UNITED NATIONS (AP) — The Ebola epidemic in West Africa can be defeated by the end of the year if the number of new cases drops to zero and stays at zero, the head of the World Health Organization said Thursday.

Dr Margaret Chan told the UN Security Council there have been no new cases in Liberia and only a total of three cases in Sierra Leone and Guinea in each of the last two weeks, “the lowest numbers seen in well over a year.”

Fears that the Ebola virus could be permanently established in humans in the region have also been defeated. “At the same time, I must caution against a false sense of security,” Chan said.

The outbreak has killed more than 11,280 people, according to the latest WHO figures. Chan called the outbreak “by far the largest, longest and most severe ever known.”

She predicted that there would be further setbacks like the flare-up of Ebola at the end of June in Liberia, nearly two months after the country was declared Ebola-free.

But Chan said that as a result of the unprecedented response to last year’s outbreak “we have a very good picture of current chains of transmission and know how to break them.”

She added, “If the current intensity of case action and contact tracing is sustained, the virus can be finally defeated by the end of this year.”

A panel selected by WHO’s executive board to examine its response to the outbreak has criticized the UN health agency’s organizational failings, saying senior WHO leaders let political considerations sway their decision-making.

Chan told the Security Council that she is personally overseeing reforms including the establishment of a global health emergency workforce which can quickly shift into “high gear,” and performance benchmarks to ensure this happens.

Dr David Nabarro, the UN Ebola chief, said human security depends on being able to anticipate such outbreaks and react quickly.