While the overall number of new MERS cases reported by the Saudi MOH has been pretty low the past couple of months, for the third time in 4 weeks we have a reported household cluster.
- In week 52 the Saudi MOH reported on an 80 y.o. female in Sajir City who was hospitalized with MERS and was apparently a secondary household contact of a 45 y.o. male (with camel contact) who was hospitalized in mid-December.
- In week 01 Saudi MOH reported a second household cluster - this time in Riyadh - involving a 56 year-old male (primary) and an 86 year-old male (secondary, household) infection. Three days later a third Case (33, male) was reported to be in home isolation.
Household clusters of MERS are not particularly rare, although 3 in just over a month is a bit unusual, particularly given 70% of the cases over that time period are part of one of these clusters.
Clusters - whether in homes or healthcare settings - illustrate that in close quarters and under the right conditions, MERS transmits reasonably well between humans.The saving grace so far has been that in the broader community, overt transmission appears far less common, although there are concerns over undetected chains of (asymptomatic or mildly symptomatic) transmissions.
The yardstick by which disease transmissibility is measured is its R0 (pronounced R-nought) or Basic Reproductive Number. Essentially, the number of new cases in a susceptible population likely to arise from a single infection.
In the simplest of terms, with an R0 below 1.0, a virus (as an outbreak) begins to sputter and dies out. Above 1.0, and an outbreak can have `legs’.
While MERS has yet to become transmissible enough (at least in a community setting) to spark a genuine epidemic, a 2017 study (see A Pandemic Risk Assessment Of MERS-CoV In Saudi Arabia) provided a detailed statistical analysis of MERS-COV cases reported in Saudi Arabia from May 2013 to May 2015.
This study reported a sub-critical R0 (R: 0.85–0.97) across all regions, but found in central and western regions that at times the R0 flirted with epidemic sustaining numbers.All of which means - while apparently still not ready for prime time - MERS-CoV wouldn't need to gain a whole lot more transmissibility to become a genuine pandemic threat.
Three small household clusters in a month does not an epidemic make, but we need to remain vigilant, keeping watch for any signs that the virus might be evolving into more of a threat.