Panel: Aerosols Driving COVID-19 Spread

Compelling evidence indicates that COVID-19 transmission via small-particle aerosols, not droplets, may be driving the pandemic, a panel of aerosol researchers said on Thursday.

At a virtual press conference of the American Association for Aerosol Research, researchers argued the reason both the World Health Organization (WHO) and the CDC are reluctant to embrace aerosol transmission is in part due to "historical bias," despite outsized evidence of aerosol transmission.

Linsey Marr, PhD, of Virginia Tech University in Blacksburg, noted the CDC's recent acknowledgement that SARS-CoV-2 can "sometimes" be spread through aerosols, though she crossed out the word "sometimes" to write "commonly."

"The definitions are still resting on epidemiology, but it doesn't really reflect what we understand about mechanisms" of transmission, she said. "Understanding mechanisms is important [for] how we can best apply interventions and slow this down."

As evidence of historical bias, Jose-Luis Jimenez, PhD, of the University of Colorado in Boulder, cited research from as recently as 1985 that measles, for which airborne aerosol spread is now commonly acknowledged, was thought to mainly be spread by large respiratory droplets, and "close contact" between patients was required.

In fact, he argued there may be more evidence for aerosol spread of COVID-19 than via droplets, noting there is no evidence for droplets in super-spreading events, impact of reduced ventilation, or transmission by asymptomatic people without a cough.

Jimenez stressed the case study of the COVID-19 super-spreading event from the choir practice in Skagit County, Washington to make his case about aerosol spread, mainly by ruling out other methods of transmission.

A two and a half hour rehearsal with one index case of COVID-19 produced 52 new infections, some standing as far away as 13 meters behind. He ruled out fomites, because not only are they "inefficient" for SARS-CoV-2 transmission, according to the CDC, but the index case didn't touch any objects.

Jimenez also ruled out droplets, because the index case "didn't talk to others," and there was no way to impact droplets on eyes, nostrils, and mouths of 52 people.

But the choir room had poor ventilation, and people were there a long time with no masks, and singing and talking intermittently, which points to aerosol spread, he said.

Marr emphasized the importance of ventilation and crowded spaces in facilitating COVID-19 spread. She discussed Japan's "3 Cs" for infection control that the country's government emphasized during the COVID-19 pandemic: closed spaces with poor ventilation, close contact settings, and crowded spaces.

While face masks are an intervention that helps prevent both aerosol and droplet transmission, Marr sounded a note of caution about face shields as a substitute for masks.

"Those are effective for large droplets that land on our eyes, nose and mouth but they don't help with aerosols," she said. "There are localities where a face shield is considered an acceptable alternative to a mask, but they are two different things."

Jimenez also pointed out that, given aerosol transmission, wearing a mask is critical, even if people are maintaining social distance.

"We need to have increased ventilation and air cleaning and do as much of these as we can," he said.

Marr said if a place feels "stuffy or you smell things," that is an indication that "ventilation is insufficient" in the space.

When asked how dangerous the recent presidential debate was for COVID-19 transmission, given President Trump tested positive for COVID-19, experts said, "the more people without a mask in a small space ... makes the situation more dangerous for everybody."

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