Figure 1. Emergence of COVID-19, Colorado, USA, 2020, showing policy-based responses (A) and definition of 4 distinct social distancing phases (B) corresponding with early closures (phase 1, March 17‒25); statewide stay-at-home (phase 2, March 26‒April 26), statewide partial transition to safer-at-home (phase 3, April 27–May 8); statewide safer-at-home (phase 4, May 5–June 3). Social distancing parameters were estimated at 4 points during March‒June by using model fitting procedures and reported case data (fits 1 and 2) and hospital census data (fits 3 and 4). In light of the 5.1 day mean incubation period, the ≈7-day lag between symptom onset and case report, and the ≈8-days between symptom onset and hospitalization based on state records, there are ≈12 and 13 day lags between infection and case report, and infection and hospitalization, respectively (gray boxes). Thus, at each model fit, we could estimate social distancing parameters reflecting transmission conditions up to 12 (fits 1 and 2) or 13 (fits 3 and 4) days before the fit date. Asterisks (*) indicate estimate generated for only part of the period. COVID-19, coronavirus disease; P, phase.