"The change in behavior that we've been hoping for, working for, anticipating, is now happening everywhere," Dr. David Nabarro told Reuters in an interview.
"The facilities to treat people are available everywhere," he said. "Safe burial teams are providing safe and dignified burial services everywhere and the result is that we're seeing the beginnings of the outbreak slowing down."
Nabarro declined to predict when the outbreak of the virus could be definitively over.
The hemorrhagic fever is spread through contact with bodily fluids of infected people or the highly contagious body of someone who has died of the virus. Nabarro said burial practices that involved people touching and cleaning bodies of Ebola victims had helped fuel the outbreak.
The worst Ebola outbreak on record infected about 21,200 people in Liberia, Sierra Leone and Guinea since it was detected in March, according to the World Health Organization.
The government of Liberia said earlier on Thursday that it could be free of the virus by the end of next month after success in curbing transmission. It said the country had only 10 confirmed Ebola cases as of Jan. 12.
"It's an incredible drop," Nabarro said, adding that he believed the Liberian figures were "absolutely correct." Nabarro described "a remarkable collective change in patterns of behavior" and said Liberia had "come to terms with the reality that the outbreak of Ebola is being driven by the way in which people behave."
More than 3,500 of the 8,400 dead were from Liberia.
Liberia, Sierra Leone and Guinea now have capacity to quickly set up mobile centers to handle localized outbreaks.
"Those who are involved in the response have worked out that they can organize rapid mobile responses in case there's a flare up anywhere, so they can set up small temporary treatment facilities wherever they are needed," Nabarro said.
He said the US, British and French military, which built treatment centers, had played a crucial role.
"This external help was absolutely vital in bolstering and supporting the capacity of the people in the country to make the changes," Nabarro said, adding that "much of the external help came from within Africa."
Asked about lessons to be taken and suggestions that the WHO had been slow in sounding an alarm about Ebola, Nabarro said: "I hope that the result of this epidemic and the investigations done ... will better enable us to work out the way in which to predict" an outbreak.