Prepared by the Colorado COVID-19 Modeling Group
Colorado School of Public Health: Andrea Buchwald, Elizabeth Carlton, Debashis Ghosh, Irina Kasarskis, Jonathan Samet, Laura Timm, Emily Wu; University of Colorado School of Medicine: Kathryn Colborn; University of Colorado-Boulder Department of Applied Mathematics: Sabina Altus, David Bortz; University of Colorado-Denver: jimi adams; Colorado State University: Jude Bayham

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Key messages in this report are:


This report provides the results of epidemic models for regions of Colorado, using methods similar to that used for the state-level model. Estimates are presented for the 11 Local Public Health Agencies (LPHAs) regions in the state and for 8 selected counties with populations that are sufficiently large to allow for county-level estimates. The model results are subject to greater uncertainty than those for the entire state because there are fewer hospitalizations and cases in each region than in the state as a whole. Estimates are most uncertain for the regions with the smallest population size. We use the model as well as COVID-19 hospital,and vaccination data to generate four measures for each region. These measures can be used to gauge the current state of SARS-CoV-2 in each region.

Table 1. The estimated effective reproductive number, prevalence of infections, percent of the population immune and vaccinated to date by region. These metrics are estimated using hospitalization data from the Colorado COVID Patient Hospitalization Surveillance (COPHS). Effective reproductive number (Re) reflects hospitalization data through 03/08/2021. The prevalence of infections and percent of the population immune is estimated for 03/01/2021. The percent vaccinated estimates are based on data provided by CDPHE through 3/14/2021 and reflect the proportion of the population that has received at least one vaccine dose as of that date.

Are infections increasing or decreasing?
How many people are infectious?
Re Infections are… Prevalence per 100,000 People infectious Percent of Population Estimated Immune Percent of Population Vaccinated
LPHA Regions
Central 0.7 Decreasing 351 1 in 285 18.5 17.2
Central Mountains 1.2 Increasing 307 1 in 326 18.6 26.4
East Central 0.5 Decreasing 609 1 in 164 40.2 13.5
Metro 0.9 Decreasing 568 1 in 176 22.7 20.4
Northeast 1.1 Increasing 562 1 in 178 24.5 19.6
Northwest 0.5 Decreasing 476 1 in 210 22.8 22.2
San Luis Valley 0.7 Decreasing 946 1 in 106 20.5 24.5
South Central 1.0 Flat 1,266 1 in 79 36.6 22.3
Southeast 0.9 Decreasing 1,341 1 in 75 42.1 18.0
Southwest 0.8 Decreasing 289 1 in 346 18.9 29.1
West Central Partnership 0.8 Decreasing 283 1 in 354 18.5 26.9
Eight select counties
Adams 1.0 Flat 983 1 in 102 28.5 15.8
Arapahoe 0.9 Decreasing 687 1 in 145 24.7 18.7
Boulder 0.6 Decreasing 268 1 in 372 18.6 22.9
Broomfield 0.6 Decreasing 11 1 in 9414 16.3 23.8
Denver 0.9 Decreasing 840 1 in 119 28.6 20.2
Douglas 0.9 Decreasing 447 1 in 224 17.2 20.5
El Paso 0.7 Decreasing 339 1 in 295 19.1 16.5
Jefferson plus 0.7 Decreasing 218 1 in 458 18.2 24.5
Due to the small population sizes of Gilpin and Clear Creek counties, these counties are combined with Jefferson County. Jefferson County comprises 97% of the population in the Jefferson plus county cluster.
Due to lags between infection and hospitalization, the estimated effective reproductive number (Re) reflects the spread of infections approximately two weeks prior to the data of the last observed hospitalization.
Immunity from vaccination is based on assumptions that the Moderna and Pfizer vaccines are 52% effective after 14 days and 90% effective a week after the second dose; the Johnson and Johnson vaccine is assumed to be 72% effective 28 days after the first dose. In the model, it is assumed that immunity due to infection wanes after 6 months if an individual has an asymptomatic infection, 12 months if symptomatic. Age distribution of vaccination by region is inferred from state-level age distribution data.

Figure 1. Map showing the 11 LPHA regions for which estimates were generated. Regions in yellow (effective reproductive number > 1), orange (prevalence > 1%), or red (both effective reproductive number > 1 and prevalence > 1%) indicate areas of concern.

Figure 2. Map showing the proportion of the population vaccinated by LPHA region. Estimates are based on data provided by CDPHE through 3/14/2021 and reflect the proportion of the population that has received at least one vaccine dose as of that date. Darker colors indicate a higher proportion of vaccinated residents.

Effective Reproductive Number

The figure below shows the estimated effective reproductive number for each region since March.

The effective reproduction number (Re) is a measure of how rapidly infections are spreading or declining in a region at a given point in time. When the effective reproductive number is below 1, infections are decreasing. When the effective reproductive number is above 1, infections are increasing.

The effective reproductive number is estimated using our age-structured meta-population model fit to hospitalization data. Because we base our parameter estimates primarily on COVID-19 hospitalization data, and hospitalizations today generally reflect infections occurring approximately 13 days prior, our most recent estimates of the effective reproductive number likely reflect the spread of infections occurring on approximately 02/23/2021.
Figure 3. Estimated effective reproductive number (Re) over time in the 11 LPHA regions in Colorado, and 8 selected counties and county clusters. Estimates shown using COVID-19 hospitalization data through 03/08/2021.

Infection prevalence

Infection prevalence provides an estimate of the proportion of the population that is currently infected with SARS-CoV-2 and capable of spreading infections. At higher levels of infection prevalence, individuals are more likely to encounter infectious individuals among their contacts. Because many people experience no symptoms or mild symptoms of COVID-19, many infections are not identified by surveillance systems. The estimates we present here are intended to provide an approximation of all infections, including those not detected by the Colorado Electronic Disease Reporting System (CEDRS).

The figure below shows the estimated infection prevalence per 100,000 individuals for each region. These are estimated from the age-structured meta-population model.

Figure 4. Estimated prevalence per 100,000 population for each of the 11 LPHA regions (top), plus the 8 selected counties and county clusters (bottom). All prevalence values over 1,000 per 100,000 are shown in dark red. Prevalence values estimated up to 03/01/2021.

COVID-19 hospitalizations

The figures below show the daily number of individuals hospitalized with COVID-19 from each region. Hospitalization data are from the COVID Patient Hospitalization Surveillance (COPHS) maintained by the Colorado Department of Public Health and the Environment (CDPHE). Each COVID-19 patient is assigned to a region based on their home zip code. COVID-19 hospitalizations are shown per 100,000 population to allow comparability across regions.

COVID-19 hospitalizations are a sensitive measure of SARS-CoV-2 transmission and are an important indicator of the severity of infections in a region. While many SARS-CoV-2 infections are not captured by state surveillance systems, we expect that almost all COVID-19 hospitalizations are identified.