March 14, 2020


The Elegant Mathematics of Social Distancing (AARIAN MARSHALL, 03.14.2020, Wired)

"We all have to make contacts with people while we live our lives, what we should aim to do is to limit them, and certainly not to add more," says William Hanage, an associate professor of epidemiology at Harvard's Chan School of Public Health. "This may seem silly if your community is not yet reporting infections, but it is best to get used to thinking this way."

From a mathematical perspective, determining how big a crowd is safe depends on a couple of key questions: How many people in a given area are infected with the disease? And how big is the event? If you know those things, you can estimate the probability of someone getting infected at the event. An elegant "Covid-19 Event Risk Assessment Planner" by the Georgia Tech quantitative biologist Joshua Weitz makes the following calculation: If, say, 20,000 cases of infection are actively circulating the US (far more than are known so far), and you host a dinner party for 10 folks, there's a 0.061 percent chance that an attendee will be infected. But if you attend a 10,000-person hockey match, there's a 45 percent chance. Hence the suspension of the NHL season, along with the NBA, March Madness, and Major League Baseball.

Unlike in a flu epidemic, there's no underlying immunity in the population, meaning if you come in contact with the fluids of an infected person, you're likely to get sick. In light of these sorts of calculations, and the fact that the virus seems to be spreading throughout a number of American communities, "it makes sense to do things like cancel mass gatherings and schools," says Lopman.

Public health experts like social distancing for three reasons. For one, it likely "flattens the curve," or decreases the number of infections at one time, or even overall. That helps prevent overloading the health care system, with its limited number of doctors, nurses, beds, and equipment like ventilators. Also, it buys time for a vaccine to be developed, says Catherine Troisi, an infectious disease epidemiologist at the University of Texas Health Science Center at Houston and the former assistant director of the Houston Health Department.

Posted by at March 14, 2020 9:42 AM