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Today the Saudi MOH announced the fourth MERS case in less than a week from the capital city of Riyadh, although they have provided no information regarding their possible routes of exposure. A second case from Dammam, is also announced, again with very limited information.
Although we have a pretty fair idea how MERS spreads in a hospital environment, and suspect a small percentage of community acquired cases are the result of exposure to infected animals (primarily camels), for the vast majority of community acquired cases we still don’t know their route of exposure.
The lack of obvious epidemiological links between many cases raises the question of transmission of the virus from mildly symptomatic (or asymptomatic) carriers of the virus. We know from limited testing of contacts of known cases that some percentage (roughly 20%) can be infected and show little or no outward signs of illness.
Whether these people can transmit the virus onward isn’t known, but we’ve seen some hints that it may be a factor (see Study: Possible Transmission From Asymptomatic MERS-CoV Case).
Normally we’d get answers to these questions from a well mounted case-control study, where epidemiologists would compare laboratory-confirmed cases to a large number of controls, matched for age, sex, and by neighborhood. By examining their respective exposures against their outcomes, patterns of risk are often revealed.
Despite repeated promises to conduct this type of study (see KSA Announces Start To Long-Awaited MERS Case Control Study), for whatever reason, we’ve yet to see one published.
All of which means - that more than three years after the first Middle Eastern outbreak of MERS - we still don’t know how it is spreading, and the best ways to stop it.