Is the novel MERS virus a serious threat to the globe? It's a perplexing question that scientists are scrambling to answer. French experts suggest that the answer is no, after reviewing the data from multiple clusters of the disease, but they caution that milder cases have possibly gone unnoticed.

With 62 out of the 77 confirmed cases, the Kingdom of Saudi Arabia has been hit hardest by the new coronavirus, who is a close cousin to the SARS strain that threatened the world back in 2002-2003. Some argue that Saudi Arabia has been more diligent with surveillance and that other nations are likely to harbor undetected cases of the virus.

However, few cases and even fewer deaths have been reported outside of this Middle Eastern nation.

So is MERS, which causes severe respiratory and kidney disease, on the verge of becoming an international pandemic? Epidemiologists at the Pasteur Institute have devised a model of the worse case scenario by calculating the virus's basic reproduction number or 'R0' (pronounced "R-naught").

MERS-CoV Case Map (June 25, 2013) from the California Department of Public Health. Forty-two out of 77 cases have died as of July 3, 2013.

R0 predicts the number of people who are likely to catch a disease after being exposed to an infected individual. Like a speedometer, it provides readout for how fast an epidemic is spreading.

The analysis was conducted at the end of June by collecting the detailed clinical information on 55 of the confirmed patients, all from the Middle East. (At the time, there were only 64 reported cases). They also included seven additional suspected but unconfirmed cases from Saudi Arabia.

With this data, they were able to build webs to connect the dots between cases in the Middle East, as well as those that occurred abroad in Europe.

In the end, they calculated that the R0 for MERS, at its worse, has been 0.69. The R0 for SARS during its early stages was estimated at 2.2 to 3.7. In other words, a person with SARS was expected to spread the disease to between two to four people.

So far the largest count of secondary infections attributed to an initial MERS case is seven, which the authors consider to be an outlier or extreme example. To reach pandemic potential, the authors argue this value would have to exceed eight people in their worst-case scenario.

"Despite sharing many clinical, epidemiological, and virological similarities with SARS, the two viruses have distinct biology, such as the use of different receptors to infect cells in human airways," explained lead author Dr. Arnaud Fontanet, a senior epideimiologist at the Pasteur Institute.

"MERS-CoV has not spread as rapidly or as widely as SARS did. SARS' adaption to humans took just several months, whereas MERS-CoV has already been circulating more than a year in human populations without mutating into a pandemic form."

In comparison, measles has had one of the highest R0s in history (12-18), whereas the R0 for 1918 Spanish flu (2-3) mirrored what was seen with SARS.

MERS is clearly dangerous, having killed more than half of its reported cases, but clearly MERS, at least for the moment, is attacking the human body and spreading between people in a manner that differs from SARS.

But the experts warn that the world isn't free from danger. Many have called for broader surveillance, as people with milder symptoms have possibly been overlooked. Millions are descending on Saudi Arabia for the Hajj, which could lead to fresh escape routes for MERS from Saudi Arabia.

"One of the main lessons of the SARS pandemic has been that early control of the virus (while it was still confined to southeast China) might have prevented its global spread. We recommend enhanced surveillance, active contact tracing, and vigorous searches for the MERS-CoV animal hosts and transmission routes to human beings," Fontanet mentioned.

Bats and civets were deemed the likely source of SARS, but we still don't know where MERS originated, suggesting more research is desperately needed to crack this mystery.

"To maximise our chances of containing MERS -CoV infection, we need continuing research, including updated R0 estimates and methodological refinements," argued mathematician Dr. Chris Bauch from the University of Waterloo. Bauch and his colleague Tamer Oraby wrote a commentary on Fontanet's study that was also published today in Lancet.

"MERS-CoV has probably been transmitted from an unknown animal host to human beings repeatedly in the past year. However, the analysis by Breban and colleagues concludes that MERS-CoV-in its current guise-is unlikely to cause a pandemic," Bauch added.

Sources: Breban R, Riou J, Fontanet A. Interhuman transmissibility of Middle East respiratory syndrome coronavirus: estimation of pandemic risk. Lancet. 2013.

Bauch CT, Oraby T. Assessing the pandemic potential of MERS-CoV. Lancet. 2013.