* Middle East virus has lower pandemic risk than SARS

* But experts caution the new virus may mutate

* Latest toll is 40 deaths from 77 confirmed cases

By Kate Kelland

LONDON, July 5 (Reuters) - The Middle East coronavirus that has killed 40 people since emerging late last year has not yet reached pandemic potential and may simply die out, according to new estimates of how easily it is transmitted.

In a study in The Lancet medical journal, researchers from France's Institut Pasteur in Paris analysed data on Middle East respiratory syndrome (MERS) clusters and found its likelihood of developing into a SARS-like worldwide epidemic was low.

The MERS coronavirus, which can cause coughing, fever and pneumonia, emerged last year and has spread from the Gulf to France, Germany, Italy, Tunisia and Britain. The World Health Organisation puts the latest global toll at 40 deaths from a total of 77 laboratory-confirmed cases.

It is related to SARS, or severe acute respiratory syndrome, because the virus that causes it is from the same coronavirus family. SARS emerged in China in 2002 and then spread around the world, killing about a tenth of the 8,000 people it infected.

But Arnaud Fontanet, who led the latest published research on MERS, said despite sharing "many clinical, epidemiological, and virological similarities with SARS, the two viruses have distinct biology".

He said one distinction was their use of different receptors to infect cells in human airways - a key factor in how easily a virus is passed from person to person.

To analyse whether MERS poses a similar threat as SARS did, Fontanet's team looked at data from 55 cases of MERS infection and calculated some called a basic reproduction number, or R0.

The basic reproduction number of an infection is the average number of secondary cases a single infected case will cause in a population with no immunity to the disease.

Even in their worst case scenario, MERS had a lower R0 (at 0.69) compared with pre-pandemic SARS (at 0.80).

When the R0 is above 1.0, epidemic potential has been reached, the researchers explained in their study. And the R0 for pandemic SARS was estimated to be between 2.2 and 3.7.

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

Ian Jones, a professor of virology at Britain's University of Reading, said the findings confirmed what appeared to be happening on the ground - "that the current MERS coronavirus transmits poorly, below the threshold required to become widely spread".

Benjamin Neuman of Reading University's microbiology research group, said it appeared from the research that the MERS virus is "slowly dying out".

But he cautioned that other studies into the biology of the virus suggest it is changing. "That change makes it difficult to predict the future of MERS," he said.

An international team of infectious diseases experts who had been to Saudi Arabia to investigate MERS said last month that it poses a serious risk in hospitals because it is relatively easily transmitted in healthcare environments. (Editing by Michael Roddy)