United Kingdom

Experts warn CDC estimate that less than 10%' of COVID-19 cases occur outdoors is exaggeration

The Centers for Disease Control and Prevention's (CDC) estimate of the risk of COVID-19 transmission outdoors is misleading, public health experts warn.

When the federal health agency released new guidelines on masks for fully vaccinated people last month, director Dr Rochelle Walensky stated that 'less than 10 percent' of coronavirus spread was occurring outside.   

Epidemiologists and infectious disease scientists tell DailyMail.com that although the figure is not incorrect, the true risk is believed to be much lower.   

The number of COVID-19 cases linked to outdoor spread is below one percent, and may even be as low as 0.1 percent.

It comes as New York Times critic David Leonhardt criticized the agency on Tuesday using mischaracterized data - from studies at construction sites in Singapore - to tell Americans to keep their masks on outside rather than following other interventions to help drive down infection rates.

Last month, the CDC said 'less than 10%' of all coronavirus transmission is occurring outdoors. Pictured: Guests stop to take a selfie at Magic Kingdom Park at Walt Disney World Resort, July 2020

Several public health experts argue that the true risk of outdoor spread is believed to be below 1%, and may even be as low as 0.1% (above)

Prior to the CDC releasing its estimate last month, officials had not stated a concrete number on the risk of outdoor transmission.

But, according to The Times, when the agency did settle on 'less than 10 percent,' it cited studies showing a large share of COVID-19 cases contracted outside occurred at construction sites in Singapore. 

For example, in one study from the London School of Hygiene & Tropical Medicine, 95 cases out of 10,926 documented around the world were classified as outdoors.

All 95 were linked to a cluster at a building site in Singapore.

The data comes from Singapore's Ministry of Health, but officials there don't classify the cases as 'outdoor transmission.'

'We didn't classify it according to outdoors or indoors,' spokesman Yap Wei Qiang told The Times.

'It could have been workplace transmission where it happens outdoors at the site, or it could also have happened indoors within the construction site.'  

New York Times critic David Leonhardt criticized the agency for publishing guidelines partially based on mischaracterized data. Pictured: Leondhart, September 2015

The Times discovered that, in many of the cases, the workers had been in close contact with each other and had either met or eaten in enclosed outdoor spaces. 

What's more, several locations that researchers had defined as outdoors for their academic papers were actually a mix of indoors and outdoors as outdoors, meaning many of the Singapore transmissions could have actually occurred inside.

However, even if all the Singapore cases were classified as outdoor transmission, that still represents fewer than one percent of the total cases. 

In another study, in the Journal of Infectious Diseases, researchers conducted a systematic review of the cases in Singapore and other around the world, and states they found that the proportion of global infections that occurred outdoors is below 10 percent.  

'In the paper, they write there is limited data but certainly less than 10 percent,' Dr Aaron Richterman, an infectious disease physician at the University of Pennsylvania, told DailyMail.com.

'You don't want to overstate a claim in academic journals, so that's probably where the 10 percent figure came from.

'That 10 percent then got translated to the guidance, even though when you look at it, it's much less than one percent.'  

Other studies, such as one from Wuhan, China, found just one of 7,324 infections was linked to outdoor transmission.

Another, from Ireland, found one case of COVID-19 in every thousand traced to outdoor spread. 

'There has been an overwhelming number of studies looking at the risk factors of transmission and transmission patterns,' Richterman said.

'And there are really no well-documented cases of casual transmission cases like another person [transmitting the virus by] walking by another person on a street.'  

Richterman said the CDC's guidance had led to a distraction from interventions that could actually help drive down case rates.

'That's where you have circumstances of a person wearing a mask on the street, walk into a restaurant to meet a friend, and take off the mask to sit down for dinner,' he said.

'And the second event has much more of a risk for transmission.'

In The Times article, Leonhardt also argued that the CDC's 'exaggeration' about the risks of outdoor transmission has led to confusion about which activities are dangerous. 

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

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