What We Do and Don’t Know About the MERS Virus

MERS-CoV was first identified in Saudi Arabia in 2012Over the past few decades, human activity has accelerated the proliferation of pathogens, the speed with which they mutate, and their capacity to adapt to new environments and hosts. In the past three decades alone, scientists have described 33 new human pathogens, including the human immunodeficiency virus. Among the most recently identified viruses is the Middle East respiratory syndrome coronavirus (MERS-CoV). Just some weeks ago, a traveller from the Middle East spread the virus to South Korea, sparking a MERS outbreak of troubling proportions: 175 cases have been confirmed, 27 people have died, and more than 2,800 patients have been placed in quarantine.

What We Know

The case fatality rate for MERS—36%, according to the World Health Organisation (WHO)—is worryingly highMERS-CoV was first identified in Saudi Arabia in 2012. The virus causes a respiratory illness that is mild or even asymptomatic in some patients but can be fatal in others. The case fatality rate for MERS—36%, according to the World Health Organisation (WHO)—is worryingly high (in Ebola virus disease, for example, this figure is 60%). As with Ebola, as well as other coronavirus infections in humans, there are no approved antiviral drugs or vaccines against MERS-CoV. In three years, there have been 1,333 cases of MERS worldwide and 471 people have died. (As one expert noted, H5N1 bird flu has killed the same number of people in 11 years.) The vast majority of MERS cases have been reported in the Middle East, particularly in Saudi Arabia, although imported cases have been reported in 26 other countries. The last countries to report MERS cases were South Korea (site of the largest outbreak outside of the Middle East) China (where one case, imported from South Korea, has been reported) and Thailand (one imported case from Oman). The recent outbreaks show that, despite its limited geographic range, MERS-CoV is a public health threat of global significance. And let us not forget that 2-3 million people from more than 100 countries make a pilgrimage to Saudi Arabia each year.

What We Don’t Know

We do not know exactly how the virus spreads from person to person, although respiratory droplets and direct or indirect contact are thought to be possible modes of transmissionThere is evidence—albeit inconclusive—that camels are the animal reservoir for human infections (MERS-CoV antibodies have been found in camels in the Middle East and even in the Canary Islands) and that the virus occasionally jumps to humans. A recent study found that some people who work with camels carry MERS-CoV antibodies despite showing no symptoms of illness. These findings suggest that asymptomatic individuals could transmit the virus to the rest of the population; this would explain why so many patients have been infected despite having had no contact with camels. One problem is that we do not know exactly how the virus spreads from person to person, although respiratory droplets and direct or indirect contact are thought to be possible modes of transmission. Because the virus is relatively stable, it could also be transmitted via contaminated surfaces. What is clear is that transmission is most common in hospital settings. With the exception of a few people infected by close relatives, all of the South Korean patients were infected in hospitals where other patients were being treated for MERS. Fortunately, unlike the severe acute respiratory syndrome (SARS) coronavirus, transmission of MERS-CoV does not appear to be associated with air travel. Therefore, although MERS appears to be less contagious than other respiratory diseases—it has a basic reproduction number of around 1, compared with 3 to 4 for SARS—it appears to spread more easily in hospitals.

Reasons for Concern?

Experts say that the current MERS outbreak in South Korea is not a global health threatIn the Middle East, the incidence of MERS has increased in the spring every year for the past three years. This year, however, for reasons unknown, the seasonal peak came earlier and in more areas of Saudi Arabia. In a notable example of international collaboration, a team of researchers recently sequenced the genome of the MERS virus found in South Korea and China but found no evidence of mutations that could explain the increase in transmission. Experts say that the current MERS outbreak in South Korea is not a global health threat, for several reasons. First, MERS-CoV is primarily an animal virus that occasionally jumps to humans. Second, MERS-CoV is less transmissible than viruses like SARS because it infects deeper areas of the lungs and is therefore not easily expelled by coughing. Only in hospital settings, where aerosols are generated by procedures such as mechanical ventilation, is the transmission rate higher. Third, the current MERS outbreak is relatively small. In the spring of 2014, a larger outbreak in Jeddah, Saudi Arabia, led to 255 cases. The large number of cases in South Korea may be due to the thoroughness of the country’s response: all contacts of infected patients are being traced and all cases are being detected, even those with mild symptoms.

Still, when it comes to viruses, nothing is set in stone. We have no way of knowing whether MERS-CoV will mutate in a way that increases its transmission rate or its virulence. Therefore, regardless of how the current outbreak unfolds over the next few months, clinicians, epidemiologists and scientists need to work closely together to answer questions about the transmission and epidemiology of MERS before it is too late. In an encouraging trend, the WHO—in an attempt to prevent emergencies like the Ebola crisis—is currently working to identify pathogens that warrant more in depth research with a view to ensuring that treatments or vaccines will be available for large-scale testing in the event of a new outbreak. It is good to know that MERS-CoV is one of the candidates for their list.