BACKGROUND: Weeks after issuing social distancing orders to suppress severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission and reduce growth in cases of severe coronavirus disease 2019 (COVID-19), all U.S. states and the District of Columbia partially or fully relaxed these measures. 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 estimated the extent to which relaxation of social distancing affected epidemic control, as indicated by the time-varying, state-specific effective reproduction number (Rt). RESULTS: In the eight weeks prior to relaxation, mean Rt declined by 0.012 units per day (95% CI, -0.013 to -0.012), and 46/51 jurisdictions achieved Rt < 1.0 by the date of relaxation. After relaxation of social distancing, Rt reversed course and began increasing by 0.007 units per day (95% CI, 0.006-0.007), reaching a mean Rt of 1.16 eight weeks later, with only 9/51 jurisdictions maintaining Rt <1.0. Parallel models showed similar reversals in the growth of COVID-19 cases and deaths. 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 SARS-CoV-2 epidemic suppression after relaxation of social distancing measures across the U.S. Premature relaxation of social distancing measures undermined the country's ability to control the disease burden associated with COVID-19
