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(CBS) -- The experimental Ebola drug known as ZMapp has healed all 18 monkeys infected with the deadly virus in a new study. If subsequent studies prove as promising, the treatment may help fight the outbreak raging through West Africa -- once more of the drug can be made.

For the study, published online Friday by the journal Nature, researchers gave the monkeys ZMapp three to five days after they were infected with the virus and when most were already showing symptoms. That is several days later than any other experimental Ebola treatment tested so far.

"For animal data, it's extremely impressive," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which had a role in the work.

In another study, the drug also completely protected six other monkeys given a slightly different version of it three days after infection in a pilot test. These two studies are the first monkey tests ever done on ZMapp, which is a compound of three antibodies that attach to cells infected with Ebola, helping the immune system kill them.

It's not known how well the drug would work in people, who can take up to 21 days to show symptoms and are not infected the way these monkeys were in a lab. Several experts said it's not possible to estimate a window of opportunity for treating people, but that it was encouraging that the animals recovered when treated even after advanced disease developed.

More developments in the Ebola coverage:

Read more about the test.

CDC worker returns after Ebola exposure

Outbreak could affect 20,000

U.S. begins human trial

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