Researchers in the United States and Canada report they have successfully tested a vaccine against the Ebola and Marburg viruses, two of the world's deadliest infectious diseases. The news comes as health officials in Angola battle a Marburg outbreak that has killed nearly 350 people this year.

In a study published in the current issue of the journal Nature Medicine, scientists report that they have successfully tested two new vaccines on macaque monkeys.

Dr. Thomas Geisbert, one of the study's authors and a virologist at the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick, Maryland, says the success in the macaque trials is a promising indicator of the vaccines' potential for use in protecting humans against the Ebola and Marburg viruses.

"These viruses are even more virulent and more pathogenic in non-human primates than in humans," he says. "So, for example, where you have got 80 to 90 percent mortality in humans, in non-human primates it's 100 percent lethal. "

Ebola and Marburg are highly contagious viruses spread through bodily fluids. The disease begins with typical flu-like symptoms that quickly debilitate the patient, causing multiple organ failure and often death.

Dr. Stephen Jones, a researcher with the Public Health Agency of Canada, explains that the vaccine is a live virus, which, instead of carrying one of its own genes, is rewired to carry one from the Ebola or Marburg virus.

"And to the immune system it looks like a little Ebola or Marburg particle but it is not capable of causing disease," he says. "You get, wow, there's a virus, and it has the correct sort of protein on its surface, but it's not going to do you any serious damage. But your body responds to it very rapidly, produces very potent antibody and cellular responses which subsequently mean that you're able to survive infection with the real thing."

The monkeys in the study were vaccinated against either Ebola or Marburg, and then 28 days later were subjected to a massive dose of the corresponding virus. None of the monkeys showed any sign of the disease, or any harmful side effects.

The researchers say their goals are two-fold, to protect citizens and soldiers against bioterrorism, and to prevent outbreaks like the current Marburg epidemic in Angola.

Dr. Jones says he thinks the vaccine has good prospects for eventually stopping human transmission.

"One of the insidious things about this virus is if there's a case in a family, it very often is transmitted to your loved ones who are caring for you whilst you are sick," he says. "And you can see it go from one family member to another to another which is awful. So if you know there's a case in a family, you could vaccinate the close contacts of that person in an attempt to stop that cycle of transmission.

The Canadian and American researchers began working on the vaccines intensively in 2001. They say it could be a couple of years before they will be able to perform human trials, and maybe six years before the vaccines will be licensed for widespread use.