Abstract
BackgroundMany countries worldwide are faced with the choice between the (re)surgence of COVID-19 and endangering the economic and mental well-being of their citizens. While infection numbers are monitored and measures adjusted, a systematic strategy for balancing contact restrictions and socioeconomic life in the absence of a vaccine is currently lacking.MethodsIn a mathematical model, we determine the efficacy of regional containment strategies, where contact restrictions are triggered locally in individual regions upon crossing critical infection number thresholds. Our stochastic meta-population model distinguishes between contacts within a region and cross-regional contacts. We use current data on the spread of COVID-19 in Germany, Italy, England, New York State and Florida, including the effects of their individual national lockdowns, and county population sizes obtained from census data to define individual regions. As a performance measure, we determine the number of days citizens will experience contact restrictions over the next 5 years (‘restriction time’) and compare it to an equivalent national lockdown strategy. To extract crucial parameters, we vary the proportion of cross-regional contacts (between 0% and 100%), the thresholds for initiating local measures (between 5 and 20 active infections per 100,000 inhabitants) as well as their duration after infection numbers have returned below the threshold (between 7 and 28 days). We compare performance across the five different countries and test how further subdivision of large counties into independently controlled regions of up to 100,000 or 200,000 inhabitants affects the results.FindingsOur numerical simulations show a substantially reduced restriction time for regional containment, if the effective reproduction number of SARS-CoV-2 without restrictions, R0, is only slightly larger than 1 and the proportion of cross-regional contacts (the so-called leakiness) is low. In Germany, specifically, for R0=1.14, a leakiness of 1% is sufficiently low to reduce the mean restriction time from 468 days (s.d. 3 days) for the national containment strategy to 43 days (s.d. 3 days across simulations) for the regional strategy, when local measures are initiated at 10 infections per 100,000 inhabitants in the past 7 days. For R0=1.28, the allowed leakiness for minimal restriction time reduces to approximately 0.3%. The dependence of the restriction time on the leakiness is threshold-like only for regional containment, due to cooperative effects. It rises to levels similar to the national containment strategy for a leakiness > 10% (517 days national vs. 486 days regional for leakiness 32% and R0=1.14). We find a strong correlation between the population size of each region and the experienced restriction time. For countries with large counties, this can result in only a mild reduction in restriction time for regional containment, which can only be partly compensated by lower thresholds for initiating local measures and increasing their duration. In contrast, further subdividing large counties into smaller units can ensure a strong reduction of the restriction time for the regional strategy.InterpretationThe leakiness, i.e. the proportion of cross-regional contacts, and the regional structure itself were crucial parameters for the performance of the regional strategy. Therefore, regional strategies could offer an adaptive way to contain the epidemic with fewer overall restrictions, if cross-regional infections can be kept below the critical level, which could be achieved without affecting local socioeconomic freedom. Maintaining general hygiene and contact tracing, testing should be intensified to ensure regional measures can be initiated at low infection thresholds, preventing the spread of the disease to other regions before local elimination. While such tight control could lead to more restrictions in the short run, restrictions necessary for long-term containment could be reduced by up to a factor of 10. Our open-source simulation code is freely available and can be readily adapted to other countries.FundingThis work was supported by the Max Planck Society.
Highlights
While the levels of daily new infections with COVID-19 around the world are still at an all-time high at the time of this study (June 2020) [1], many countries that were hit early on by the epidemic have demonstrated that its control is possible through populationwide contact bans and radical restrictions on public and economic life [1,2], which were deemed necessary despite their socioeconomic cost [3,4]
We found a steep decline of the required restriction time when the proportion of cross-regional contacts is reduced, which is due to cooperative effects of individual local containment measures and the discrete nature of infection events in the small-number regime
Note that we focus on the crucial parameters necessary to meaningfully compare the country-wide and the local strategy under equivalent conditions, and that, we intentionally neglect several other factors which could change the course of the epidemics between countries or over time
Summary
While the levels of daily new infections with COVID-19 around the world are still at an all-time high at the time of this study (June 2020) [1], many countries that were hit early on by the epidemic have demonstrated that its control is possible through populationwide contact bans and radical restrictions on public and economic life [1,2], which were deemed necessary despite their socioeconomic cost [3,4] The effects of these drastic interventions have been tracked and assessed in multiple ways using computational modeling and parameter inference [5À10] and several crucial factors for their efficacy have been identified, including, but not limited to, contact tracing [11], mobility [12] and age [13]. Maintaining general hygiene and contact tracing, testing should be intensified to ensure regional measures can be initiated at low infection thresholds, preventing the spread of the disease to other regions before local elimination While such tight control could lead to more restrictions in the short run, restrictions necessary
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