The experimental Ebola drug ZMapp shows it is 100% effective in monkey studies, even in later stages of the infection, the only clinical trial data showed.

The study’s findings have been published in Nature.

Yet the limited supplies will not help the 20,000 people predicted to be infected during the outbreak in West Africa.

Two out of seven people given the drug, have later died from the disease.

ZMapp has been dubbed the “secret serum” as it is still in the experimental stages of drug development with, until now, no public data on effectiveness.

Doctors have turned to it as there is no cure for Ebola, which has killed more than 1,500 people since it started in Guinea.

Researchers have been investigating different combinations of antibodies, a part of the immune system which binds to viruses, as a therapy.

ZMapp shows it is 100 percent effective in monkey studies, even in later stages of the Ebola infection

Previous combinations have shown some effectiveness in animal studies. ZMapp is the latest cocktail and contains three antibodies.

Trials on 18 rhesus macaques infected with Ebola showed 100% survival.

This included animals given the drug up to five days after infection. For the monkeys this would be a relatively late stage in the infection, around three days before it becomes fatal.

Scientists say this is significant as previous therapies needed to be given before symptoms even appeared.

One of the researchers, Dr. Gary Kobinger from the Public Health Agency of Canada, said this was a huge step up from previous antibody combinations.

“The level of improvement was beyond my own expectation, I was quite surprised that the best combination would rescue animals as far as day five, it was fantastic news.

“What was very exceptional is that we could rescue some of the animals that had advanced disease.”

However, there is always caution when interpreting the implications for humans from animal data.

A Liberian doctor, one of three taking the drug in the country, and a Spanish priest both died from the infection despite ZMapp treatment.

The course of the infection is slower in humans than macaques so it has been cautiously estimated that ZMapp may be effective as late as day nine or 11 after infection.

But Dr. Gary Kobinger said: “We know there is a point of no return where there is too much damage to major organs, so there’s a limit.”

The group wants to start clinical trials in people to truly assess the effectiveness of the drug.