Prepared by the Colorado COVID-19 Modeling Group
Colorado School of Public Health: Elizabeth Carlton, Debashis Ghosh, Irina Kasarskis, Talia Quandelacy, Jonathan Samet; University of Colorado-Boulder Department of Applied Mathematics: Sabina Altus, David Bortz; Colorado State University: Jude Bayham; University of Maryland School of Medicine: Andrea Buchwald

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Summary

Key messages in this report are:

Introduction

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 09/13/2021. The prevalence of infections and percent of the population immune is estimated for 09/06/2021. The percent vaccinated estimates are based on data provided by CDPHE through 09/19/2021 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 areโ€ฆ Prevalence per 100,000 People infectious Percent of Population Estimated Immune Percent of Population Fully Vaccinated
LPHA Regions
Central 1.0 Flat 1,464 1 in 68 69.9 50.9
Central Mountains 1.1 Increasing 513 1 in 195 65.4 63.6
East Central 0.6 Decreasing 3,050 1 in 33 75.0 36.2
Metro 0.9 Decreasing 851 1 in 117 72.7 62.3
Northeast 0.9 Decreasing 1,745 1 in 57 71.3 51.7
Northwest 1.1 Increasing 1,179 1 in 85 67.5 44.9
San Luis Valley 1.1 Increasing 549 1 in 182 58.7 46.2
South Central 1.0 Flat 1,890 1 in 53 73.9 45.0
Southeast 1.8 Increasing 1,118 1 in 89 55.7 34.6
Southwest 0.6 Decreasing 583 1 in 171 66.3 56.2
West Central Partnership 1.0 Flat 673 1 in 149 57.2 48.2
Eight select counties
Adams 0.9 Decreasing 1,047 1 in 95 80.0 55.8
Arapahoe 0.7 Decreasing 949 1 in 105 73.9 59.4
Boulder 1.1 Increasing 732 1 in 137 71.2 68.6
Broomfield 0.1 Decreasing 133 1 in 752 67.1 68.3
Denver 1.0 Flat 688 1 in 145 78.1 64.8
Douglas 0.8 Decreasing 691 1 in 145 65.6 61.1
El Paso 1.0 Flat 1,685 1 in 59 72.8 51.1
Jefferson plus 1.1 Increasing 552 1 in 181 66.7 64.7
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 09/19/2021 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 fully vaccinated by age group and LPHA region. Estimates are based on data provided by CDPHE through 09/19/2021 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 4. Map showing the percent of the population immune by LPHA region. Estimates are based on data provided by CDPHE through 09/06/2021. 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 08/31/2021.