Thursday, 17 April 2014

MERS-CoV numbers by week...

There is a lot of buzz about the Middle East respiratory syndrome coronavirus (MERS-CoV) right now.

Laboratory-confirmed MERS-CoV Cases per week
(dark green, left y-axis) and the cumulative average of cases
(pale green; right y-axis). Note: recent cases have yet to 

have date of onset reported. Last 2 points may drop as 
cases are assigned to onset in earlier weeks; trend 
will likely remain.
Click on image to enlarge.
This is because the number of human cases appear to be sky-rocketing. Charting those numbers, as I've done here with a new cases/week and accompanying cumulative average (at each weekly data-point I calculate the average of all cases before it and plot that average). The chart does support the feeling of a spike although see my caveat in the chart's legend.

But there are a few things to remember when looking at these charts:

  1. We're talking about ~270 cases in a region  (the "Middle East) with ~400 million[1] inhabitants; just 0.0000675% of people are noted as positive for this virus. While the recent surge in cases makes the chart look steep (and is definitely worthy of being watched, analysed and some work done to understand it), laboratory confirmed MERS-CoV cases are still a drop in the ocean of humanity
  2. The surge in numbers is being driven by two clusters; One in the United Arab Emirates involving ~12 healthcare workers (HCWs) and an unidentified source (proposed as 1 or more ill patients) and the other in Jeddah, Kingdom of Saudi Arabia involving ~38 cases (from 2,517 suspect cases and contacts having been tested) in a hospital outbreak, also with many HCWs.[2]
  3. In these 2 clusters, as in other MERS clusters, we may be seeing a much more representative spectrum of the clinical impact which infection by MERS-CoV is capable of. This, in my opinion anyway, is important because it tells a very different story to that of the severe disease we have mostly seen in cases among older males weakened by pre-existing disease, a group in which the virus wreaks a special kind of havoc
  4. We still have very little to no data on what "normal" respiratory viruses do among older males with underlying disease in the KSA. That will significantly inform our understanding of the capacity of MERS-CoV to cause disease. Perhaps many of the >200 human respiratory viruses we know of cause just as much severe disease as MERS-CoV seems to. Don't bother pointing and laughing - show me the data to support your argument against that statement.
  5. Has testing changed and is it contributing to the numbers? Early on, contacts of a confirmed case were "observed" (looked at for overt signs of disease - a sometimes subjective  process) and now they are being more actively sampled and tested using laboratory methods; the only way we know what virus is present in a patient...when the sample is correct and they work and other caveats. It would be helpful to know when that testing process changed and whether testing bias, along with sporadic clusters, is a big contributor to the continually climbing cumulative average seen in the chart above.
Finally just a comment. 2-years after the discovery of MERS-CoV we are still seeing large scale hospital-related outbreaks like this one in Jeddah and the one among HCWs in the UAE. 

Potentially these outbreaks are triggered when 1 case infects many people. Perhaps these are so-called "super-spreader" events (1 person infects many). I need to read more to understand those better and whether they happens with any virus but we focus more on it in emerging virus outbreaks. 

But why is this happening? It's not as though hospitals are not well aware of standard infection prevention and control practices for handling patients with respiratory illnesses of unknown origin. Respiratory illnesses make up a big part of hospital business. MERS-CoV is not the only, nor the most frequent, pathogen in this class. I suspect seasonal respiratory viruses kill many people in the region too. But do we know that do or that they don't when compared to MERS-CoV? Some contrast here would be very valuable. Studies of respiratory viruses, using PCR-based methods to look at cases of pneumonia, from the Arabian peninsula are more rare human cases of MERS-CoV. 

What's more scary than rising case numbers for me is the fact the these hospital clusters keep on happening.