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Volume 27, Number 9—September 2021

Estimating the Impact of Statewide Policies to Reduce Spread of Severe Acute Respiratory Syndrome Coronavirus 2 in Real Time, Colorado, USA

Andrea G. Buchwald1, Jude Bayham, Jimi Adams, David Bortz, Kathryn Colborn, Olivia Zarella, Meghan Buran, Jonathan Samet, Debashis Ghosh, Rachel Herlihy, and Elizabeth J. CarltonComments to Author 
Author affiliations: Colorado School of Public Health, Aurora, Colorado, USA (A.G. Buchwald, O. Zarella, M. Buran, J. Samet, D. Ghosh, E.J. Carlton); Colorado State University, Fort Collins, Colorado, USA (J. Bayham); University of Colorado, Denver, Colorado, USA (J. Adams); University of Colorado, Boulder, Colorado, USA (D. Bortz); University of Colorado School of Medicine, Aurora (K. Colborn); Colorado Department of Public Health and Environment, Denver (R. Herlihy)

Main Article

Table 2

Model-estimated levels of social distancing, mask wearing and other parameter values at 4 time points over the course of the SARS-CoV-2 epidemic, Colorado, USA, 2020*

Characteristic Range of possible values and sources (ref.) Fitted value as of Apr 3† (95% CI) Fitted value as of Apr 16† (95% CI) Fitted value as of May 15† (95% CI) Fitted value as of Jun 16† (95% CI)
Estimated effectiveness of social distancing
Phase 1: early closures, Mar 17‒25, %‡ 10‒70 45 (42‒53) 65 (63‒72) 52 (49‒66) 52 (52‒53)
Phase 2: state-wide stay-at-home, 
Mar 26–Apr 26, % 50‒99 NA 76 (72‒77) 80 (80‒83) 81 (80‒82)
Phase 3: half of state under stay-at-home, 
half transitioned to safer at home, 
Apr 27–May 8, % 45‒99 NA NA 80 (78‒84) 85 (83‒90)
Phase 4: statewide safer at home, 
May 9–Jun 3, %§
90 (85‒93)
Proportion of population wearing masks 
starting Apr 4
Other parameter values
Rate of infection 0.2‒0.6 (24) 0.41
Reduction in infectious contacts due to 
symptomatic persons who self-isolate after 
March 5 0.3‒0.8 (15) 0.38
Ratio of infectiousness for symptomatic vs. 
asymptomatic persons 1.0‒4.0 (25,26) 2.27
Probability that symptomatic cases are identified by state surveillance 0.05‒0.6 (24) 0.28

*COVID-19, coronavirus disease; NA, not available; ref, reference; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2. †Fit 1 uses all reported SARS-CoV-2 cases as of Apr 3 with an onset date of Mar 26 or earlier; fit 2 uses all reported cases as of Apr 16 with an onset date of Apr 8 or earlier; fit 3 uses COVID-19 hospital census through May 15; fit 4 uses hospital census data through Jun 16. ‡For the purpose of model fitting, phase 1 social distancing was modeled by using a start date of Mar 17, which corresponded with the closure of bars, restaurants, casinos, and many public schools in the state. §The statewide Safer at Home policy remained in effect through the summer, but because of the ≈5.1-d mean incubation period and the 8-d lag between symptom onset and hospitalization, we were able to generate transmission estimates through Jun 3.

Main Article

1Current affiliation: University of Maryland School of Medicine, Baltimore, Maryland, USA.

Page created: June 02, 2021
Page updated: August 17, 2021
Page reviewed: August 17, 2021
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