Prepared by the Colorado COVID-19 Modeling Group
Colorado School of Public Health: Andrea Buchwald, Elizabeth Carlton, Debashis Ghosh, Irina Kasarskis, Jonathan Samet, Emily Wu; University of Colorado School of Medicine: Kathryn Colborn; University of Colorado-Boulder Department of Applied Mathematics: Sabina Altus, David Bortz; 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 04/05/2021. The prevalence of infections and percent of the population immune is estimated for 03/29/2021. The percent vaccinated estimates are based on data provided by CDPHE through 4/11/2021 and reflect the proportion of the population that is fully vaccinated 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 Fully Vaccinated
LPHA Regions
Central 1.1 Increasing 676 1 in 148 24.7 17.8
Central Mountains 0.9 Decreasing 175 1 in 572 25.1 28.7
East Central 0.9 Decreasing 1,568 1 in 64 46.7 14.0
Metro 1.0 Flat 685 1 in 146 28.8 21.4
Northeast 1.0 Flat 662 1 in 151 30.6 21.3
Northwest 0.7 Decreasing 471 1 in 212 28.8 24.4
San Luis Valley 0.5 Decreasing 408 1 in 245 28.7 23.8
South Central 1.6 Increasing 974 1 in 103 37.1 22.1
Southeast 1.0 Flat 990 1 in 101 41.8 17.7
Southwest 1.2 Increasing 562 1 in 178 28.0 31.2
West Central Partnership 1.0 Flat 247 1 in 405 28.0 29.2
Eight select counties
Adams 1.1 Increasing 1,161 1 in 86 32.7 16.3
Arapahoe 1.0 Flat 804 1 in 124 31.1 19.3
Boulder 1.1 Increasing 538 1 in 186 26.9 24.8
Broomfield 1.2 Increasing 326 1 in 307 24.1 24.8
Denver 0.9 Decreasing 817 1 in 122 34.7 21.6
Douglas 1.0 Flat 510 1 in 196 24.3 21.7
El Paso 1.1 Increasing 697 1 in 143 25.2 17.1
Jefferson plus 1.0 Flat 192 1 in 520 26.2 25.4
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 percent of the population fully vaccinated by LPHA region. Estimates are based on data provided by CDPHE through 3/28/2021 and reflect the proportion of the population that has received one dose of Johnson and Johnson or two doses of Pfizer or Moderna vaccines as of that date. Darker colors indicate a higher proportion of vaccinated residents.

Figure 3. Map showing the percent of the population immune by LPHA region. Estimates are based on data provided by CDPHE through 03/29/2021. 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 03/23/2021.

Figure 4. 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 04/05/2021.

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 5. 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 03/29/2021.

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.