This Map Shows the COVID-19 Risk for Gatherings in Each California County

This post was updated on November 19 at 12 p.m.

• Skip to: See the COVID-19 risk map

As Thanksgiving draws closer and a huge proportion of California's population moves back to the most restrictive COVID-19 guidelines due to a rapid uptick in cases, many people are finding themselves making tough decisions about whether or not to visit family for the holidays.

Nationally, The Centers for Disease Control and Prevention has strongly recommended that people stay home for Thanksgiving.  In California, nonessential travel outside of the state was already strongly discouraged by Gov. Gavin Newsom's recent travel advisory, which asks people to self-quarantine for 14 days after arriving from another state or country. This state guidance comes on the heels of recommendations released by Bay Area health officials, which state that "traveling outside the Bay Area will increase your chance of getting infected and spreading the virus to others after your return."

It's no surprise that many people are canceling family Thanksgiving plans outright. But for others wanting to research ways to make informed choices about traveling over the holidays, a new map created by a team at the Georgia Institute of Technology aims to show the risk level of attending an event for every county in the United States, based on how many people would be gathering there.

The COVID-19 Event Risk Assessment Planning Tool allows users to choose an event size and location, and see the percentage risk level that at least one person infected with the coronavirus would be present.

For a Thanksgiving-sized gathering of 10 people, for example, the map projects that on Nov. 17, the likelihood in San Francisco that one or more people present has COVID-19 is 5%. Across the bay in Alameda County, it's 4%. In Napa County, the risk jumps to 11%.

By default, the map also assumes that there are five times more COVID-19 cases than are actually being reported, based on data from coronavirus antibody surveys as well as increases in testing. (You can also set this assumption — called "ascertainment bias" — to 10:1 rather than 5:1, if you're curious about areas with far less testing availability).

The COVID-19 Event Risk Assessment Planning Tool

This embedded map displays the risk level for a gathering of 10 people, with an ascertainment bias of 5:1. To adjust gathering size and bias level parameters, explore the full tool here.

The map project was led by professors Joshua Weitz and Clio Andris at Georgia Tech, in collaboration with researchers at the Applied Bioinformatics Laboratory and Stanford University. Andris said that so far more that 3 million people have used the map in the run-up to the Thanksgiving holiday.

The data, which Andris said is updated "very regularly" to reflect new case rates, is drawn from the New York Times's COVID-19 statistics that show reporting from state and local health agencies.

Using This Map to Make Choices?

What should anyone using this map know, in order to interpret its data in informed way? Andris said it's important to realize that the percentage risk it shows "is not the percentage of the chance that you might be infected with COVID-19" at an event, but instead reflects the percentage chance that one or more people might have COVID-19 at such a gathering.

The map also doesn't distinguish whether an event is indoors or outdoors, whether participants are wearing face coverings or if people gathered are maintaining 6 feet of distance between each other. It purely reflects the number of people gathered in the same place, and the statistical likelihood that one or more of those people has COVID-19 — so you should bear this in mind when using the map to make any decisions about holiday gatherings.

The makers of the map also stress that its results are not meant to replace or supersede usual COVID-19 risk reduction tactics like wearing a face covering, distancing and gathering outdoors in smaller groups.

"We want everyone to be safe and to limit their travel during the holiday season," Andris said. "We want people to stay at home as much as possible. And if you're going to be in a group, make sure that that group is small, and that the people that you're with have been very responsible over time."