As a rule most vaccines are used to prevent a disease but a few such as those given for smallpox and rabies, are used after infection to help the immune system fight off the invading pathogen.
With filoviruses such as Ebola, Marburg and Lassa fever, this is crucial because the viruses act by first suppressing the immune response.
Ebola usually begins suddenly with symptoms such as fever, headache, sore throat, weakness, joint and muscle aches, diarrhoea, vomiting and stomach pain; a rash, red eyes and bleeding may also occur.
In as many as 90% of people who catch the illness it is fatal and this means the Ebola virus is extremely dangerous.
Therefore the production and testing of a vaccine is an extremely challenging and slow process and few facilities exist which are highly contained with staff capable of doing the testing.
According to Dr. Anthony Sanchez, from the Centers for Disease Control (CDC) in Atlanta, more and more naturally occurring human outbreaks of the deadly disease are occurring.
Dr. Sanchez says worldwide air travel and tourism, means the virus can now be transported to and from remote regions of the world which has presents a huge potential for bioterrorism; Ebola is a Biosafety Level 4 threat, and Dr. Sanchez says a protective vaccine is desperately needed.
To date there have been more than 1,500 reported cases of Ebola haemorrhagic fever in humans.
An experimental Ebola vaccine developed by Canadian scientists last year which has been tested in three types of animals showed the vaccine kept at least half from dying when it was administered after infection.
Four of eight primates injected with a lethal dose of Ebola virus survived when they were given the vaccine within 30 minutes of exposure.
That vaccine could eventually become the first treatment for people newly infected with the deadly virus.
The research, reported in a January 2007 issue of the journal Public Library of Science Pathogens, was the first time anything has been shown to improve survival after infection with an Ebola virus and the next step will be to test the vaccine on people for the first time.
Dr. Sanchez presented an overview of the Ebola vaccine's development at the Society for General Microbiology's meeting in Edinburgh, Scotland, this week.
