"This is the first time that all three countries—Guinea, Liberia and Sierra Leone—have stopped the original chains of transmission that were responsible for starting this devastating outbreak two years ago," Dr Matshidiso Moeti, WHO's regional director for Africa said in a statement.
"I commend the governments, communities and partners for their determination in confronting this epidemic to get to this milestone. As we work towards building resilient health care systems, we need to stay vigilant to ensure that we rapidly stop any new flares that may come up in 2016."
Sierra Leone is currently in its 90-day period of enhanced observation, which if successful, should finish on February 5, 2016. Despite having previously been declared Ebola free, Liberia is now under closer watch, after a survivor became infectious again.
The next few months will be "absolutely critical" in making sure countries are ready to detect and respond to any new cases of the illness, according to WHO's special representative of the director-general for the Ebola response, Dr Bruce Aylward.