The Ebola outbreak sweeping through West Africa will get significantly worse before it subsides, infecting as many as 20,000 people, the World Health Organization said Thursday, even as U.S. researchers announced plans to begin human safety trials next week in a race to develop an effective vaccine.

Adding more urgency to the crisis, new research detailed how the virus at the heart of the outbreak has mutated repeatedly in recent months, a fact that could hinder diagnosis and treatment of the devastating disease the longer the crisis stretches on. Five of the paper’s 50 co-authors died of Ebola before they could see their findings about the sequencing of the virus’s genome published.

Despite the bad news continuing to flow out of West Africa — of overfilled and understaffed treatment centers, of airlines suspending service into affected areas, of controversial quarantines, of body counts climbing by the day — Thursday also brought a sense that the international response to the crisis, widely criticized as slow and inadequate, is shifting into higher gear.

WHO issued a “roadmap” aimed at stopping the current Ebola outbreak within the next six to nine months. It includes dramatically scaling up efforts to contain the spread of the disease and treat those stricken by it, increased resources at hospitals and isolation centers, ensuring safe burials and more aggressive public awareness campaigns. The agency said it also will work to clear logistical “bottlenecks” that have made it difficult to get disinfectants, body bags, gloves and other medical supplies to the areas where they are desperately needed.

The plan will cost an estimated $490 million over the next six months and require thousands of experts and local volunteers, the agency said. That doesn’t include support for other essential services or helping West African health systems recover from the epidemic.

As the worst Ebola outbreak in history unfolds in West Africa, The Post's Joel Achenbach explains how the deadly virus wreaks havoc on the human body. (Davin Coburn/The Washington Post)

The WHO also offered sobering new numbers of Ebola’s toll in West Africa. At least 1,552 people have now died and 3,069 have been infected in the four countries battling the virus — Guinea, Liberia, Sierra Leone and Nigeria — though the actual totals almost certainly are higher.

Scientists from the National Institutes of Health, in collaboration with British drugmaker GlaxoSmithKline, said the experimental vaccine that will undergo human trials beginning next week has shown promising results in non-human primates. If the trials are successful, officials said, they could have 10,000 doses available in short order to immunize health workers and others at risk of contracting Ebola.

‘Safety is paramount’

The trial, which will take place at NIH’s Clinical Center in Bethesda, will involve 20 healthy adults between ages 18 and 50 and will test both the safety of the vaccine and its ability to generate an immune response in patients. Participants will be evaluated over a 48-week period, but officials said they hope to have initial safety data by the end of the year — a scientific and ethical necessity before distributing any new drug widely.

“Safety is paramount; it’s absolutely paramount,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told reporters Thursday. “The data [on the vaccine] in non-human primates is really quite impressive. But in science, you never know, and that’s why we do the trials.”

Fauci said he couldn’t predict when the drug might be available for medical workers and others at risk of contracting Ebola in West Africa, but he said researchers would work as quickly as possible to make that happen.

The NIH and other groups, such as the British-based public health charity Wellcome Trust, are trying to line up similar human Ebola vaccine trials in the United Kingdom, Gambia and Mali beginning as early as next month. And the Centers for Disease Control and Prevention is in talks with Ni­ger­ian officials about conducting trials in that country.

Fauci said that while it might be ideal to conduct trials in countries being hit hardest by the current outbreak, such as Liberia or Sierra Leone, “the infrastructure in those places would not allow the kind of careful Phase I studies that you would like to do.”

The Ebola epidemic over time

The vaccine being tested at NIH next week is based on a chimpanzee cold virus called chimp adenovirus type 3. The virus is used as a carrier to deliver pieces of genetic material from two Ebola species: Sudan and Zaire, the type responsible for the current outbreak in West Africa. It is designed to deliver one part of Ebola’s genetic material to human cells, eventually triggering an immune response in the patient. The vaccine, however, does not allow Ebola genes to replicate and cannot cause infection, Fauci said.

NIH’s trial comes amid a flurry of efforts to ramp up testing and production of various Ebola treatments and vaccines developed in recent years, primarily by small biotech firms and governments such as the United States and Canada.

Origin of an outbreak

As researchers work toward effective treatments for Ebola, the study published Thursday in Science offered new insights into the origins of the outbreak and how it ended up in West Africa. By genetically sequencing samples of the Ebola Zaire strain — one of five types known to infect humans — researchers said the virus appears to have diverged about a decade ago from a related strain in Central Africa, where previous outbreaks have occurred.

Ebola’s arrival in Sierra Leone this spring appears to have begun with a single funeral, according to the findings. A young woman who had recently suffered a miscarriage arrived at Kenema Government Hospital in Sierra Leone in late May with a high fever. During her treatment, doctors discovered she had been infected with Ebola, becoming the country’s first diagnosed case.

The woman eventually recovered, but health workers who traced her contacts discovered that she and more than a dozen other women recently had attended the burial of a traditional healer who had been treating Ebola patients near the Sierra Leone-Guinea border. All had been infected.

“They realized she was not an isolated case,” said Pardis Sabeti, an associate professor at Harvard whose lab sequenced the Ebola virus genomes from 78 patients and quickly made public the data earlier this summer.

Thursday’s study also details more than 300 genetic mutations that make the current Ebola outbreak different from any in the past. While viruses routinely mutate and some of those changes are most likely insignificant, others have the potential to affect the accuracy of diagnostic tests or the effectiveness of vaccines and treatments under development for the disease.

Sabeti said researchers are expecting to receive additional Ebola samples soon from Nigeria. They plan to sequence those, as well, and release the data as soon as possible, in hopes of speeding up understanding of the disease and efforts to contain it.

Sabeti said she and colleagues already have heard from companies developing treatments and diagnostic tests, who want to understand how the mutations could affect those efforts. Only through such collaboration, she said, can scientists tackle the outbreak with the speed it deserves.

“There’s nothing you should crowdsource more than an epidemic,” she said. “It has this urgency where we need every person working on it.”

Abby Phillip, Caelainn Hogan and Lenny Bernstein contributed to this report.