Scientists at the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) and the Public Health Agency of Canada's National Microbiology Laboratory reported in the British medical journal The Lancet that monkeys given a high dose of Marburg virus - normally 100 percent fatal at that dose - survived the virus when treated with the vaccine.
"I'm still excited about it, to see that basically none of these animals got severely ill, keeping in mind that this was a uniformly lethal challenge," says Heinz Feldmann from Canada's Public Health Agency. "Never, to my knowledge, has an animal survived such a uniformly lethal challenge."
An outbreak of the Marburg virus, or it's deadly cousin Ebola, could cause hemorrhagic fevers with fatality levels nearing 90 percent. Until now, no treatments other than intensive supportive care have been available. For these reasons, both viruses, which occur naturally in Africa, are also considered prime bioterror threats.
A study published by the same researchers in June of 2005, reported that the vaccine was effective in preventing Marburg or Ebola infection in monkeys when given prior to exposure. In this new study, they demonstrated that in addition to being a highly effective preventative measure, it is also 100 percent effective as a treatment for Marburg in non-human primates. When the vaccine was administered 20 to 30 minutes after exposure to the virus, all of the treated monkeys survived.
"They didn't develop any manifestations or any clinical symptoms associated with infection, like bleeding, like coagulation disorders, or no changes in liver function, so there were absolutely no problems," says USAMRIID's Thomas Geisbert.
The scientists made the vaccine with a technique called "reverse genetics." They began with a human virus called Vesicular Stomatitis Virus (VSV), which infects people rarely enough that most of the population would not yet have immunity to it. They removed a disease-causing gene from VSV and replaced it with a Marburg virus surface protein. The surface protein is harmless on its own, but can successfully trigger an immune response, thus allowing people's bodies to build-up protective antibodies.
While several years of testing are needed before the vaccine can be approved for people, the experimental version could be deployed in the event of a bioterror attack. Lab personnel who work with the virus, who are currently at highest risk for exposure outside of Africa, could also petition to use it after an accidental exposure. "I would most definitely want to take it if there were a way that I were allowed to take it," says Geisbert.
The researchers are also developing a similar vaccine against the Ebola virus, which has been shown to prevent infection by the virus when given prior to exposure. However they don't yet know if it will be effective as a post-exposure treatment.
