Please Note

The Washington Post is providing this important information about the coronavirus for free. For more free coverage of the coronavirus pandemic, sign up for our Coronavirus Updates newsletter where all stories are free to read.

With the coronavirus spreading uncontrolled across nearly the entire country, many public health experts are urging Americans to cancel their Thanksgiving gathering plans.

But not everyone is taking this advice to heart: Roughly 40 percent plan to attend a Thanksgiving gathering with 10 or more people, according to a recent survey commissioned by Ohio State University’s Wexner Medical Center.

Peer-reviewed risk assessment data produced by researchers at the Georgia Institute of Technology suggests that many of those big events may become coronavirus super-spreaders. Drawing on public data sets, the Covid-19 Event Risk Assessment Planning Tool produces daily county-level estimates of the odds of encountering at least one coronavirus-positive person at a gathering of 10 or more people.

At the county level nationwide, the average estimated risk of running into a coronavirus-positive person at a 10-person gathering is just a hair under 40 percent. That’s a pretty high number — if you take five of next week’s Thanksgiving gatherings, you can expect that a coronavirus-positive person will be at two of them.

This risk varies a lot by county. Coastal cities tend to be a little safer at the moment — the risk is generally between 10 and 20 percent, depending on where you are.

But in some parts of the country — particularly the Upper Midwest — the risk is much higher. In Cook County, Ill. (home to Chicago), the risk approaches 60 percent. If you attend two Thanksgiving dinners in Cook County, in other words, odds are you’ll encounter a coronavirus-positive person at at least one of them.

In El Paso, the risk is nearly 90 percent. Across much of North Dakota, the odds are anywhere from 95 to 100 percent.

To calculate these risks, the researchers combine data on the number of people confirmed to be infected in a county, along with what’s called the “ascertainment bias” — an estimate of how much confirmed case counts undercount the actual case numbers. This latter part is necessary, because many people who contract the coronavirus never experience symptoms of the disease it causes, covid-19, and many other people are contagious before they become symptomatic.

At the project’s website, you can toggle some of these variables, such as event size and ascertainment bias, to see how they affect the risk of encountering an infected person in your area. This underscores a key caveat: These are estimates subject to a lot of variance, given all that we still don’t know about this virus and how it spreads.

These numbers also shouldn’t be read as odds that you, personally, will contract the virus if you attend an event. The risk of transmission in a given situation depends on a lot of additional factors, including social distancing, mask use, humidity, whether the event is indoors or outdoors, and so on. Those are all outside the scope of this particular tool.

One final caveat: As confirmed case counts skyrocket across the country, it’s likely these numbers will only get worse between now and Thanksgiving.

Nevertheless, it appears certain that millions of Americans are determined to not let a deadly global pandemic put a damper on their Thanksgiving plans. Many of them will hold large, unmasked indoor events with friends and family, virtually guaranteeing that large-scale coronavirus transmission will occur over the holiday.

If Thanksgiving does become a nationwide superspreader event, as many medical workers fear, the consequences for a medical system already stretched thin by the pandemic are likely to be catastrophic.