Below is the latest version of the nosocomial cluster diagram using data from the MERS-CoV DON released today. Click-through for high-resolution! (A few thoughts follow.)
1. We’re starting to see some tertiary (sustained) transmission, which has been pretty uncommon thus far in the short history of MERS. A tertiary case cluster worth watching (initiated by Case #16, who was originally infected by index Case #1) is boxed in the upper-right corner of the diagram.
2. The index case has (so far) infected 27 people, which places him on the far right-tail of the secondary transmission distribution for MERS-CoV (namely, the number of secondary infections caused by any one primary case)*. Statistically speaking, this suggests that this 27-person secondary case cluster is an improbable (but high-impact) event. The tertiary cluster mentioned above also falls into this category, though it’s not likely to be quite as rare as the secondary cluster; however, the bigger it gets, the less probable it becomes. *The mean of this distribution is typically defined as the basic reproductive number of the disease in question.
3. I’ve been receiving a lot of questions about why the secondary case cluster (i.e. infections caused by index Case #1) has gotten so massive (and so fast). One popular theory is that some people have an unusually high viral load after infection and/or shed the virus at an unusually high rate. This may certainly be a contributing factor. However, it is important to remember that a super-shedder can only become a super-spreader in settings where infection control and prevention (ICP) is sub-standard. Context matters!