the virus reappeared twice since then, with the latest flare-up in November.

This largest and most complex Ebola outbreak claimed the lives of more than 11,300 people and infected over 28,500, according to WHO.

"Detecting and breaking every chain of transmission has been a monumental achievement," said 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."

Rick Brennan, Director of WHO Emergency Risk Management and Humanitarian Response said good leadership, community engagement, collaborative efforts from many countries as well as people in Western Africa contribute to the end of the unprecedented Ebola outbreak over last four decades.

"We are appreciating the facts that China deployed a large number of health care providers and public health specialists, they played important roles in treatment of individuals, investigating cases and tracking down contacts," Brennan told Xinhua.

However, WHO cautioned that the three countries remain at high risk of additional small outbreaks of Ebola, like the most recent one in Liberia.

According to WHO, the job is not over, more flare-ups are expected and that strong surveillance and response systems will be critical in the months to come.

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 showed 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 one year, and in rare instances, be transmitted to intimate partners.

WHO and partners are working with the governments of Guinea, Liberia and Sierra Leone to help ensure that survivors have access to medical and psychosocial care and screening for persistent virus, as well as counseling and education to help them reintegrate into family and community life, reduce stigma and minimize the risk of Ebola virus transmission.