Stopping the SuperSpreader Epidemic: the lessons from SARS (with, perhaps, applications to MERS)

Wick, W. David

I discuss the so-called SuperSpreader epidemic, for which SARS is the canonical examples (and, perhaps, MERS will be another). I use simulation by an agent-based model as well as the mathematics of multi-type branching-processes to illustrate how the SS epidemic differs from the more familiar uniform epidemic (e.g., caused by influenza). The conclusions may surprise the reader: (a) The SS epidemic must be described by at least two numbers, such as the mean reproductive number (of"secondary"cases caused by a"primary case"), R0, and the variance of same, call it V0; (b) Even if R0>1, if V0>>R0 the probability that an infection-chain caused by one primary case goes extinct without intervention may be close to one (e.g., 0.97); (c) The SS epidemic may have a long"kindling period"in which sporadic cases appear (transmitted from some unknown host) and generate a cluster of cases, but the chains peter out, perhaps generating a false sense of security that a pandemic will not occur; (d) Interventions such as isolation (or contact-tracing and secondary case isolation) may prove efficacious even without driving R0 below one; (e) The efficacy of such interventions diminishes, but slowly, with increasing V0 at fixed R0. From these considerations, I argue that the SS epidemic has dynamics sufficiently distinct from the uniform case that efficacious public-health interventions can be designed even in the absence of a vaccine or other form of treatment.