Last week, I took another (more informed) shot at exploring the relationship between comorbidity and mortality among MERS cases – both during this spring’s outbreak and prior to it. I ended the post by promising further insight into two potential confounders – age & sex. We’ll start our discussion on age here today… But I present the following information with a very important caveat. Earlier this week, 113 previously unannounced cases (and 92 deaths) were added to the KSA MERS tally – but we don’t know much about them yet. Because of this, I couldn’t include these newly reported cases in the analysis below; when we learn more about them, I’ll revisit and revise as necessary. The situation is constantly changing and what we think we know now may be flipped on its head come the morning!
With this in mind, let’s now take a closer look at the age distribution of MERS cases in KSA, as well as how age interacts with comorbidity and mortality:
From these charts, a couple of general trends jump out immediately (based off of the sample represented):
1. The older a person is, the higher his/her chance of dying from MERS*.
2. Likewise, the younger a person is, the less likely he/she is to have comorbidities*.
3. Youth (≤18 years old) are largely under-represented; this is especially the case for children under 5.
Observations 1 & 2 aren’t really that unusual; they make sense intuitively**. However, Observation 3 is perplexing… Children under 5 get sick often due to their still-under-developed immune systems… And because they’re constantly putting things in their eyes, ears, and mouths that they really shouldn’t be. This is particularly relevant in the context of acute respiratory infections (ARIs), which remain the leading cause of illness and death among children in this age group – worldwide… Now, given that MERS is a respiratory infection – and that nearly a third of the Saudi population is under the age of 14 and more than 10% are under the age of 5 – shouldn’t we be seeing a much larger number of cases in young children..?
Though they may not be herding camels and working in hospitals, I find it hard to believe that only 4 children under 5 have contracted MERS so far in Saudi Arabia. After all, the middle-aged and elderly individuals who have made up the bulk of reported KSA MERS cases thus far would have inevitably exposed the young people in their homes to MERS-CoV before receiving treatment… And as it stands, there exists no sound mechanistic explanation for why young children may be protected from contracting the virus^. So for the time being, the question isn’t whether or not there are more cases under 5, but rather – where are they?
…I guess we’ll just have to wait and find out^^.
*Of the 4 children under 5 that have been reported as KSA MERS cases, 2 had comorbidities; 1 of these children died, but the second death occurred in an infant who was otherwise healthy (2 deaths total). At 50% each, the fatality and comorbidity rates are quite high; however, because there are only 4 cases in this group, the impact of comorbidity on mortality in children under 5 is inconclusive.
**This said, if the age distribution today is substantially different from what it was a year ago, it would have serious implications on how the impact of comorbidity on mortality has changed over time. I’ll explore this in depth at a later date. Check back soon!
^However, because children have less developed immune systems, their immune reactions may not be as strong against MERS-CoV infection… Resulting in asymptomatic and mild cases that would likely go undetected.
^^In the past, SARS has also demonstrated an unusually low rate of detections in children. However, a well-established mechanism for why this might be the case has yet to surface in the literature. The way I see it, SARS and MERS are anomalous when compared against other ARIs in children under 5.