If you're heading out to meet a group of ten people in someone's backyard in San Francisco tonight, the chances that one of those people will be currently infected with the coronavirus is 15 percent. Raise the group number to 25, and the probability rises to 34 percent.
These are the estimates being made in a new, interactive data-mapping tool from Georgia Tech, highlighted today by the Chronicle and others. It takes publicly available data on COVID-19 prevalence in every county in the U.S., and it extrapolates that into a map that shows where it's most dangerous to congregate in groups of various sizes. As the team explains, disease prevalence has been multiplied by 10, given that the CDC believes that — on average — actual case numbers are about ten times higher than are currently being tested and counted.
The team acknowledges that in counties where testing is widely available, the prevalence multiplier might be a lot lower — which is likely the case in San Francisco, though nothing is for certain.
The tool shows just how dangerous a group of 50 or more people is for disease transmission in the Bay Area, at present. If you raise the meter to 50, the odds of an infected person being there are 56 percent in San Francisco, and about the same in the East Bay and South Bay as well. Raise the event size to 100, and there is an 80-percent or greater chance you will have a COVID-person there in nearly every Bay Area County.
In Los Angeles County, in groups of 50 people, you'd have an 87-percent chance of finding an infected person right now. In New York City, where case counts have slowed considerably since the spring, you're looking at a 25-percent chance in groups of 50, and in Chicago it's currently a 50-percent chance.
"We want people to be informed about the risk — that it’s real, it’s elevated, and many people may be asymptomatic," says one of the tool's creators, biological sciences professor Joshua Weitz, speaking to the Chronicle. "It reinforces the need to wear masks."
Case counts may or may not be slowing down in many parts of California, however a technical issue with the state's lab-reporting tool currently has these totals being undercounted, as we learned yesterday.
So where your masks! And if you are going to the park or to a backyard BBQ, realize that there's a decent chance one of your friends or friends of friends could be infected!