Ebola
Simplified vaccine against Ebola virus developed
By PLoS Medicine
Jun 10, 2006, 13:21

Humans who get infected with Ebola virus develop an illness called Ebola hemorrhagic fever (EHV), which is one of the most deadly viral diseases known; 50%�90% of all ill patients die, and there is no available treatment for EHV. Scientists think that the occasional outbreaks of the disease occur because the virus �jumps� from an infected animal to a person (a rare event) and then is transmitted between people by direct contact with infected blood or other body fluids or parts. Several strains or variants of the Ebola virus exist. Most outbreaks have been caused either by the Zaire strain or by the Sudan/Gulu strain (so-called because that is where the particular virus was first isolated). Scientists are working on a vaccine against Ebola that could be given to people before they get infected and then protect them when they come in contact with the virus. A number of candidate vaccines have been developed and tested in animals.

The researchers who did this study are working on a vaccine that consists of two particular parts of the virus. One part is called GP (which stands for glycoprotein) and is from the outer coat of the virus; the other, NP (nucleoprotein), is from its inside. Without the rest of the virus, GP and NP cannot cause EBV. However, the hope is that giving these parts of the virus to an individual can educate their immune system to build a response against GP and NP, which would then recognize the virus should the vaccinated person become infected with the whole virus, and destroy it before it can cause disease. To get the GP and NP parts into the body so that they can cause a strong immune response (which is what effective vaccines do), the researchers used a manmade version of another, harmless virus called recombinant adenovirus 5 (or rAd5) to carry the NP and GP. The researchers have shown previously that this strategy for introducing a vaccine works in animals. The vaccine�i.e., the combination of the rAd5 virus and the two Ebola virus parts�can protect animals against subsequent infection with real Ebola virus that would otherwise kill them. However, during these earlier studies, the researchers had noticed that the GP part, when present at high levels, seemed to make human cells sick. They had not seen any similar problems in the experimental animals, but to be on the safe side they decided to see whether they could change the GP part so that it would still be effective as a vaccine but no longer make human cells sick.

They changed the GP part of the vaccine in different ways so that it would no longer make human cells sick and then tested whether the resulting vaccines (combined with the original NP part and the Ad5 virus) could still protect monkeys from EHF after they were infected with Ebola virus. They found that some of the new GP versions made the vaccine less effective, but others did what they had hoped for; namely, they gave the same level of protection as when the original GP part was present. While doing these experiments, the researchers also found that the NP component seemed unnecessary and in some cases even weakened the vaccine's effect.

The researchers have now developed a simplified vaccine against Ebola virus that is effective in monkeys. This vaccine consists of only a modified GP component (which is well tolerated by human cells even at high concentrations) and the rAd5 component. This vaccine is not the only candidate currently being developed against Ebola, but it seems likely that it is one of a few that will be tested in human volunteers in the near future. The initial clinical trials will test whether the vaccine is safe in humans, and whether it can cause the immune system to produce an immune response that is specific for the Ebola virus. Assuming that the outcomes of these trials are positive, the next question is whether the vaccine can protect humans against Ebola disease. Because Ebola is so dangerous and outbreaks are relatively rare, the vaccine will likely be tested only during an actual outbreak. At that time, an experimental vaccine might be given to people at immediate risk of becoming infected, especially health-care workers who, because they take care of infected patients, are themselves at very high risk of becoming infected. In addition to trials in humans, the scientists will also explore whether this vaccine, which was developed based on the GP component of the Zaire strain, can protect monkeys against infections with other strains of the Ebola virus.

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