MERS doesn't yet have pandemic potential, but future of virus unknown: study
A colourized transmission of the MERS coronavirus that emerged in 2012 is shown. (THE CANADIAN PRESS/HO, National Institute for Allergy and Infectious Diseases)
Published Thursday, July 4, 2013 6:43PM EDT
TORONTO -- The new MERS coronavirus currently doesn't spread well enough among people to trigger a pandemic, says a new study that calculates the rate at which the virus is transmitting person to person.
But the senior author says the pattern of how the virus is spreading now cannot be used to predict whether MERS will become a bigger threat in the future.
"There is absolutely no guarantee that this virus will stay as it is. It could very well follow the same path as SARS did 10 years ago," Dr. Arnaud Fontanet, who heads the emerging diseases epidemiology unit at the Institut Pasteur in Paris, said in an interview.
Others too say the study should not be used to write off the new virus.
"The virus has shown a potential for human-to-human transmission. And whether such transmission is sustained depends on the intensity of control measures as well as the characteristics of the people involved in transmission," said Marc Lipsitch, an infectious diseases specialist who teaches at Harvard University.
"For that reason, I think it's premature to say that this virus does not present a pandemic threat."
The study, published in the journal The Lancet, analyzes what is known about how often people who have been infected with the MERS virus spread it to one or more other people. The authors used the data to calculate what is called the basic reproduction number, known in the parlance of infectious diseases as the R nought.
In order for a disease to achieved sustained spread, the average infected person must spread a bacterium or virus to at least one other person. That is an R nought of one. A pathogen with a basic reproduction number of less than one would peter out.
The measles virus, which is very contagious, has a basic reproductive number of between 12 and 18, meaning that among people who are susceptible to the virus, each infected person would be expected to pass the virus to between a dozen and 18 other people. In a 2003 study in the journal Science, Lipsitch and colleagues estimated the R nought of SARS to be three.
Using the publicly available data on MERS cases, Fontanet and his co-authors set out to figure out what the basic reproduction number for the new coronavirus has been to date.
There are many holes in the available data. For instance, Saudi Arabia, which is responsible for 63 of the 77 confirmed MERS cases, often does not disclose if new infections have links to previous ones -- which might mean they caught the virus from another person -- or are what are called sporadic cases, people thought to have been infected by an animal or exposure to the virus in the environment.
The authors tried to work around the gaps by calculating best- and worst-case scenarios. Both, it turns out, came up with a reproductive number of less than one, which suggests the virus doesn't yet have pandemic potential, they said. Those rates were 0.60 and 0.69 respectively.
The authors noted that in the early stages of SARS it had a reproductive number of 0.80, closer to one than what is currently seen with the MERS virus. But they suggested that even though the viruses are cousins, people should be cautious about drawing too many comparisons because the viruses are different and have cropped up in different parts of the world.
SARS emerged from crowded southern China, where a taste for wild meat brought people in contact with SARS-infected civet cats. So far MERS infections have been traced only to Middle Eastern countries. While the ultimate reservoir of the virus is presumed to be bats, it is also thought that some animal species is acting as a bridge, bringing the virus from bats to people. So far, though, the source of the virus has not been identified.
Experts familiar with MERS or with the process of calculating a basic reproductive number echoed Fontanet's suggestion that people should not over-interpret the study's findings.
"The difference between what's going on with MERS now and SARS then is only that MERS doesn't seem to be moving at the same pace," said Dr. Allison McGeer, an infectious diseases expert who was involved in Toronto's battle against SARS and who has travelled twice to Saudi Arabia to help investigate its MERS outbreak.
"I just don't think there's any way of knowing" what the future holds for MERS, McGeer said. "It's going to do what it likes."
She had some concerns about the designation of cases as sporadic or linked in the Lancet paper, noting the authors suggested an outbreak in a hospital in Dammam in eastern Saudi Arabia was not linked to another large outbreak in several hospitals in Al-Hasa.
In fact, the Dammam cases are an extension of the Al-Hasa outbreak, she said, though she acknowledged that information has not been clearly stated. McGeer helped investigate the Al-Hasa outbreak and was a major author on a recent New England Journal of Medicine article on it.
Meanwhile, a commentary published with Fontanet's study stresses that the MERS virus could evolve by developing mutations that allow it to be more transmissible among people. That is known to have happened with the SARS virus.
And the commentary, by researchers from the applied mathematics department of the University of Waterloo in Waterloo, Ont., said the future behaviour of the virus could also be affected by factors like changes in the season, or events like the Hajj, in which as many as three million pilgrims from around the world flock to the Muslim faith's holiest site, Mecca.
"In terms of what the public should take home from this, it's that we're definitely not out of the woods," said Waterloo's Chris Bauch.
"It could go up not only because it (the virus) evolves but because the Hajj is coming up. And not only are you going to have slaughter of (potentially) infected animals, but you're also going to have a possibility for transporting it to other countries."
Fontanet said the priority now should be finding the source of the virus and limiting the chances it has to spread to people. The more times the virus infects people, the more chances it has to develop mutations that would help it adapt to spreading easily among humans, he said.
"It's precisely because it is not yet that transmissible that it's the perfect timing for identifying the animal reservoir and stopping it at the source," he continued.
"If that had been done for SARS 10 years ago when it was circulating in southeast China before moving to Hong Kong and the rest of the world, it might have prevented a pandemic."