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 11/08/2021. The prevalence of infections and percent of the population immune is estimated for 11/01/2021. The percent vaccinated estimates are based on data provided by CDPHE through 11/12/2021 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 0.7 Decreasing 1,317 1 in 76 55.5 54.4
Central Mountains 1.1 Increasing 204 1 in 490 46.6 67.4
Metro 1.0 Flat 1,142 1 in 88 54.8 65.1
Northeast 1.0 Flat 2,187 1 in 46 58.0 54.5
Northwest 1.2 Increasing 1,798 1 in 56 50.7 47.4
San Luis Valley 0.4 Decreasing 640 1 in 156 41.1 49.3
South Central 1.1 Increasing 2,626 1 in 38 56.9 48.5
Southeast 1.1 Increasing 1,363 1 in 73 45.9 37.5
Southwest 0.7 Decreasing 827 1 in 121 48.7 58.5
Eight select counties
Adams 1.0 Flat 1,924 1 in 52 63.4 59.5
Arapahoe 0.8 Decreasing 1,016 1 in 98 57.0 62.3
Boulder 1.3 Increasing 882 1 in 113 51.3 70.4
Broomfield 1.4 Increasing 505 1 in 198 46.5 70.7
Denver 1.1 Increasing 1,420 1 in 70 58.3 67.4
Douglas 0.9 Decreasing 742 1 in 135 48.4 63.8
El Paso 0.7 Decreasing 1,509 1 in 66 58.2 54.6
Jefferson plus 0.9 Decreasing 838 1 in 119 48.9 67.3
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 11/12/2021 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 11/12/2021 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 11/01/2021. 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 10/26/2021.

Figure 5. 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 11/08/2021.

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 6. 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 11/01/2021.

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.

Figure 7. The daily number of people hospitalized with COVID-19 per capita for the 11 LPHA regions and the 8 selected counties and county clusters in Colorado over the past 12 weeks. Hospitalization data are from the COPHS hospital census data through 11/08/2021.

Figure 8. The daily number of people hospitalized with COVID-19 hospitalizations per capita for the 11 LPHA regions and the 8 selected counties and county clusters in Colorado since the first case was reported in March 2020. Hospitalization data are from the COPHS hospital census data through 11/08/2021.

Technical Summary

Within the State of Colorado, as for other states, the spread of SARS-CoV-2 varies across the regions of the state, differing between urban and rural locations and resort and non-resort areas, for example. The modeling carried out by the Colorado COVID-19 Modeling Group was initiated at the state level providing a picture that does not give detail at the regional or county-level. Such detail is needed for public health planning and action.

To provide the needed detail, the Modeling Group developed regional-level models, using approaches similar to those for the overall Colorado model. To date, we have used scaled-down versions of the state-wide SEIR model to generate estimates of the effective reproductive number, infection prevalence, and the cumulative number of people infected in the 11 LPHA regions and 8 select counties with larger population size.

Model updates this week.

Model updates were implemented this week.

Many factors, such as obesity and diabetes, can greatly increase the risk of hospitalization or death due to COVID-19. The prevalence of these risk factors varies regionally, and we therefore modify the age-specific rate of hospitalization accordingly within each region. From the extensive list of such risk factors and corresponding hazard ratios provided here. We estimate that an individual with one or more of these factors is twice as likely to be hospitalized. The percent of the population assumed to fall into this category in a given region is aggregated from CDPHE’s county-level estimates of adults with fair to poor health, provided through the CDPHE Open Data Portal.

Doubling the likelihood of hospitalization for this portion of the population produces a regional hazard ratio from which the region-specific increase or decrease in hospitalization rate is calculated as the percent increase or decrease from the statewide hazard ratio. The table below provides, for each region, the combined hazard ratio and the factor by which hospitalization rates are multiplied in the model.

Methods. We use data on COVID-19 hospitalizations, mobile-phone based mobility data and an age-structured meta-population model of SARS-CoV-2 transmission to estimate the current state of infections in each region. The model is similar in many ways to the state-wide model, in that it accounts for differences in the severity of infection by age and uses Colorado-specific demographic and clinical data. For example, the length of time a COVID-19 patient spends in the hospital varies by age and is based on data provided by Colorado hospitals. A full description of the model and estimation approaches are provided in the documentation, available here. Prior modeling reports and documentation can be found here.

Data Sources: COVID-19 hospitalizations are obtained from COPHS reported through 11/12/2021.Due to lags in reporting, these data are cleaved 4 days prior such that hospitalizations through 11/08/2021 are used in modeling and shown in this report. A new case definition for COVID-19 hospitalizations was introduced on August 13, 2021 and was applied retroactively to all previously reported hospitalizations. Patients admitted or under observation status for 24 hours or more and meet one of the following criteria: admission diagnosis of COVID-19, discharge diagnosis of COVID-19, or received positive COVID-19 diagnostic test (antigen, rapid molecular, or PCR) during the hospitalization are reported to COPHS as a COVID-19 hospitalization. SARS-CoV-2 vaccination data by county are provided by CDPHE weekly. Age distribution of vaccination by region is inferred from state-level age distribution data.

Case data are based on CEDRS through 11/13/2021. Due to lags in reporting, these data are cleaved ten days prior on 11/03/2021. These data sources are provided by CDPHE.

Population movement between regions is estimated from weekly aggregated cell-phone data, provided by xmode, through 11/07/2021.

Limitations: The regional model estimates are subject to greater uncertainty than those for the entire state because there are fewer hospitalizations in each region than in the state as a whole. Estimates are most uncertain for the regions with the smallest population size. This model uses real-time mobility to account for the flow of populations between regions. It remains an open question in the scientific community how to use measures of population movement to accurately model the spread of infections

Region Population Counties
LPHA Regions
Central 801,025 Chaffee, El Paso, Lake, Park, Teller
Central Mountains 179,767 Eagle, Garfield, Grand, Pitkin, Summit
East Central 41,837 Cheyenne, Elbert, Kit Carson, Lincoln
Metro 3,274,440 Adams, Arapahoe, Boulder, Broomfield, Clear Creek, Denver, Douglas, Gilpin, Jefferson*
Northeast 764,381 Larimer, Logan, Morgan, Phillips, Sedgwick, Washington, Weld, Yuma
Northwest 202,304 Jackson, Mesa, Moffat, Rio Blanco, Routt
San Luis Valley 45,988 Alamosa, Conejos, Costilla, Rio Grande, Saguache
South Central 242,903 Custer, Fremont, Huerfano, Las Animas, Pueblo
Southeast 46,073 Baca, Bent, Crowley, Kiowa, Otero, Prowers
Southwest 100,155 Archuleta, Dolores, La Plata, Montezuma, San Juan
West Central Partnership 106,628 Delta, Gunnison, Hinsdale, Mineral, Montrose, Ouray, San Miguel
Eight select counties
Adams 520,268 Adams
Arapahoe 656,618 Arapahoe
Boulder 328,562 Boulder
Broomfield 72,481 Broomfield
Denver 736,522 Denver
Douglas 360,940 Douglas
El Paso 728,076 El Paso
Jefferson plus 599,049 Clear Creek, Gilpin, Jefferson

Table 2: Description of the regions used in this report, including the 11 LPHA Regions. Population estimates are based on 2020 US Census Projections provided by the Colorado Demography Office.

*Clear Creek, Gilpin Counties and Jefferson counties are modeled as a single unit due to the small population of Clear Creek (9,379 residents) and Gilpin (5,924 residents) – populations too small to allow for stable estimation.

Figure 9. Comparison of daily active COVID-19 hospitalizations by data source.