Abstract
Background All fifty U.S. states and the District of Columbia implemented social distancing measures that interrupted transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and reduced morbidity and mortality from coronavirus disease 2019 (COVID-19). All 51 jurisdictions have since begun to relax these measures, albeit in an uncoordinated fashion.
Methods We obtained state-level data on implementation and relaxation of social distancing measures. We fitted mixed-effects linear regression models with a random effect for state, specifying the time-varying, state-specific effective reproduction number (Rt, the expected number of secondary infections generated by each index case given date-specific population susceptibility and risk behavior). Explanatory variables included time in days, implementation period (1-21 days prior to first relaxation of any statewide social distancing measure vs. time from relaxation through May 28, 2020), and a time-by-period product term.
Results During the three weeks prior to relaxation, the estimated Rt was declining by an average of 0.0016 units per day (95% confidence interval [CI], −0.0025 to −0.0008). At the time of initial relaxation, states had reported a median 7,628 cases (range, 337-361,313) and 289 COVID-attributable deaths (range, 7-28,663), and the median estimated Rt was 1.0 (range, 0.8-1.3) (i.e., the median rate of epidemic growth across states was estimated to be zero). After the first relaxation of statewide social distancing measures, the estimated Rt began increasing by an average of 0.0047 units per day (95% CI, 0.0037 to 0.0058) compared with the pre-relaxation period. If these trends continue, the estimated state-level Rt would reach an average value of 1.05 (95% CI, 1.03-1.07) by 30 days after relaxation and 1.15 (95% CI, 1.11-1.18) by 60 days. A similar upward trajectory in the estimated Rt was observed after relaxation of statewide restrictions on internal movement.
Conclusions Relaxation of U.S. statewide social distancing measures was associated with a subsequent reversal of an earlier downward trend in transmission of SARS-CoV-2, particularly in states with less active epidemics at the time of relaxation.
Competing Interest Statement
The authors have declared no competing interest.
Funding Statement
Sir Henry Dale Fellowship, Wellcome Trust (UK); Sullivan Family Foundation
Author Declarations
I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.
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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:
This ecological analysis was based on publicly available data and was exempt from ethical review.
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Data Availability
The data on social distancing policies are contained within the Supplementary Appendix Table. The data on COVID-19 cases and deaths are publicly available from https://github.com/nytimes/covid-19-data.