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CDC: “less than 10 percent” is scarier than 0 percent, so let’s run with that

Back in August 2020, I reviewed scientific research into the question of “asymptomatic transmission” of Covid-19. The concluding section of my review begins under the header “Your risk from a chance encounter is indistinguishable from zero.” Here is a relevant quotation:

There’s nothing scientific about quarantining the healthy or forcing them to wear face masks. … The most reliable science on COVID-19 tells us that the more you’re in close contact with someone infected and symptomatic, and the more severe the case, the higher your risk of contracting it.

On the other hand, your risk from a brief encounter with someone at a grocery store, someone walking from the door to the restaurant table, or someone going past on a public sidewalk is indistinguishable from zero.

The brief was to provide factual information to an already terrified public whose fears were being inflamed by irresponsible politicians and media.

Yesterday, Maggie Haberman of the New York Times did the rare thing and questioned a statistic used by the Centers for Disease Control and Prevention (CDC): “less than 10 percent of documented transmission [of Covid], in many studies, have occurred outdoors.”

How much would that be? Generally, the “less than x percent” formulation implies that the number less than x is close to x. So people hearing that statistic would assume it’s probably over 9 percent.

In the same fashion, you wouldn’t say that UNC “cracked the top 10” if they were ranked No. 1. It would be technically accurate but rather misleading.

But as it turns out, zero percent is also less than 10 percent.

Haberman writes:

That benchmark “seems to be a huge exaggeration,” as Dr. Muge Cevik, a virologist at the University of St. Andrews, said. In truth, the share of transmission that has occurred outdoors seems to be below 1 percent and may be below 0.1 percent, multiple epidemiologists told me. The rare outdoor transmission that has happened almost all seems to have involved crowded places or close conversation.

Saying that less than 10 percent of Covid transmission occurs outdoors is akin to saying that sharks attack fewer than 20,000 swimmers a year. (The actual worldwide number is around 150.) It’s both true and deceiving. …

They continue to treat outdoor transmission as a major risk. The C.D.C. says that unvaccinated people should wear masks in most outdoor settings and vaccinated people should wear them at “large public venues”; summer camps should require children to wear masks virtually “at all times.”

These recommendations would be more grounded in science if anywhere close to 10 percent of Covid transmission were occurring outdoors. But it is not. There is not a single documented Covid infection anywhere in the world from casual outdoor interactions, such as walking past someone on a street or eating at a nearby table.

“Not a single documented case.”

Well, that’s less than 10 percent, sure.

Jon Sanders / Research Editor and Senior Fellow, Regulatory Studies

Jon Sanders studies regulatory policy, a veritable kudzu of invasive government and unintended consequences. As director of regulatory studies at the John Locke Foundation, Jo...