Regional COVID-19 spread despite expected declines: how mitigation is hindered by spatio-temporal variation in local control measures.

Successful public health regimes for COVID-19 push below unity long-term global Rt -- the average number of secondary cases caused by an infectious individual. Most assessments use local information. Populations differ in Rt, amongst themselves and over time. We use a SIR model for two populations to make the conceptual point that even if each locality averages Rt