About the Model
This model was developed in collaboration with Carl Bergstrom and Ryan McGee from the University of Washington. This model can be used to simulate infection dynamics of SARS-CoV-2, and evaluate the impact different testing strategies and turnaround time has on outbreaks in populations. This model is used in Color’s Return to On-site SARS-CoV-2 Testing Protocol, you can learn more about the model and proactive testing here:
SEIR models are epidemiological models that are used to model the spread of disease in a population. Standard SEIR models are compartmental models, meaning they track the proportion of the population in different disease states over time. SEIR models include compartments for susceptible (S), exposed (E), infectious (I), and recovered (R) disease states.
The SEIRS+ model is an extended SEIR model, which incorporates the effects of stochastic dynamics, network structure, SARS-CoV-2 testing, and additional interventions in a population.
You can learn more about the SEIRS+ model here: https://github.com/ryansmcgee/seirsplus.
The parameter values and descriptions used in the model are listed in the table below.
1.0, 1.5, 2.0, 2.5
The R0, or reproductive number, is the expected average number of secondary infectious cases produced by a single infectious case.
The time from exposure to when the individual becomes infectious to others.
The period when an individual infected with SARS-CoV-2 is contagious but has not yet developed symptoms.
The total time from exposure to symptom onset — this is the sum of the latent period and presymptomatic period.
The time period during which an infected individual is infectious to others. For symptomatic cases, this includes the presymptomatic period.
76% while presymptomatic, 80% during first 5 days of infectious period, and decreasing thereafter10,11
Probability that a single test will correctly identify an infectious individual as having SARS-CoV-2.
Percentage of individuals infected with SARS-CoV-2 who do not develop symptoms.
Percent symptomatic who self-quarantine
Percentage of symptomatic individuals who develop sufficient symptoms (i.e., fever) that they call in sick and stay home from work.
Test turnaround time
1 day, 3 days, 5 days
Length of time between testing and isolation for individuals who receive positive results.
Modeling can be extremely important to help us understand epidemic progression, however, all models have assumptions, limitations, and biases that make them imperfect estimates. While we do our best to pick the most accurate and evidence-based parameters about SARS-CoV-2 disease spread, estimates for these parameters vary and may change as we learn more about SARS-CoV-2. Because these parameter choices can have significant impacts on model outcomes, we cannot guarantee our choices are always correct and any results produced by this model should not be interpreted to predict exact numbers of cases or outcomes.
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