Dr. Bruce Ribner, medical director of the infectious disease unit at Emory's hospital, credited aggressive supportive care and the fact that both Brantly and Writebol were healthy and well-nourished with helping them recover.
The pair received an experimental therapy called ZMapp, a cocktail of antibodies made by tiny California biotech Mapp Biopharmaceutical. Health experts cautioned against declaring the drug a medical breakthrough based on two patients.
"The honest answer is we have no idea," Ribner said, when asked if the experimental drugs helped the missionaries' survival. He said early studies in primates suggest the drug has few long-term side effects.
The scale of the Ebola outbreak in West Africa, the largest in history with 2,473 people infected and at least 1,350 dead, has prompted a scramble for experimental drugs, most of which have only been tested in monkeys and cell cultures.
Last week, the World Health Organization backed the use of untested drugs and vaccines, but the scarcity of supplies has raised questions about who gets the treatments.
ZMapp was also given to a third patient, a Spanish priest, who has now died from his infection, as well as two doctors in Liberia and a nurse. Sources in Liberia told the WHO that two of those patients have shown marked improvement following their treatment.
But about 50 percent of people survive Ebola anyway, even under poor medical conditions, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease at the National Institutes of Health, said on MSNBC on Thursday.
"I'd say we have a couple of people who've recovered, they've gotten excellent medical care and the specific therapy, ZMapp ... may have had a role in it but we don't know," Fauci said.