Thoughts and (many) questions on the 4 MERS-CoV UAE cases. [AMENDED]

Thanks to Prof Andrew Rambaut for pointing out my incorrect usage of R₀

4 new infections were detected from health-care workers (HCWs) in contact with a previously confirmed 82-year old male MERS patient-we've covered this previously.

My main question is: Does that mean the basic reproduction number (R₀) for MERS-CoV is creeping towards being greater than 1? If in fact 4 new infections arose from every case, on average, that would put the MERS-CoV up there with influenza, SARS-CoV and mumps.

While the R₀ is not >1, this was an informative small cluster of human infections. Basic protective measures may not have been taken in this cluster and presumably we can exclude pre-existing immunity as a moderating factor (although do we know that yet?).

I'm also assuming these 4 are otherwise "normal healthy" people, not suffering any major underlying disease(s), and so I'm having trouble imagining what stops this from happening all the time. Perhaps co-infections in which the MERS-CoV is the super-infecting (2nd) virus such that an innate and moderating immune response is already underway?

Perhaps the virus has recently changed to become more easily transmissible - we need more sequences from recent strains and more isolates in animal studies to confirm that or course. I'm not holding my breath that either of those will be achieved rapidly-although perhaps I should be holding my breath.

To me, and feel free to educate me on any of this post via Twitter (@MackayIM), this means we should expect to see more viral RNA-positives (regardless of symptoms since 2/4 here were asymptomatic) in contacts...doesn't it? What is the status of testing "less close" contacts? How far does the testing net get thrown I wonder?

Nonetheless, we don't see 4 new community infections from every case in the KSA. I'm assuming that the KSA MOH lab(s) uses the same PCR assay as that recommended by the WHO. 

Others have already noted that, on the data we have, MERS-CoV is unlikely to cause a pandemic.The most recent announcement of 2 cases of MERS-CoV provides an answer to that, one that is a typical KSA MOH site's communication:

It is worth mentioning that, the MOH has tested 64 samples recently, given that, all of such samples have been proved negative, May Allah be praised, except the cases that have been published.

No contacts positive there. So, at the end of the day, we can't extrapolate much from clusters, because when we consider the entire MERS-CoV population and all of it's tested contacts, we have an R₀ value <1. Still, the clusters do happen and this does highlight that numbers, tools and hypotheses are good as guides - they should not to be used without full regard to each specific pandemic, epidemic, outbreak, cluster or even individual case.