GENEVA (Reuters) - It will be complicated and perhaps impossible to use a vaccine to tackle Democratic Republic of Congo’s new Ebola outbreak, the World Health Organization’s emergency response chief said on Thursday.

An Ebola outbreak was declared in northeastern Congo’s North Kivu province on Wednesday, and almost 20 WHO officials are already on their way to the epicenter, Peter Salama told Reuters at WHO’s headquarters in Geneva.

Officials have not yet confirmed which type of Ebola is causing the outbreak. That information, expected within days, is crucial for the vaccine strategy.

It could be the Zaire, Sudan or Bundibugyo strain of Ebola, Salama said, although the high death rate pointed towards Zaire, the same kind of Ebola that caused Congo’s last outbreak, which was declared over just last week.

That effort relied heavily on Merck's MRK.N vaccine, which was given to contacts of Ebola patients, and contacts of contacts, to ring-fence the disease and stop it spreading.

“If this (outbreak) turns out to be Ebola Zaire, then certainly that would bring that option into play,” Salama said. “If it doesn’t we are going to have to look at much more complex options, and we may not have any vaccine options.”

Merck’s vaccine is only effective against the Zaire strain, and is the only vaccine that has gone through Phase 3 efficacy trials, a spokesman for the GAVI global vaccine alliance said.

Another vaccine is being developed by Johnson & Johnson JNJ.N to protect against multiple related diseases, including the Sudan strain.

“However it has not been through Phase 3 and it would have to be deployed as part of a clinical trial,” the GAVI spokesman said. “While it is promising, we do not yet have reliable data on its efficacy.”

Ring-vaccination depends on tracing all potential Ebola sufferers, which might be impossible in northeast Congo, where Salama said there were over 100 armed groups. Instead, WHO might consider “a more homogenous geographical strategy”, he said.

Security concerns will be assessed on a day-to-day-basis and may oblige WHO to hold staff back in the cities of Goma and Kinshasa until their safety can be assured, he said.

Armored personnel carriers and support from U.N. peacekeepers may be needed, making it very difficult to deploy the kind of far-reaching contact tracing used in the previous outbreak.

WHO dealt with an outbreak in the same area around a decade ago, “where we had to negotiate with many armed factions”, Salama said.

“So there is experience in the organization. That’s not to say this is going to be simple.”