A genetically engineered virus may offer the first effective vaccine against Lassa fever, a sometimes deadly hemorrhagic fever common in West Africa, U.S. and Canadian scientists said on Monday.

The vaccine successfully protected four monkeys against Lassa, a virus that sometimes causes high fever, internal bleeding and which kills at least 5,000 people a year.

“This is the first vaccine platform shown to completely protect nonhuman primates from Lassa virus,” said Dr. Thomas Geisbert of the U.S. Army Medical Research Institute of Infectious Diseases in Frederick, Maryland.

Lassa is a hemorrhagic virus like Ebola, Marburg and yellow fever but is far less likely to be fatal. It is, however, far more common than Ebola or Marburg .

“We are hopeful that the VSV strategy, which we have successfully demonstrated for Marburg, Ebola and now Lassa virus, could have utility against other hemorrhagic fevers as well,” Geisbert added in a statement.

The VSV vaccine is named for vesicular stomatitis virus, which is genetically engineered to carry some of the genetic material from the Lassa virus.

They immunized four rhesus monkeys with a single dose of the Lassa vaccine and gave non-engineered VSV to two others. All six monkeys were then infected with Lassa.

None of the four immunized vaccinated monkeys became ill but the other two died, the researchers reported in the journal Public Library of Science Medicine.

“Lassa fever poses a huge public health threat in Western Africa,” said Dr. Heinz Feldmann of the Public Health Agency of Canada.

“While the mortality rate of this virus is not as high as with some viral hemorrhagic fevers, there are many more cases of Lassa fever and a great number of survivors are permanently affected by complications such as hearing loss, so this vaccine may have a much broader application.”

Lassa fever is mild in about 80 percent of cases but it can cause epidemics of severe disease. The World Health Organization estimates that as many as 300,000 people are infected in West Africa each year and 5,000 people die of it.

There are rival vaccines against Lassa in development.

Dutch-based Crucell has a U.S. government contract to develop a Lassa vaccine. Peregrine Pharmaceuticals Inc. is working on an antibody-based drug called Tarvacin to attack “enveloped” viruses such as Lassa, the AIDS virus, Hepatitis B and C, West Nile and SARS.