The new MERS coronavirus that has claimed dozens of lives in the Middle East does not yet have the ability to trigger a pandemic, but vigilance is needed in case it mutates, French scientists said on Friday.
"Our analysis suggests that MERS-CoV does not yet have pandemic potential," they reported online in The Lancet.
But they urged health watchdogs to keep up their guard and pursue the search for the virus' bolthole in nature.
"We recommend enhanced surveillance, active contact tracing and vigorous searches for the MERS-CoV animal hosts and transmission routes to human beings," said the Pasteur Institute team which conducted the research.
A respiratory virus that can cause fever and pneumonia, MERS is a cousin of the deadly SARS virus which erupted in southern China in 2003 and snowballed into a global health scare.
Since Middle East Respiratory Syndrome (MERS) first came to light in 2012, there have been 43 recorded deaths, 36 of them in Saudi Arabia.
The death rate is high -- at roughly one in every two patients -- although doctors say there may be other cases that are not diagnosed as MERS or do not cause grave illness.
The trio of French epidemiologists looked at 55 MERS cases for evidence of "clusters" in which the virus had been passed on from one person to another rather than stopping with the patient who had fallen ill.
The point was to calculate the "basic reproduction number" -- known in an equation as R -- that is essentially a benchmark of contagiousness. When R is above 1, a virus or bacteria has epidemic potential.
But the worst-case scenario for MERS was an R of 0.69 and the most optimistic scenario was 0.60, the team found.
By comparison, Severe Acute Respiratory Syndrome (SARS) had an R of 0.8 at the pre-pandemic phase. The disease, which leapt from animals to animals, eventually killed 800 people.
Like SARS, MERS appears to cause a lung infection, with patients suffering from fever, coughing and breathing difficulties. But it differs in that it also causes rapid kidney failure, other research has found.
Study leader Arnaud Fontanet said that despite the encouraging news about contagiousness, health monitors must keep their eyes peeled.
"The R can change swiftly if the virus mutates or if there are exceptional events such as mass gatherings, like the pilgrimage to Mecca," he told AFP.
The 2012 Hajj drew 3.1 million people, and virologists note that this year's event likewise occurs in October, as the northern hemisphere slides into the season for coughs and sneezes.
World Health Organisation (WHO) head Margaret Chan sounded the alarm to ministers at the UN agency's annual congress in May, urging them to provide sound, factual advice for pilgrims.
"One of the main lessons from SARS is that if it had been confined at an early stage, this would have prevented it spreading globally," said Fontanet.
Figuring out the animal reservoir for MERS and how the virus is transmitted to humans is a priority, he said.
"We have a window of opportunity, and it's now that we have to act, before it adapts to humans."
The World Health Organization (WHO) said on June 26 that 77 laboratory-confirmed cases had surfaced worldwide with 40 deaths.
Saudi Arabia announced two further deaths on Wednesday and on Thursday Britain said a 49-year-old Qatari man who had been airlifted to London for treatment last September had died last week.