The combination household/hospital cluster of MERS cases in Wadi Al Dawasir we've been following for a week has grown to 10 cases; with the addition of a second asymptomatic health care worker.
The most recent report - technically yesterday's - was posted overnight on the Saudi MOH website. It also announces one death, a 71 year old male from Al Baha.
Of the 10 cases we've seen in this cluster 2 are listed as `Under Review' (possibly household members), 4 are listed as `Secondary Hospital Acquired (patients)', 2 are described as `Secondary Household contacts' and we now have 2 asymptomatic HCWs.
Last summer we saw a large hospital outbreak (see Saudi MOH On KKUH MERS Outbreak & Increased Asymptomatic Detections) in Riyadh, which involved at least 28 cases - 21 of whom were asymptomatic.
While asymptomatic (or perhaps very mild) cases have been reported previously by the Saudi MOH, their incidence was roughly 1 in 10 cases. So the KKUH outbreaks was quite a departure from the norm.
Most of those were detected through RT-PCR testing of close contacts of symptomatic patients, although until late summer of 2015, the Saudi MOH didn't appear to be aggressively looking for asymptomatic cases.
This came to a head very publicly in September of 2015 with the WHO Statement On The 10th Meeting Of the IHR Emergency Committee On MERS, which rebuked the Saudis for their handling of asymptomatic cases (among other issues).
Since then, the Saudis have been far more diligent in seeking out, and reporting, asymptomatic cases. But this does make it difficult to compare rates today to rates reported in previous years.
Like most viral infections, MERS-CoV can produce a wide range of illness, and so it is likely that many mild or asymptomatic cases go undetected. For now, the role of mild or asymptomatic cases in spreading the virus remains suspected, but largely unproven.