The Ebola virus could eventually infect 20,000 people and the actual number of current cases may already be two to four times higher than reported, the World Health Organization says.

The United Nations health agency released a road map on Thursday with its $490-million US plan to try to contain the outbreak in nine months.

The number of estimated cases is now 3,069, including 1,552 deaths, in Liberia, Sierra Leone, Guinea and Nigeria as of Aug. 26, according to the WHO.

The true number of cases could be two to four times higher in some areas because of limits to the reporting capacity, Dr. Bruce Aylward, WHO's assistant director general and a Canadian epidemiologist, told reporters.  

WHO Ebola

World Health Organization Assistant Director General Bruce Aylward says the Ebola outbreak is a global health security issue. (Pierre Albouy/Reuters)

"We're going to need 750 internationals at least and 12,000 nationals — not in capital cities, not in emergency co-ordination rooms … but being right out there at the field level in the districts actually running the operations," Aylward said.

The 20,000 estimate of potential cases doesn’t necessarily mean that that many are expected, Aylward said, but it illustrates the scale of the system that needs to be in place.

Doctors Without Borders said it’s important to act on WHO’s plan.

"Huge questions remain about who will implement the elements in the plan," said MSF operations director Brice de le Vingne. "None of the organizations in the most-affected countries … currently have the right setup to respond on the scale necessary to make a serious impact."

The U.S. National Institutes of Health announced Thursday it is launching the safety trial on a vaccine developed by the agency's National Institute of Allergy and Infectious Diseases and GlaxoSmithKline. It will be tested on 20 healthy adult volunteers in Britain and the U.S. to see if the vaccine is safe and triggers a protective immune response.

The GSK vaccine consists of a common cold virus, called an adenovirus that has been engineered to include two genes of the Ebola virus.

U.S. researchers also plan to study other Ebola vaccines, including one developed by scientists at the National Microbiology Laboratory in Winnipeg licensed to NewLink Genetics.

Halifax-based Immunovaccine has reformulated an American Ebola vaccine to enhance it, which resulted in a stronger, longer-lasting immune response in monkeys.

"The vaccinated monkeys all survived whereas the control monkeys that didn't receive the vaccine succumbed to the infection within a week," said Marion Stanford, the company’s director of research.

Immunovaccine is looking for a partner to test the vaccine for safety in humans.

Even if studies suggest a vaccine or drug is safe and effective against the Ebola virus, it’s not clear whether enough doses could be manufactured quickly.

With files from CBC's Karen Pauls, The Associated Press and Reuters