The outcome was stunning. Within 24 hours of the March 12 lab accident, an experimental vaccine — never before tried on humans — was on its way to Germany from a lab in Canada.

Within 40 hours, the at-risk scientist, a 45-year-old woman whose identity has not been revealed, was injected with the vaccine.

So far, so good.

If the woman is still healthy on April 2, she can consider herself safe.

Ebola hemorrhagic fever, mostly seen only in Africa, is one of the world's most feared diseases. It begins with flu-like symptoms, followed by bloody diarrhea and vomiting. Days later, some victims begin bleeding through the nose, mouth and eyes. Depending on the strain of virus, it can kill up to 90 percent of victims. It has no cure. It is spread through direct contact with the blood or secretions of an infected person.

Dr. Stefan Guenther, head of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, where the researcher was working, said tests so far show that the scientist is healthy and free of the virus.

The peak period for an outbreak in the 21-day Ebola incubation period passed this week, he said.

"We are now on the downside," Guenther told The Associated Press, noting that with each passing day the chance of infection taking root diminishes.

One complicating factor is that it's not entirely clear the researcher was actually infected with the virus. At the time of the accident, she was wearing three layers of protective gloves. Though the needle stuck her, the plunger was not pushed so it's not certain the virus entered her bloodstream.

If she doesn't become infected, scientists may not know if it was the vaccine or luck.

There are two other known accidents involving researchers who came into direct contact with a similar strain of the virus. A Russian researcher died from it. A British scientist became ill but survived.

When the accident happened, Guenther knew that he had to act swiftly.

He sent out an e-mail to fellow scientists in the Ebola research community. One was Dr. Heinz Feldmann, chief of the virology laboratory at the Rocky Mountain Laboratories, a U.S. National Institutes of Health research facility in Hamilton, Montana, who sprang into action.

"We considered this as serious as (the Russian) case, in terms of the exposure," Feldmann told the AP in a telephone interview last weekend.

Feldmann was part of an international group of experts from the US Centers for Disease Control and Prevention; the Fort Detrick, Maryland-based US Army Medical Research Institute of Infectious Diseases; the Canadian Public Health Agency, Boston University and the University of Texas Medical Branch.

The night of the accident, Feldmann, Guenther and several other experts in Ebola research were on the teleconference debating the options: a live vaccine developed by Feldmann and colleagues, but untried on humans; a treatment designed to interfere with the virus's ability to multiply; and the use of an anti-coagulant that Army research found had saved the lives of monkeys exposed to Ebola.

The one that emerged as the strongest, Guenther said, was the vaccine developed by Feldmann and collaborating researchers at several institutions. Much of the key works was done about nine years ago in Winnipeg, at a microbiology research lab run by the Canadian government, where Feldmann worked at the time.

Although the vaccine is based on a different kind of virus, researchers used genetic engineering to make the virus look like Ebola, triggering an immune system response.

In a 2008 study, Feldmann and other researchers showed that when given 20 minutes after a lethal dose of Ebola virus, four of eight monkeys survived. There were no vaccine side effects, Feldmann said.

"The group came very swiftly to agreement that this vaccine would be the best ... because it had showed an effectiveness when used after exposure," Guenther said, referring to its use in experiments.

In the end, the woman whose life was at risk made the decision herself. She took the vaccine about 40 hours after the accident. Within 12 hours she had a headache, muscle pain and a fever, but recovered quickly.

"Those are normal reactions to live attenuated vaccines," Feldman told AP.

If no symptoms occur in the remaining several days, "she was either not infected or the treatment was successful," Feldmann said. He noted that while future study of her immune response may help to clarify the question, it will most likely remain open to interpretation.

"It's hard to tell whether we'll ever know if she was infected or not."

Either way, Feldmann said scientists cannot draw conclusions about the experimental vaccine's safety or effectiveness. "It was to save her life, that was the priority," he said.

Even so, it might have been five years or more before the vaccine would ever have been tried on a human, because of additional animal studies and production issues, he said.

Guenther expressed hope that the situation might attract more money for more research in the field of such vaccines.

"Of course, we can't just go to Africa, now that we have seen such a vaccine went well when used on a human ... but it is also a direction that we will have to pursue," Guenther said.

"Perhaps this will be like a little push, where one says, yes, it's possible. Everything that we are doing on an experimental level can actually be put into practice."