COVID-19 transmission in the U.S. before vs. after relaxation of state social distancing measures
Abstract
Background
Weeks after issuing social distancing orders, all U.S. states and the District of Columbia at least partially relaxed these measures. Critical unanswered questions remain about the timing of relaxation, and if and how unregulated social distancing measures can be sustained while effectively maintaining epidemic control.
Methods
We identified all statewide social distancing measures that were implemented and/or relaxed in the U.S. between March 10-July 15, 2020, triangulating data from state government and third-party sources. Using segmented linear regression, we evaluated the extent to which social distancing measure relaxation affected epidemic control, as indicated by the time-varying, state-specific effective reproduction number (R t ).
Results
In the eight weeks prior to relaxation, mean R t declined by 0.012 units per day (95% CI, -0.013 to -0.012), and 46/51 jurisdictions achieved R t < 1.0 by the date of relaxation. After relaxation of social distancing, R t reversed course and began increasing by 0.007 units per day (95% CI, 0.006-0.007), reaching a mean R t of 1.16 eight weeks later, with only 9/51 jurisdictions maintaining R t <1.0. Indicators often used to motivate relaxation at the time of relaxation (e.g. test positivity rate <5%) predicted greater post-relaxation epidemic growth.
Conclusions
We detected an immediate and significant reversal in epidemic growth gains after relaxation of social distancing measures across the U.S. These results illustrate the potential pitfalls of premature relaxation of social distancing measures in the U.S.
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