Fundamental limits on inferring epidemic resurgence in real time

Parag, K. V.; Donnelly, C. A.

We find that epidemic resurgence, defined as an upswing in the effective reproduction number (R) of the contagion from subcritical to supercritical values, is fundamentally difficult to detect in real time. Intrinsic latencies in pathogen transmission, coupled with often smaller incidence across periods of subcritical spread mean that resurgence cannot be reliably detected without significant delays, even if case reporting is perfect. This belies epidemic suppression (where R falls from supercritical to subcritical values), which can be ascertained 5-10 times more rapidly. These innate limits on detecting resurgence only worsen when spatial or demographic heterogeneities are incorporated. Consequently, we argue that resurgence is more effectively handled proactively, at the expense of false alarms. Responses to recrudescent infections or emerging variants of concern will more likely be timely if informed by improved syndromic surveillance systems than by optimised mathematical models of epidemic spread.