MERS update: two unsettling developments

Things have generally been quiet on the MERS front in the last several weeks. Authorities are discouraging Muslims from making the hajj pilgrimage this year, for fear of a large MERS outbreak in close quarters. In the meantime, medical researchers have been hard at work, and in the past few days two interesting results have come out.

First, a researcher has discovered MERS virus fragments airborne in a camel barn in Saudi Arabia. One of the happy surprises about MERS has been that (unlike its predecessor SARS) it is difficult to pass on to others; it did not seem to be airborne, and even person-to-person contact didn’t often cause it to spread. So this latest research night be a sign that it’s mutating into an airborne version. That could be very bad, though often when viruses mutate to a more easily transmissible version they become less dangerous at the same time: more people will get infected but fewer will get sick and die.  Now this is early research and needs to be replicated before it’s trustworthy, so don’t panic yet.

Second, an epidemiologist has done an analysis of the spread of MERS so far, and she came to a very interesting conclusion.  In general, MERS is confusing because it isn’t following the pattern of spread that we would expect from a virus.  The researcher puts together a very informative table listing various aspects of the MERS cases that have appeared to-date and classified them as to the patterns they fit:

 

Supporting evidence of the epidemiologic pattern of MERS-CoV

Supporting evidence of epidemiologic pattern Epidemic (human to human) Sporadic, animal source Sporadic, deliberate release
Low case numbers for 12 months  ✓  ✓
Low estimates of R0  ✓  ✓
Long persistence despite low R0  ✓  ✓
Some person-to-person transmission documented  ✓ ? ?
Hospital outbreaks  ✓ ?a
No epidemics arising from mass gatherings  ✓ ?
Evidence of multiple introductions in a single outbreak  ✓
Several cases without an identified epidemiologic link to a human case of MERS-CoV  ✓  ✓
Several cases without an identified epidemiologic link to a zoonotic source ?  ✓
Several cases with no link to human OR zoonotic source  ✓
MERS-CoV identified in camels  ✓
Multiple genetic strains in a single hospital outbreak at Al Ahsa Hospital ?b  ✓
Active surveillance had not found evidence of a high proportion of undetected cases  ✓  ✓

So let’s walk through this. Scientists classify infectious diseases as either:

  • epidemic: defined by “a rapid rise in cases over time, not by the total number of cases.”  One would expect to see a normal curve of cases as it spreads through a population and eventually hits its saturation point. Small epidemics are “outbreaks” and large ones are “pandemics.”
  • sporadic: occurring occasionally and with low case numbers.
  • endemic: present for long periods of time a population but don’t display rapid rises over a short period of time (e.g. malaria).

There’s no evidence that MERS is endemic, since it was first seen only two years ago. And MERS isn’t fitting the model of an epidemic disease: whereas SARS took off and largely died out within a period of 8 months, MERS hung around at barely noticeable levels for over two years before it had any kind of spike at all.

sars_vs

source: C. Raina MacIntyre, http://link.springer.com/article/10.1007/s10669-014-9506-5/fulltext.html

What’s even more strange is that when it did start to gain some traction, it did so in some very peculiar ways. There was a hospital outbreak, but only one, and several of the people who were infected had no clear tie back to any other infected patient or animal. What is even stranger was that among the patients in the hospital outbreak there were three distinct variations of MERS present. So the only thing that strongly links those patients is the timing of their infections. And let’s not forget the point I raised earlier: the hajj in Saudi Arabia should led to a huge outbreak of MERS in 2012 and 2013, but it didn’t.

Now you might argue that there could be a large number of infected people who are non-symptomatic — walking carriers. But testing has not shown that to be the case. So the number of cases we’ve seen is fairly representative of the real number of cases.

So MERS fits closest, but certainly not perfectly, in the “sporadic” category. Within “sporadic” the researcher looks at two subdivisions: the virus can be traced back to an animal source, and the virus was released into the population by humans (the researcher says “deliberately” but I’m not willing to go there yet). Both are plausible but still not perfect fits — refer back to the table above.

The recent news I referred to at the beginning about the possibility that MERS has become airborne increases the plausibility of “sporadic, animal sourced” but it still doesn’t explain all of the known cases.  Conversely, the fact that it’s found in camels with a connection to some of the cases argues against “sporadic, released.” Though the researcher does point out that if it was originally cultured from camels and released into the population, it is not at all far-fetched to assume that it would eventually find its way back to the camel population and infect it widely.

So what should we take away from this? More than anything, that we really don’t understand MERS — right now it isn’t following a pattern that is at all familiar to us. And probably no potential origin should be eliminated at this point; no case is strong enough. Certainly no one should declare that MERS is a bioterrorist attack and start a panic, because the evidence does not support that conclusion. This isn’t good news, and it’s always unsettling when someone suggests that we can’t rule out the possibility that someone deliberately released a virus, but in the end this just points out how much we still don’t understand about MERS even 27 months after its first appearance.

We will know more in the coming months. For now, keep washing your hands, and practice good hygiene if you travel to the middle east.

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