How many COVID-19 cases are actively circulating in your community?

This section was inspired by a question my friend Abby messaged me yesterday. She asked:

How come there don’t seem to be any stats on active cases? Obviously it’s important to track new cases, but what I mostly want to know is, what is the likelihood that, if I run into someone on the street, they have COVID-19, and it doesn’t seem like new cases tells me that.

In response, I explained that active cases are pretty difficult to track in a country that hasn’t even managed to set up robust contact tracing at national or state levels. To keep tabs of active cases, a public health department would essentially need to call all infected people in its jurisdiction at regular intervals. Those people would need to answer questions about how they’re doing, what symptoms they have, and if they had gotten tested recently. This type of tracking might be doable for some smaller counties, but it’s challenging in larger counties, areas with swiftly rising COVID-19 case counts, areas without sufficient testing capacity, areas with health disparities where some residents aren’t likely to answer a call from a contact tracer… you get the idea.

But it’s still possible to model how many people sick with COVID-19 are likely present in a community at a given time. Epidemiologists and statisticians can use a region’s new case rate—the number of people recently diagnosed with COVID-19—and other COVID-19 metrics, along with population density and demographic information, to estimate how many people in that region are currently infected. A recent analysis in the New York Times used this type of method to estimate how many infected students might come to schools across the country.

If you’d like to see the likely infection rate in your area, check out the COVID-19 Event Risk Assessment Planning Tool developed by researchers at the Georgia Institute of Technology and Applied Bioinformatics Laboratory. Select a state and an event size, and the tool will tell you how likely it is that someone sick with COVID-19 is at this event. For example, at a 50-person event in New York: 2.2% risk. At a 50-person event in Florida: 21.3% risk.

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