An American doctor who was exposed to Ebola in Sierra Leone and put into isolation at the National Institutes of Health doesn’t have the virus after all and has been sent home, the NIH said Tuesday.

The patient, a doctor working to help fight the Ebola epidemic, was kept under observation as part of a clinical trial to try and find out just what happens to patients with dangerous diseases such as Ebola. The idea is to watch the infection develop from the very beginning, to help doctors trying to fight outbreaks.

“The patient has given NIH permission to release the following information: The high-risk exposure was a needle stick injury,” NIH said in a statement. “Because he had an exposure and a fever, there was a greater chance he had active infection, thus the reason for isolation. Currently, the patient feels well and has no fever. CDC guidelines are being followed and the patient will remain in his home, check his temperature twice a day, until the completion of a 21-day observation period from the time of reported exposure. No additional details about the patient are available at this time.”

The doctor, who has asked for privacy, would have been the sixth American treated for Ebola in the United States. Three — Dr. Kent Brantly, Nancy Writebol and Dr. Rick Sacra — have recovered. NBC News freelance camera operator Ashoka Mukpo is being treated at the University of Nebraska Medical Center in Omaha, and an unnamed U.S. doctor is being treated at Emory University Medical Center. A man from Liberia who developed symptoms after arriving in the U.S., Thomas Eric Duncan, is being treated in Dallas.

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— Maggie Fox