By Ben Hirschler

DAVOS, Switzerland (Reuters) - United Nations agencies need a final $1 billion to fight West Africa's deadly Ebola epidemic as experts move to a new phase involving a massive detective operation to trace remaining cases, the U.N. Ebola chief said on Wednesday.

David Nabarro estimated that an overall total of $4 billion in new money, equivalent to all the aid committed so far, was needed by relief agencies and the worst affected countries themselves to end the epidemic and "help these countries to get back to the economic trajectory they had".

The unprecedented outbreak has so far killed more than 8,400 people, overwhelmingly in Liberia, Guinea and Sierra Leone, and the international response has included balance of payments and military support as well as medical help.

So far, U.N. bodies have received around $1 billion, two-thirds of a target set last year. Nabarro, who heads the U.N. response, said at the World Economic Forum in Davos that the World Health Organization, UNICEF and the World Food Programme now needed the same amount again.

Although the rate of infection is slowing, experts in Davos said tracking down remaining cases so that the disease cannot flare up again will be a major task.

Jeremy Farrar, director of Britain's Wellcome Trust international medical charity, said the epidemic would have "a long and bumpy tail".

A priority for the next few months will be scaling up efforts to trace all the contacts of infected people. Nabarro said that would require perhaps 1,000 epidemiologists, since there are still around 50 micro-outbreaks in the region:

"I can't say how long it will take, but it is that last part of getting down to zero that might be the most difficult."

Drugmakers and research institutions are meanwhile racing to develop Ebola vaccines, which could yet have a role to play in protecting healthcare workers and other at-risk groups, depending on how much longer the epidemic lingers.

 

(Editing by Kevin Liffey)