Service Estimates COVID-19 Exposure Risk at Gatherings

Curious what the odds are someone at your neighborhood block party or corner bar has COVID-19?

A service from Georgia Tech’s Applied BioInformetics Laboratory uses countywide case rates and gathering sizes to produce an estimate of the chance an individual with a confirmed case of COVID-19 would be in the crowd.

The mapping tool helps illustrate the dramatic disparity between Wisconsin counties in the disease’s spread. An event with 10 attendees in Milwaukee County has a 30 percent chance of having someone with the disease, while Langdale County, would present only a one percent risk.

What if the event size grows from one to 100 people? Milwaukee’s risk measure jumps to 97 percent, Langdale’s stays put at one percent.

The planning tool does not estimate how likely one is to become infected with the disease. A variety of factors, including whether an individual is indoors or outdoors and how long they’re exposed, influence the probability.

It also does not account for demographic data. The majority of new cases in Wisconsin for the past month have been confirmed in individuals under the age of 40.

“The model is simple, intentionally so, and provided some context for the rationale to halt large gatherings in early-mid March and newly relevant context for considering when and how to re-open,” says the project’s About page. “Precisely because of under-testing and the risk of exposure and infection, these risk calculations provide further support for the ongoing need for social distancing and protective measures. Such precautions are still needed even in small events, given the large number of circulating cases.”

The service does filter out cases deemed “recovered,” leading counties with large, prolonged outbreaks but smaller, recent caseloads, notably Racine and Rock counties in Wisconsin, to have lower estimated risks. The service does not estimate the impact of testing volumes and reflects only confirmed cases.

The open-source tool indicates gatherings are currently safer in Chicago and Minneapolis than Milwaukee, but only on a relative basis. Gatherings of 100 people or larger in each of the three host counties are given an over 68 percent chance of having an individual with the disease present.

The entire codebase for the project is available for review on Github. The service relies on data from The COVID Tracking project and the United States Census Bureau.

Other Services

Ten days ago we reported that open-source platform Rt.live estimated Wisconsin’s COVID-19 reproductive rate was estimated at 1.39 (the number of newly-infected people from each case). The platform estimated, at that point, Wisconsin had the fastest-growing spread in the country.

Data released since then has backed up the estimate, including a weekly Milwaukee County epidemiological report. Released yesterday the Milwaukee County report gave the weekly reproductive rate at 1.4.

The Rt.live service now estimates the Wisconsin reproductive rate to be 1.09, the 22nd fastest-growing rate in the country.

You can continue to monitor all of Urban Milwaukee’s COVID-19 coverage on our Coronavirus topic page.