How do you estimate Covid-19 cases?
Our latest COVID-19 incidence estimation model is built from Biobot’s dataset. We mined this dataset to derive an empirical relationship between the concentration of virus in wastewater samples and the number of new cases reported in the associated communities. This means that our model provides an estimate of the number of daily new reported cases in your community on the sampling date.
More specifically, we correlate the effective virus concentrations in wastewater samples with the 7-day rolling average of daily new reported COVID-19 cases per 100,000 population in the corresponding communities on the date of sample collection.
What is the formula you use to calculate case estimates?
We convert the estimated incidence to cases using your supplied catchment population size:
- Estimated number of new cases in your catchment on the day of sampling = (estimated Covid-19 incidence) x (catchment population)
Our model is built in part on data from clinically confirmed cases. Reported cases include only patients who sought out Covid-19 testing and received a positive test result. This population does not include most people with asymptomatic infections or people without access to testing. Because reported cases are an undercount of the true number of infected people, this estimate is likely an underestimate of the true number of newly infected people in your population. We are working actively to update our model based on the latest science and datasets available.
What factors affect your calculation for COVID-19 cases? What information do you need from customers to improve your case estimates?
In previous versions of our case estimate derivations, our calculations relied on customer-reported flow rates. Our current methodology no longer depends on flow. Instead, we derive our case estimates from effective concentrations, which accounts for factors including population size, wastewater flow rates, and dilution due to inflow and infiltration.
How do you calculate the COVID-19 incidence estimate?
To estimate Covid-19 incidence, we divide the effective viral concentration by a fixed parameter that relates the effective concentration to Covid-19 incidence, as derived from our wastewater dataset:
- Estimated Covid-19 incidence = effective SARS-CoV-2 concentration / fixed parameter