French scientists reported their findings on May 29 in The Lancet.
Since Sept. 2012 when the World Health Organization (WHO) started tracking the new SARS-like virus, 49 people have had laboratory-confirmed cases, including five new cases reported in Saudi Arabia on May 29, according to a new statement.
The MERS (Middle East respiratory syndrome) virus is most closely-related to a bat virus, and is part of a family of viruses that cause the common cold and SARS, the latter of which killed nearly 800 people in a 2003 outbreak.
The majority of cases have been seen in Saudi Arabia, with a smaller number appearing in other countries in the Middle East including Jordan, Qatar, Tunisia, and United Arab Emirates. Cases have also appeared in the U.K. and France.
A 65-year-old Frenchman who had previously traveled to Dubai in April died Tuesday. He marked France's first case of MERS. His hospital roommate had also developed the disease but did not have previous contact with the Middle East, evidence of the disease spreading among those in close contact.
The new Lancet study provides the first detailed clinical descriptions of MERS, and was completed by scientists who visited the infected French patients. The hospital roommates shared the same room for three days, they reported. The doctors discovered the virus in the second patient had an incubation period of nine to 12 days. An incubation period is the amount of time it takes from exposure to an infectious organism to symptoms appearing.
Previously the incubation was thought to be one to nine days.
"Our findings suggest that the virus's incubation period could reach 9-12 days, a longer period than what was previously recorded, with clinical implications for the duration of quarantine," wrote the researchers, led by Dr. Benoit Guery of Calmette Hospital in Lille, France.
Doctors have previously reported evidence of the disease spreading among people in close contact in a health care facility in the Eastern part Saudi Arabia where at least 21 infections were reported since mid-May.
The five new Saudi cases are not tied to this facility, the WHO said Tuesday.
Dr. Margaret Chan, Director-General of the WHO, singled out MERS in a speech on Monday in Geneva to the WHO assembly.
"We understand too little about this virus when viewed against the magnitude of its potential threat," Chan said. "We do not know where the virus hides in nature. We do not know how people are getting infected. Until we answer these questions, we are empty-handed when it comes to prevention. These are alarm bells. And we must respond."
The WHO has more information MERS.