MERS outbreak in Korea possibly caused by a "superspreader"

MERS (Middle East Respiratory Syndrome) is back in the news thanks to a recent outbreak in South Korea that has resulted in 9 deaths and over 100 cases (Reuters). Some have claimed that the virus may have mutated and a pandemic is possible. Let's calm down and look at the facts.

Last summer, MERS made the headlines with an outbreak in Saudi Arabia resulting in nearly 700 cases and approximately 300 deaths (Figure 1). Saudi Arabia has been the epicenter of MERS since originating there in 2012. Almost all new cases are located in Saudi Arabia perhaps because of a persistent population of infected individuals and the presence of the natural reservoir of the virus (bats and camels). Thus, in my post on MERS last year, I concluded with the statement: "Unless you plan to travel to the Middle East (i.e. Saudi Arabia) you don't have to worry about MERS."

Unfortunately, this statement is no longer true at this moment because MERS has spread to a second country, Korea. However, before getting into the details of the recent outbreak, let's first review some salient background information:

1. MERS is caused by coronavirus which is the same general type of virus that causes some common colds (link) and that is closely related to the SARS virus which was quite deadly (NYTimes). In 2002 in China, "SARS infected 8,273 people in 37 countries and killed 775 of them, a mortality rate of nearly 10 percent, before basically disappearing in early 2004."

2. The mortality rate for MERS is as high as 30%. "Patients usually die from high fevers and pneumonia. The disease appears more likely to kill people with underlying conditions like diabetes, kidney failure and breathing problems." There is no cure or vaccine and so the symptoms are treated.

3. Most of the spread appears to be among close family members and within hospitals. It is not considered highly infectious; the reproduction number for MERS is 1, i.e. the typical infected person will spread MERS to one other person. By comparison, the reproduction number for influenza and Ebola is about 2, and for polio and smallpox, it is around 5 to 8. For measles it is much higher, somewhere between 10 and 20. MERS mainly infects those who breathe in large amounts of virus from an infected person.

Apparently last month, a businessman, age 68, picked up MERS in the Middle East and brought it back to South Korea. It was the first time MERS was in the country. He got sick with what was thought to be pneumonia and visited several hospitals before he was correctly diagnosed. During this time he may have infected more than 20 people.

There are two general explanations for this new outbreak. One possibility is that the virus has mutated to become as contagious as a cold (as noted above some colds are caused by a different type of coronavirus) which could be a frightening prospect. Another possibility is that the outbreak was caused by a "superspreader" who possessed a high concentration of virus in his body, and could therefore spread it more easily than a person who a lower level of virus.

An NPR article addresses these two possibilities interviewing several medical experts. It turns out the "mutated virus" scenario is more scary but less likely:

"It's always possible that a virus can change. That's a general rule," says Christian Drosten, a virologist at the University of Bonn in Germany. But it takes not just one but several changes for a virus to become more contagious. "And the probability for these to happen together is really very, very low," he says.

The more likely possibility was that the man who originally contracted the virus in Saudi Arabia and brought it back to Korea was a "superspreader".

"If we look at data that we have [from Saudi Arabia] that are not published yet, what we can say is there are some patients that have extraordinarily high viral loads," Drostan says. And when these so-called superspreaders cough, he says, they can infect many people, sometimes a dozen or more. "Maybe the index case in Korea was one of those superspreaders," Drosten says.

Indeed according to reports (NYTimes), the man who originally contracted the virus visited 4 different hospitals during a 9 day period before doctors realized that he had MERS. Korean hospitals are notoriously crowded, and thus he was in close proximity with many people some of whom may have had weakened immune systems. He could have infected dozens of people who in turn could have infected dozens more before they were alerted to the danger.

The good news is that the superspreader scenario makes it more likely the outbreak will be contained once the superspreader has been identified and quarantined along with those he has infected. Then the number of new cases is likely to drop as long as the public health system moves quickly to isolate any new patients along with anyone else who might have been in contact with an infected individual (Reuters):

'Stephen Morse, professor of epidemiology at Columbia University, added that measures being taken such as isolating suspected cases and monitoring those in contact with patients should stop the outbreak. "There will be a time lag, since the disease takes some days to develop, but if the proper measures are being taken, there should soon be a large falloff in the number of new cases," he said.'

Thus, one can expect the MERS outbreak to be contained in the next month or so, making it safe for people in Korea and for those planning to visit Korea this Summer.

Figure 1. Number of MERS cases by month. The outbreak in Saudi Arabia last summer (April, 2014) is clearly indicated. The current outbreak in Korea has not been recorded on this graph since most of the cases have occurred in June (Lancet, via Vox.com).