A vaccine that protects monkeys completely from the deadly Ebola and Marburg viruses has been developed by researchers. The announcement comes at a time when both viruses are on the rampage in Africa.
Marburg and Ebola are closely-related haemorrhagic fevers. Their normal host is unknown but they are thought to live in wild animals and rarely jump to people, while people pass them on only through close contact with bodily fluids. But both cause horrific bleeding and are lethal in up to 90% of cases. There is no vaccine or cure.
The largest outbreak of Marburg known so far started in northern Angola in March 2005. Doctors are still struggling to get it under control and the World Health Organization has recorded 399 human cases, 355 of them fatal as of 26 May. Meanwhile an outbreak of Ebola that started in May 2005 in the Republic of Congo has so far caused 12 cases, with nine deaths.
Fears that the viruses could become more contagious in humans – or be used as weapons – has motivated an intensive search for a vaccine. But efforts to produce a vaccine using whole, killed viruses, or viral DNA, have proved disappointing.
Scientists at Canada’s National Microbiology Laboratory in Winnipeg and colleagues in France and the US have now injected macaque monkeys with a completely different, live virus genetically modified to express a surface glycoprotein from either Ebola or Marburg. Each vaccine protected the monkeys completely from the virus its added protein came from, though not from the other virus.
The host virus causes a disease called vesicular stomatitis in animals and people. But a weakened strain was used in the vaccine and caused no symptoms in the animals, and there was no shedding of the live vaccine virus by the monkeys.
This is important, because the scientists admit there are “questions regarding the safety of live attenuated vectors”, chiefly that the genetically modified viruses could cause unexpected trouble, or recombine with other viruses. “But these live vaccines [stimulate much more of an immune-system reaction] than dead ones,” says Sylvain Baize, an Ebola expert at the Pasteur Institute in Lyon, France, who was not involved in the research.
Because they have the same surface protein as either Ebola or Marburg, he says, the vaccine viruses should be drawn to and replicate in the same tissues as the pathogen, eliciting immunity exactly where it is needed. And because they cause a transient blood infection, they also travel to lymph nodes where they provoke important kinds of cellular immunity.
By Debora MacKenzie