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.
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.
Today’s newsletter will be a bit longer than usual, so I can explain how the C.D.C. ended up promoting a misleading number.
The Singapore mystery
If you read the academic research that the C.D.C. has cited in defense of the 10 percent benchmark, you will notice something strange. A very large share of supposed cases of outdoor transmission have occurred in a single setting: construction sites in Singapore.
one study, 95 of 10,926 worldwide instances of transmission are classified as outdoors; all 95 are from Singapore construction sites. In another study, four of 103 instances are classified as outdoors; again, all four are from Singapore construction sites.
This obviously doesn’t make much sense. It instead appears to be a misunderstanding that resembles the childhood game of telephone, in which a message gets garbled as it passes from one person to the next.
The Singapore data originally comes from a government database there. That database does not categorize the construction-site cases as outdoor transmission, Yap Wei Qiang, a spokesman for the Ministry of Health, told my colleague Shashank Bengali. “We didn’t classify it according to outdoors or indoors,” Yap said. “It could have been workplace transmission where it happens outdoors at the site, or it could also have happened indoors within the construction site.”
As Shashank did further reporting, he discovered reasons to think that many of the infections may have occurred indoors. At some of the individual construction sites where Covid spread — like a complex for the financial firm UBS and a skyscraper project called Project Glory — the concrete shells for the buildings were largely completed before the pandemic began. (This video of Project Glory was shot more than four months before Singapore’s first reported Covid case.)
Because Singapore is hot year-round, the workers would have sought out the shade of enclosed spaces to hold meetings and eat lunch together, Alex Au of Transient Workers Count Too, an advocacy group, told Shashank. Electricians and plumbers would have worked in particularly close contact.