(CNN) - The Ebola outbreak "continues to accelerate" in West Africa and has killed 1,552 people so far, the World Health Organization said Thursday.The total number of cases stands at 3,069, with 40% occurring in the past three weeks. "However, most cases are concentrated in only a few localities," the WHO said.
The outbreak, the deadliest ever, has been centered in Guinea, Sierra Leone and Liberia, with a handful of cases in Nigeria. The overall fatality rate is 52%, the WHO said, ranging from 42% in Sierra Leone to 66% in Guinea.
The WHO issued a "road map" Thursday that "responds to the urgent need to dramatically scale up the international response" in light of the acceleration of new cases, it said.
It aims to stop Ebola transmission in affected countries within six to nine months and prevent its spread internationally.
The road map prioritizes the setting up of treatment centers, community mobilization and safe burials.
It will also try to overcome bottlenecks in vital supplies such as personal protective equipment, disinfectants and body bags.
The road map is based on an assumption that "in many areas of intense transmission the actual number of cases may be 2-4 fold higher than that currently reported," the document states.
It also acknowledges that the total number of cases "could exceed 20,000 over the course of this emergency."
An Ebola vaccine being developed by the U.S. National Institutes of Health and GlaxoSmithKline will be fast-tracked for human trials, the international consortium behind the effort said Thursday.
The experimental vaccine could be given to healthy volunteers in Britain, Gambia and Mali as early as September, according to a statement from the consortium, which provided funding.
The vaccine, which does not contain infectious virus material, has shown promise in early tests on primates to protect them from Ebola without significant adverse effects, the statement said.
The human trials will begin as soon as ethical and regulatory approvals are granted.