Not to nag, but if you’re considering seeing family for the holidays, I need you to take a look at this map. It’s a color-coded guide to Covid-19 risk level, sorted by geography (at the US county level) and crowd size. First, you choose the location where the gathering would happen. Then, at left, you can choose an event size with the slider, from 10 to 5,000 people, and watch the risk of at least one Covid-positive person being there skyrocket the bigger the gathering gets. In some locations where the virus is spreading out of control, like in parts of the Dakotas, that chance is darn near 100 percent, even if the gathering is just 10 people. Get any bigger than 10 people, and the map spits out odds calling it a virtual certainty in many places that you’ll be sharing space with a sick person.
Public health experts would really rather we not gather for the holidays, but they say that if you do go through with it, the shindig should be held outdoors, with as few people as possible, and everyone keeping their distance and wearing masks. But if you look at that map now, it shows that across the US, there’s no such thing as a perfectly safe way to gather during the pandemic. Even with all those precautions, the risk right now is huge, particularly if you’re in the Midwest or hosting anyone coming from the Midwest. For example, in Cook County (which includes Chicago), the chance of a Covid-positive person attending a gathering of just 10 people is around 50 percent. In Jones County, Iowa, that chance is a staggering 99 percent. North Dakota, South Dakota, and Kansas also have counties sitting at 99 percent.
“We highlight the fact that these sort of regional-level risks have shifted,” says Georgia Institute of Technology quantitative biologist Joshua Weitz, coauthor of a new paper in Nature Human Behaviour describing the map system. “In late summer it was in the South, Southeast. And late spring and early summer, the Northeast. So there’s definitely been regional shifts. And right now, the strongest and most worrisome rates of spread are in the Midwest plains and upper mountain region.”
Weitz and his colleagues built the map by pulling in regularly updated Covid-19 case reports from The New York Times for each county. But that doesn’t tell the whole story, because many more people are infected with the virus, but they don’t know it because they’re asymptomatic and never get tested. So if you look at the map again, at left you’ll see an option for “ascertainment bias.” Based on serological studies—that is, of people who’ve tested positive for the antibodies that indicate their immune systems mounted a defense against Covid-19, even if they never felt sick—the researchers are assuming there are actually 10 times more cases in the US than are being reported. In areas where testing is more widely available, that rate may be lower, hence the option to choose an ascertainment bias of 5 on the map.