As I’ve noted in previous posts, it seems like the comorbidity and mortality rates associated with MERS has dropped off drastically since the onset of the current outbreak this spring. The two rates themselves go hand-in-hand; people with comorbidities are 80% more likely to die from MERS than those without. This makes a lot of sense; individuals with systemic issues like diabetes, heart disease, etc. have compromised immune systems, making it harder for them to fight off the infection once they have it.
We thought for a little while that the large proportion of asymptomatic cases that have been diagnosed since recently might have been responsible for decreases in these rates. However, after taking a closer look, that turned out not to be the case.
It remains unclear why more healthy people have been getting sick during this outbreak than in the past (though it does partly explain why the case fatality rate has been significantly lower than usual these past few weeks). [And I’m still searching for hypotheses that might explain it!] So, to help us along on our collective journey, I built another visual aid. [Surprise, surprise.]
I only included symptomatic cases in this chart, and instead of using absolute numbers as I’ve done in the past, I plotted rates. [That’s what we’re interested in, after all!] A couple of things jumped out at me right away:
1. Monthly mean values for both rates were very high in the 5 months preceding the current outbreak… But since April, mean comorbidity has been cut in half and mean mortality even less so than that [and still falling]! (Nevertheless, we have to wait till the cases that are still in the hospital make it home safe before we can say anything definitive about case fatality for recent patients.)
2. …However, upon closer inspection, it seems as though the downward trajectory for both rates started several months ago – around December-2013/January-2014.
3. But perhaps most importantly, both mortality and comorbidity have historically been quite variable; there have been plenty of dips and peaks in the past. [Nevertheless, this is the lowest both have been since MERS started to enter double digit cumulative cases in January 2013.]
So, what does this all mean? To be honest, I’m not certain… But I’m starting to think that the current “lulls” we’re seeing in our rates of interest aren’t as unusual as they seem – at least, not for this particular disease. From its short but erratic history, its obvious that MERS exhibits great variability when it comes to comorbidity and mortality – some of which can most certainly be attributed to “human factors” like changes in public awareness and management of the disease. What I have yet to determine is whether or not there’s any further method to the madness… And if there is, what that method might be.
[Got a theory? Get in touch! If possible, I’ll run the numbers and see if you might be on to something.]
Note: Temporal data indicates date of onset, hospitalization, diagnosis, or public reporting.
Data Sources: KSA MoH & WHO
Please Note: Data are highly imperfect and subject to change.