There is essentially no outdoor transmission of COVID-19

There is a weird tendency in many liberal spaces — the comments here included — to conflate “trusting the science” with “trusting the specific recommendations of the CDC.” But in fact the CDC is a bureaucracy staffed by human beings, and as human beings are wont to do it is being misleading about the science because it would mean admitting error:

When the Centers for Disease Control and Prevention released new guidelines last month for mask wearing, it announced that “less than 10 percent” of Covid-19 transmission was occurring outdoors. Media organizations repeated the statistic, and it quickly became a standard description of the frequency of outdoor transmission.

But the number is almost certainly misleading.

It appears to be based partly on a misclassification of some Covid transmission that actually took place in enclosed spaces (as I explain below). An even bigger issue is the extreme caution of C.D.C. officials, who picked a benchmark — 10 percent — so high that nobody could reasonably dispute it.

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.

is isn’t just a gotcha math issue. It is an example of how the C.D.C. is struggling to communicate effectively, and leaving many people confused about what’s truly risky. C.D.C. officials have placed such a high priority on caution that many Americans are bewildered by the agency’s long list of recommendations. Zeynep Tufekci of the University of North Carolina, writing in The Atlantic, called those recommendations “simultaneously too timid and too complicated.”

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.

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Erring on the side of protection — by exaggerating the risks of outdoor transmission — may seem to have few downsides. But it has contributed to widespread public confusion about what really matters. Some Americans are ignoring the C.D.C.’s elaborate guidelines and ditching their masks, even indoors, while others continue to harass people who walk around outdoors without a mask.

All the while, the scientific evidence points to a conclusion that is much simpler than the C.D.C.’s message: Masks make a huge difference indoors and rarely matter outdoors.

As Leonhardt says, an unduly large number of the already tiny percentage of the alleged outdoor transmissions the CDC is using to justify the 10% threshold occurred at a Singapore construction site with both indoor and outdoor spaces, and most or all of the transmission probably happened indoors. There is just no good scientific justification for, say, recommending that kids wear masks outdoors at almost all times at summer camp.

In general, the American response (both in terms of policy and practice) has been too draconian about outdoor activity and not draconian enough about indoor activity. The CDC still insists on muddying the waters, but a simpler message would be more accurate and almost certainly more effective as well.