Modeling the COVID-19 pandemic in King County without Waning Immunity

This page illustrates modeling simulations for SARS-CoV-2 transmission in King County Washington after June 1, 2021. They are based on documented assumptions about circulating variants, their prevalence and infectivity, vaccination rates by age groups, vaccine efficacies against each variant, age-stratified mixing patterns and approximations of contact rates based on reactive changes in physical distancing practices and mask use in response to variation in local hospitalizations. These simulations assume no waning of immunity whether naturally or vaccine acquired. Alternatively, they may be seen as simulations with perfect boosting in which all vaccinees receive booster shots shortly before their protection starts waning.

The model was calibrated to 12 metrics (diagnosed cases, hospitalizations, and deaths for four age groups) from King County in two stages: i) for the initial epidemic period in spring 2020 to calibrate the base transmission rate, reduction in transmission due to being diagnosed, and proportion of mild and severe cases and ii) for the spring of 2021 to calibrate diagnostic rates for mild and severe illness, the relative proportion of asymptomatic cases diagnosed, and the current level of physical distancing by age. With this procedure, 100 parameterizations which minimized the distance between the normalized model outputs and normalized data for the 12 metrics (diagnosed cases, hospitalizations, and deaths for four age groups) were selected and are presented here.

The darkest model line represents the median values of all parameters across the 100 sets, and as such, SHOULD NOT be interpreted as a precise forecast. Other lines show possible outcomes with all 100 calibrated parameterizations. Projections are based on best available data but may change. New variants with increased infectivity and/or mortality may emerge. Vaccine efficacy may vary, and prior immunity may wane. In particular, human social distancing and masking behavior is difficult to predict and may alter future epidemic waves.

Model simulations were last updated on October 28, 2021. Details of model simulations and parameters can be found on the Model description page. Simulations of scenarios with different assumptions on in-person schooling and provided access of all school-age kids to vaccination can be compared on the Adjustable societal measures page.

Version

Model version 2.0

medRxiv

Modeling the COVID-19 pandemic in King County with Waning Immunity

This page illustrates modeling simulations for SARS-CoV-2 transmission in King County Washington after June 1, 2021. They are based on documented assumptions about circulating variants, their prevalence and infectivity, vaccination rates by age groups, vaccine efficacies against each variant, age-stratified mixing patterns and approximations of contact rates based on reactive changes in physical distancing practices and mask use in response to variation in local hospitalizations. These simulations include linear waning of both natural and vaccine immunity against infection to 50% of the initial level of protection over one year. The protection against hospitalization is assumed to remain constant throughout the post-vaccination period. No booster shots are simulated which makes this scenario overly pessimistic.

The simulations are based on documented assumptions about currently circulating variants, their prevalence and infectivity, vaccination rates by age groups, vaccine efficacies against each variant, age-stratified mixing patterns and approximations of contact rates based on reactive changes in physical distancing practices and mask use in response to variation in local hospitalizations.

The model was calibrated to 12 metrics (diagnosed cases, hospitalizations, and deaths for four age groups) from King County in two stages: i) for the initial epidemic period in spring 2020 to calibrate the base transmission rate, reduction in transmission due to being diagnosed, and proportion of mild and severe cases and ii) for the spring of 2021 to calibrate diagnostic rates for mild and severe illness, the relative proportion of asymptomatic cases diagnosed, and the current level of physical distancing by age. With this procedure, 100 parameterizations which minimized the distance between the normalized model outputs and normalized data for the 12 metrics (diagnosed cases, hospitalizations, and deaths for four age groups) were selected and presented here.

The darkest model line represents the median values of all parameters across the 100 sets, and as such, SHOULD NOT be interpreted as a precise forecast. Other lines show possible outcomes with all 100 calibrated parameterizations. Projections are based on best available data but may change. New variants with increased infectivity and/or mortality may emerge. Vaccine efficacy may vary and prior immunity may wane. In particular, human social distancing and masking behavior is difficult to predict and may alter future epidemic waves.

Model simulations were last updated on October 28, 2021. Details of model simulations and parameters can be found on the Model description page. Simulations of scenarios with different assumptions on in-person schooling and provided access of all school-age kids to vaccination can be compared on the Adjustable societal measures page.

Version

Model version 2.0

medRxiv

Adjustable societal measures for the COVID-19 King County model

This page allows for comparison of different scenarios for the course of the SARS-CoV-2 epidemic in King County Washington after June 1, 2021 under different assumptions of vaccination and social distancing.

Users may specify up to three combinations of parameters. These parameters include the minimum and maximum social distancing levels (applied when weekly hospital admissions exceed or retreat from specified population levels), the level of physical interaction at schools (applied September 1st, 2021), vaccine coverage in adults and teenagers (aged 12 and above) and vaccine coverage in children aged 5-11 (with protection beginning December 1st, 2021).

Simulations use the median values of 100 parameter sets obtained by calibrating to cases, deaths, and hospitalizations earlier in the pandemic. These calibrated parameter sets are based on documented assumptions about circulating variants, their prevalence and infectivity, vaccination prioritization by age groups, vaccine efficacies against each variant, age-stratified mixing patterns and approximations of contact rates based on reactive changes in physical distancing practices and mask use in response to variation in local hospitalizations

Model simulations were last updated on October 28, 2021. Details of model simulations and parameters can be found on the Model Description page. Assumptions about viral variants and vaccine efficacy against them are estimates. These estimates can be found in the table at the bottom of the Model Description page and will be updated as new data accrues.

Version

Model version 2.0

medRxiv

Results from COVID-19 model (using median of calibration parameters)

Management option A
Management option B
Management option C

Selected outcome

The selected outcome over time for a given set of simulations with or without waning of immunity under up to three different management selections. Where any of the three selection sets are identical, only the first will be drawn so sometimes there may just be one or two curves displayed.

Social distancing: contacts relative to pre-pandemic

Social distancing follows the logic of the model description. We use a social distancing parameter that varies from 0 (pre-pandemic interactivity) to 1 (complete lockdown, no contacts). This parameter captures all non-pharmaceutical interventions including closure of businesses/schools, mask wearing, sanitization, outdoor meetings, and general reduction in contacts. When hospital admissions exceed certain thresholds, we impose reductions in contacts through partial lockdowns and increase the value of the parameter.

Vaccine Coverage: vaccines administered from 6/1/2021

Our model contains several assumptions about vaccination. 1) A fixed number of doses are given each day, which corresponds to fully vaccinated individuals. 2) Coverage is not complete. Children (<20 years old) are divided into three groups: 0-5 (no vaccine), 5-11 (12/1/2021 vaccinations up to child coverage) and 12-19 (5/1/2021 vaccination up to coverage), Other age groups start 1/1/2021 and get vaccinated to the coverage specified. 3) Vaccines are prioritized to the elderly (>70 years old).

Weekly Hospital Admissions per 100k of population

Illustration of social distancing implementation. When weekly hopsital admissions rise above the lockdown threshold (red line) or drop below the reopening threshold (green line), social distancing is increased or decreased, respectively. The thresholds are evaluated weekly which means adjustments to the social distancing can be delayed by a up to a week from when the limits are exceeded.

King County COVID-19 Modeling Group

We are an academic group led by Fred Hutchinson Cancer Research center researchers. We are funded by the CTSE and the NIH.

Our work is dedicated to modeling the ongoing SARS-CoV-2 pandemic, with a particular focus on matching and projecting local data from King County Washington, USA.

The model considers the epidemiology of COVID-19 including social distancing, vaccination, and emerging variants.


Modeling team

Chloe Bracis (Grenoble)

David Swan (Fred Hutch)

Mia Moore (Fred Hutch)

Daniel Reeves (Fred Hutch)

Eileen S. Burns (Indepedent)

Dobromir Dimitrov (Fred Hutch/UW)

Joshua T. Schiffer (Fred Hutch/UW)


This website was created by Eileen S. Burns


Version

Model version 2.0

medRxiv

COVID-19 Epidemiological model

We have developed a mechanistic mathematical model to describe the epidemiological dynamics of COVID-19 since March 2020.

Mathematical details are presented below.