COVID-19 Regional Models for Colorado

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

For Contact: Jon.Samet@CUAnschutz.edu

Summary

Key messages in this report are:

• COVID-19 infections are estimated to be increasing in most major population centers in Colorado.

• COVID-19 hospitalizations and estimated new infections are increasing in five regions: Central, Central Mountains, Metro, Northeast and San Luis Valley, and are flat in five regions.

• Southeast has the lowest percentage of the population that is fully vaccinated (32%). Vaccination is highest in the Central Mountains (59%) and Metro (59%) regions.
  

• These estimates are based on hospitalization data through 08/02/2021 and vaccination data through 08/08/2021. Due to lags between infection and hospitalization, these estimates do not capture major changes in transmission after approximately July 20th.
  

• A new case definition for COVID-19 hospitalizations was introduced on August 13, 2021 and applied retroactively to all hospitalizations throughout the pandemic. Details on the new case definition can be found under data sources in the Technical Summary.

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.

• The effective reproduction number (Re) is a measure of how rapidly infections are spreading or declining in a region.

• Infection prevalence is an estimate of the portion of the population currently infected and capable of spreading infections. It can be used to gauge how risky contacts are.

• The percent of the population fully vaccinated to date is based on data provided by CDPHE and includes all people who have received one dose of Johnson and Johnson or two doses of Pfizer or Moderna vaccine. Because it takes approximately a month to develop immunity, some of these individuals may not yet be immune to the virus.

• The percent of population estimated to be immune is estimated from the number of individuals immune due to vaccination combined with the individuals immune due to infection. These estimates account for the time between vaccination and immunity and the efficacy of the vaccine. As the immune population grows, due to vaccination or prior infection, the spread of infection slows.

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 08/02/2021. The prevalence of infections and percent of the population immune is estimated for 07/26/2021. The percent vaccinated estimates are based on data provided by CDPHE through 08/08/2021 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	1.2	Increasing	669	1 in 150	45.8	47.7
Central Mountains	1.3	Increasing	136	1 in 734	51.1	58.6
East Central	1.0	Flat	966	1 in 104	63.1	33.5
Metro	1.3	Increasing	541	1 in 185	55.4	58.8
Northeast	1.1	Increasing	601	1 in 166	50.9	48.6
Northwest	0.7	Decreasing	912	1 in 110	50.5	42.3
San Luis Valley	1.1	Increasing	639	1 in 156	41.8	42.9
South Central	1.0	Flat	793	1 in 126	51.7	42.3
Southeast	1.0	Flat	667	1 in 150	41.0	31.5
Southwest	1.0	Flat	521	1 in 192	47.5	53.5
West Central Partnership	1.0	Flat	670	1 in 149	45.7	45.6
Eight select counties
Adams	1.1	Increasing	549	1 in 182	57.1	51.2
Arapahoe	1.2	Increasing	506	1 in 197	56.2	55.4
Boulder	1.0	Flat	141	1 in 710	57.4	65.7
Broomfield	1.3	Increasing	40	1 in 2481	56.3	65.1
Denver	1.1	Increasing	352	1 in 284	60.5	61.3
Douglas	1.0	Flat	433	1 in 231	54.7	59.2
El Paso	1.1	Increasing	768	1 in 130	51.6	47.8
Jefferson plus	1.5	Increasing	125	1 in 800	53.5	61.2
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 08/08/2021 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 08/08/2021 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 07/26/2021. 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 07/20/2021.