Liberia was declared free of the Ebola virus Thursday by the World Health Organization, ending the deadly outbreak in West Africa. The Ebola epidemic has killed over 11,000 people over the past two years.

The announcement was made 42 days after Liberia’s last Ebola case was identified. A country is considered free of Ebola after two 21-day incubation periods have passed since the last known case tested negative for a second time. Last year, Guinea and Sierra Leone — two other hardest-hit nations — were declared free of Ebola.

"Today the World Health Organization declares the end of the most recent outbreak of Ebola virus disease in Liberia and says all known chains of transmission have been stopped in West Africa," the U.N. health agency announced in Geneva.

However, the organization warned in a statement that "more flare-ups are expected and that strong surveillance and response systems will be critical in the months to come.”

Liberia was declared free of the Ebola virus twice last year — in May and September — but each time a new case emerged. More than 4,800 people died of the Ebola virus in Liberia.

“Detecting and breaking every chain of transmission has been a monumental achievement,” WHO Director General Margaret Chan said in the statement. “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.”

On Tuesday, in his State of the Union address, U.S. President Barack Obama commended doctors and health workers for their efforts to stop the spread of the deadly virus.

“They set up the platform that then allowed other countries to join in behind us and stamp out that epidemic. Hundreds of thousands, maybe a couple million lives were saved,” Obama reportedly said.