The Middle East respiratory syndrome (MERS) coronavirus (CoV) has now been known of for 6 consecutive Hajj mass gatherings (see the bar graphs).
To date there has not been a single MERS outbreak among the 2 million+ pilgrims either while they are onsite or after the travellers return to their many home countries.
Its fair to say that given the extensive and luxurious opportunities for spread, MERS-CoV is not a great transmitter. But we also have to say that such a comment skitters over a simple fact; we don’t really know much about how MERS-CoV moves around outside of the hospital or farm. Even in these two sites of known transmission, we have yet to confidently identify the precise mechanisms of transmission (but probably droplets and self-inoculation via contaminated surfaces). It may be that MERS-CoV has spread wildly through past Hajj gatherings, but has caused little or no obvious illness – and therefore little signal to attract testing.
In general, MERS cases and their contacts are chased down for testing because of the obviously sick initial case. We also know that MERS-CoV spreads in the household. But we have yet to see data from:
- long-term, planned prospective studies
- screening people for MERS-CoV
- sampling people without known links to MERS cases
- with no or mild acute respiratory illnesses
- who have not sought medical attention
- in the Kingdom of Saudi Arabia (KSA; where 83% of MERS detections have occurred)
We’re still unclear on how widespread MERS-CoV transmission is in the community at large in the KSA – if it is at all. Contact tracing of cases seeking medical attention often finds mild or subclinical infections; they certainly do occur. But the ongoing question is whether such ‘cold and flu-like illness’ cases contribute to the human transmission cycle of MERS-CoV? The assumption is that there is too little virus released by these mildly ill people to be of concern for community transmission. At least one study of the contacts of an asymptomatic MERS-CoV PCR positive person appeared to confirm this assumption.According to the KSA Ministry of Health, since 2015, ‘Primary’ cases have made up nearly half of all MERS cases there.(see the pie graph) That portion excludes cases acquired by contact with a known case, acquisition in hospital or in the household. The Primary category includes the animal-to-human transmission group (which have not been publicly reported in such high numbers), a group acquiring virus after exposure to humans with unreported/undocumented infection roaming the wider community and perhaps a group who self-inoculate after contact with contaminated surfaces. The first group may suffer from under-reporting or overly restrictive classification of what constitutes “contact” with animals. There remains little information on the second group or how extensive it might be. This is where mild or asymptomatic cases may theoretically contribute.
We have seen studies that use antibody testing to seek out how much MERS-CoV might have already occurred in the wider community; there is some but apparently little. There are two things that suggest antibody studies may be giving us a false sense of security. Firstly, data that show the antibody response to MERS-CoV infection can be low, delayed and short-lived.[11,16,17] Secondly, in milder cases, there may be no detectable antibody response at all.[11,12,13,16] But remember, we know that there is virus. These may be underestimates related to the insensitivity of our methods to detect low levels of antibody, or simply that this virus does not trigger higher levels of antibody.
Studies on returning Hajjis highlight that influenza viruses and rhinoviruses are the current real concern among those with symptomatic illness.[1-10,15] MERS-CoV is not.
All of that aside, we now wait for 1-2 weeks (incubation period for MERS) to see if any outbreaks occur after Hajj-2017. If they do occur – will it be bad luck, the usual failed infection control or a significantly changed virus? Let’s see what the first banner headline says.