Chimerix (Nasdaq: CMRX) says it has FDA approval to launch a Phase 2 clinical trial of its possible treatment for Ebola. The drug has already been administered to two Ebola patients, one of whom died. The new trial will test safety, tolerability and efficacy for Brincidofovir.

The company has already received Food and Drug Administration approval to accelerate Brincidofovir’s development as well as its use in the two Ebola patients in the U.S.

No effective treatment for Ebola exists.

The Chimerix news came as concerns mounting about fighting the disease, which has killed more than 4,500 people.

U.S. health officials said Thursday they still don’t know how two Dallas nurses caught Ebola from a patient.

The revelation that one of the hospital nurses was cleared to fly on a commercial airline the day before she was diagnosed raised new alarms about the American response to the Ebola outbreak in West Africa.

In Sierra Leone, the government announced the virus had infected two people in the last part of the country that had been free of the disease, in the mountainous north, despite aggressive steps to keep it at bay.

In Washington, President Barack Obama directed his administration to respond in a “much more aggressive way” to the threat and, for a second day in a row, canceled his out-of-town trips to stay in town and monitor the Ebola response. He was calling foreign leaders and U.S. lawmakers to discuss what more must be done, the White House said, and bringing his Cabinet members together on the matter.

A ban on travel to the U.S. from the Ebola-stricken countries, sought by some Republican lawmakers, is not under consideration, White House spokesman Josh Earnest said Thursday.

In a briefing for Wall Street analysts and later in a detailed press release, Chimerix spelled out next steps in its efforts as well as other drug development programs.

Chimerix says it is working with the FDA to develop a trial protocol for testing in patients who are confirmed to have Ebola. The FDA already approved an investigational new drug (IND) application, an essential step in the review process.

Brincidofovir tablets “are available for immediate use in clinical trials,” the company said Thursday.

Thomas Duncan received the drug but died in Dallas.

Journalist Ashoka Mukpo is being treated in Nebraska.

Brincidofovir has shown potential results against Ebola in tests run by the U.S. Centers for Disease Control.

“Frankly, we’ve known about the potency of Brincidofovir against the Ebola virus for a little over six weeks,” said Chimerix CEO Michelle Berrey in the presentation to analysts.

“We want to do our part first to determine if there is a role for Brincidofovir, we do have emergency INDs that are currently in place and we do have a protocol that is under review at the FDA to be finalized under a new IND by the end of this week. We are working to identify the key hospitals, the key medical centers where we believe that protocol should be implemented where we know they have adequate training; they have adequate facilities to be caring for individual who are confirmed to be infected with Ebola virus and to our appropriate candidates for Brincidofovir.”

Triangle firm BioCryst and drug giant GlaxoSmithKline also have treatments under development.

(The Associated Press contributed to this story.)