For the first time, a vaccine protected monkeys against the lethal Ebola virus, raising doctors' hopes of developing a means of inoculating people against the terrifying disease.
Four macaques that were injected with the experimental vaccine suffered no ill effects after being exposed to normally lethal doses of the virus. Four macaques that were not inoculated died within six days.
The findings mark the first time an Ebola vaccine has worked in primates, said Dr. Gary Nabel, director of the Vaccine Research Center at the National Institutes of Health and an author of the study, published in Thursday's issue of the journal Nature.
The monkeys are more closely related to humans than any other species in which an Ebola vaccine has worked. A human vaccine still could be years away, however. Among other things, questions of safety and how to deal with different strains of the virus would have to be resolved before experiments on humans could begin.
Ebola hemorrhagic fever, first recognized in 1976, kills up to 90% of its human victims within days of infection. Outbreaks so far have occurred only in Africa. An outbreak has killed 145 people in Uganda this year, and 1995 one in Zaire claimed 245 lives.
The fever's dramatic symptoms - which include severe pain, high fever, bleeding from the eyes, and rapid death - have been depicted in the book The Hot Zone and the movie Outbreak. Some fear the virus, which spread by bodily contact, could be carried elsewhere by terrorists or sick airplane passengers.
''Ebola is a difficult virus because currently available antiviral drugs have no proven effect on it and we do not know its natural reservoir, making environmental control impossible,'' said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which partially funds the Vaccine Research Center. ''A vaccine is the best hope for protecting humans from infection.
None of the primates that received the vaccine showed signs of illness during the six-month study. Three of them did not have any virus in their blood; the fourth showed low levels, but the virus disappeared after a week.
''We're encouraged that we can see any protection, because until this point it's really been impossible to develop immunity in the primate,'' Nabel said.
Vaccines attempt to marshal the body's immune system to build defenses by showing it what the targeted virus looks like. Traditional approaches involved inoculating with dead germs or live but weakened ones.
In 1997, Nabel and others developed a strategy that protected guinea pigs by using a vaccine made of DNA strands that encode Ebola virus proteins. The approach worked in rodents but was not completely effective for primates.
In the latest research, Nabel and colleagues boosted the DNA vaccine with a weakened virus that normally causes respiratory infections. The strain was modified with a protein of the Ebola Zaire strain. The one-two punch worked.
''It was really the two together that gave a very significant antibody response that I think allowed us to see the protection that we saw,'' Nabel said.
Researchers said more study is needed to figure out what immune system mechanism actually protected the animals.
''It's a good development. It's promising,'' said Dennis Burton, professor of immunology at the Scripps Research Institute in San Diego. ''They've taken it to the next step to monkeys from guinea pigs.''
Though Ebola may never become a worldwide problem, research is needed just in case and to prepare for other, yet-undiscovered viruses, he said.