Reduced susceptibility to SARS-CoV-2 in metropolitan regions

Barrett, T. J.; Patterson, K. C.; James, T. M.; Kruger, P.

The coronavirus pandemic is wreaking public health, social, and economic havoc across the globe, and to date a variety of strategies have been implemented to attempt to control the spread of disease [1, 2]. A critical unknown for policy planning is the number of people who have been infected and are no longer susceptible [3]. Tests for active SARS-CoV-2 infection or antibody presence can provide an indication, but both are prone to selection bias, under-representative population sampling and insufficient reliability [4, 5]. Here, we present an alternative to determine residual susceptibilities based on the analysis of observed population-wide disease dynamics data. For four highly-affected countries, we directly compared the dynamics in the largest metropolitan regions with the rest of the countries. We show that substantial susceptibility reductions are measurable in the metropolitan regions, which all continued in a phase of exponential growth of case numbers for a relatively longer time before public health interventions were introduced. Compared to these interventions, the reduction in metropolitan region susceptibility had a substantial role in the post-growth decline in infection rates. Reduced population susceptibility has far reaching consequences on future policy responses and disease forecasts including vaccine trial planning and, in the case of a second epidemic wave, higher population-normalised mortality rates for non-metropolitan regions.