A New York man who was admitted to hospital with a high fever and stomach problems after travelling through West Africa has tested negative for Ebola.
The patient in New York had been isolated shortly after arriving at Mount Sinai hospital on Monday.
An outbreak in West Africa has killed more than 900 people and a state of emergency has been declared in Liberia.
The US infectious disease agency is now operating at its highest emergency response in order to free up resources.
Ebola is one of the deadliest diseases known to humans, with a fatality rate in this outbreak of between 50% and 60%.
It is spread through contact with the bodily fluids of Ebola patients showing symptoms.
Two other Americans infected with Ebola were flown this week from Liberia to a hospital in Atlanta to receive treatment.
They are reportedly improving after receiving an experimental drug called ZMapp, produced by a firm in San Diego, but it is unclear if the drug is responsible for their improving health.
At least one country involved in the outbreak is interested in the drug.
Nigeria’s health minister, Onyenbuchi Chukwu, said at a news conference that he had asked US health officials about access. Nigeria has seen seven confirmed cases.
Officials said the manufacturer would have to agree. A spokesman for the Centers for Disease Control and Prevention (CDC) said director Dr. Tom Frieden “conveyed there are virtually no doses available”.
Dr. Tom Frieden was expected to testify in front of Congress about the outbreak on Thursday.
The treatment, tested only in animals, boosts the immune system’s ability to fight off Ebola through antibodies made by lab animals exposed to elements of the virus.
The World Health Organization (WHO) has planned to convene a panel of medical ethicists next week to discuss the ramifications of using an untested drug.
In a statement, it said the use of ZMapp “has raised questions about whether medicine that has never been tested and shown to be safe in people should be used in the outbreak and, given the extremely limited amount of medicine available, if it is used, who should receive it”.
Some public health officials were wary of ramping up production of the drug at the expense of traditional isolation and testing measures.
Peter Piot, who co-discovered the virus in 1976, and two other Ebola experts, urged the drug be made more widely available.
The FDA has separately given the US defense department an emergency authorization to use an Ebola diagnostic test overseas.
It will be used in labs designated by the defense department to respond to the Ebola outbreak.
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