How doctors know an Ebola patient is no longer contagious

As Dr. Kent Brantly and Nancy Writebol make their way back into the world after a three week quarantined battle with the Ebola virus, the public may be wondering how exactly their doctors determined the two patients are no longer contagious.

On Thursday, at a press conference at Emory University Hospital, where Brantly and Writebol received their care, Dr. Bruce Ribner, medical director of the infectious disease unit at the hospital, assured the public that he and his team had taken all the appropriate safety precautions to be sure both patients were safe for discharge.

"After a rigorous and successful course of treatment and testing, the Emory health care team has determined that both patients have recovered from the Ebola virus and can return to their families and community without concern for spreading this infection to others," said Ribner.

He said patients who do not show clinical symptoms of the virus for two days are believed to no longer be contagious, though he wasn't specific about which types of lab tests were conducted to make the final call, perhaps out of respect for his patients' privacy.

However, a statement released by the Centers for Disease Control and Prevention provided somewhat more detail and said blood tests indicated both patients are now free of the virus. "CDC provided consultation to the healthcare team at Emory University Hospital and conducted the laboratory testing of patients to confirm that they no longer had Ebola virus circulating in their blood," the statement said.

The CDC also made clear that contact with the recovered patients, either by their families or members of the public, does not pose a risk.

"Individuals who recover from Ebola are not contagious as far as transmitting the virus through close personal contact with blood or body fluids such as urine, feces, sweat, or vomit," the CDC statement said. The agency added that case studies of patients who have survived the disease in West Africa provide further evidence that Ebola survivors don't pose a threat to others once the virus is no longer detectable in their blood.

Dr. Amar Safdar, associate professor of infectious diseases and immunology at NYU Langone Medical Center, told CBS News that blood, or serologic, tests are typically done to assess if a patient with an infectious disease is still sick. Blood test results of someone recovering from the Ebola virus will indicate a decreased viral load compared to someone with an acute infection. Doctors will also look to see if the patient has developed certain antibodies that indicate the body is fighting off the infection.

In addition to lab work, doctors also rely on less high-tech approaches such as basic physical exams. "The standard is if the symptoms have resolved and the patient has clinically improved that means the patient has recovered from the acute infection," said Safdar.

There is some indication that a patient may continue to shed the virus even after a doctor determines the acute infection has subsided. Most likely, the rate at which the virus clears the body varies just as much as the incubation period, which is between 2 and 21 days. According to the World Health Organization, in one instance, a lab worker who contracted Ebola on the job and survived was found to have traces of the virus in his semen 61 days after the initial infection. This could theoretically mean a man could infect his partner during sexual intercourse weeks after he's declared disease-free, although no such cases have been documented.

Ribner said he doesn't think Brantly, 33, and Writebol, 59, would suffer any long-term health problems as a result of contracting the virus. "There may be some recovery time because this is fairly devastating disease," he said. "We would anticipate that any patients who have had not any significant organ damage will make a full recovery."

And they probably don't need to worry about going through this ordeal ever again. Safdar said when a person contracts the Ebola virus their immune system creates antibodies that will protect them from that strain of the virus in the future.

However, there are at least three deadly strains of Ebola. The strain of the virus that has so far killed more than 1,200 people in West Africa is believed to be Ebola Zaire, which has the highest mortality rate; as many as 96 percent of people infected with this strain of Ebola will die. Safdar said the two other strains found in Sudan and Uganda kill about a quarter of those who contract it.

"There are certain types of antibodies that have been shown to protect against certain types of Ebola infections -- neutralizing antibodies," explained Sadfar. "These are desirable to protect from further exposure."

He added that researchers are especially interested in examining blood samples of patients who have made a full recovery from the virus since a closer look at those antibodies could provide crucial information for developing an effective vaccine.