The woman had accidentally pricked her finger three weeks ago with a needle used to inject Ebola into mice. It was not known if the virus actually entered her bloodstream, but she was given an experimental vaccine just in case. The vaccine had never been tested on humans.

Scientists don't know if the vaccine saved her or if she was simply lucky not to get the disease during an excruciating 21-day waiting period.

"The patient is healthy. We're happy about that," said Dr. Stefan Schmiedel, who attended to her at a Hamburg hospital.

Doctors said they would now study her immune response for evidence of whether she actually contracted the virus, and whether the vaccine helped her defeat it.

"It's possible we'll never know with 100 percent certainty," said Dr. Stephan Guenther, head of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, where the woman worked.

On March 12, the unidentified 45-year-old nicked her finger through three layers of protective gloves while working on lab mice injected with the Ebola virus.

In the hours after the accident, an international group including virologists and biologists held a telephone conference to brainstorm a strategy that might save her life. Among several risky options, the team chose the experimental vaccine, and within 48 hours it was sent from a lab in Canada and injected into the woman.

Hours later she developed a headache, muscle pain and a fever _ symptoms that passed quickly and did not return.

"We believe she's overcome everything," Guenther said. "It's been three weeks. That's a very conservative estimate for Ebola."

Guenther said scientists will analyze the woman for antibodies in an attempt to determine whether they came solely from the vaccine or whether she was generating others, signaling that she had been infected.

"The vaccination only contained antibodies against surface Ebola proteins. If she was building antibodies to other proteins, then they must have stemmed from a true infection," Guenther said.

She may not have contracted Ebola at all.

Though the needle stuck her, the plunger of the syringe was not pushed, so it's not certain the virus entered her bloodstream.

Ebola hemorrhagic fever, seen mostly 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.

So far, there is no cure. The virus is spread through direct contact with the blood or secretions of an infected person.

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