Read part one of this blog post here.
Aerosol Transmission of Covid: More Prevalent than Presumed
For months into the pandemic, the World Health Organization (WHO) and U.S. Centers for Disease Control and Prevention (CDC) insisted close-range, large-droplet spread was driving the pandemic. Aerosol transmission, WHO stated, was limited to “specific circumstances and settings,” primarily aerosol-generating medical procedures such as intubation and CPR.
Prodded by scientists worldwide, both organizations eventually agreed aerosol transmission of SARS-CoV-2 was possible in community settings and perhaps not rare. But even today, top scientists warn the impact of long-range spread has been vastly underestimated.
“Aerosol transmission plays a significant role in indoor environments and cannot be neglected,” argues Maosheng Yao, PhD a professor of engineering at Peking University.
Covid transmission via aerosols “matters much more than has been officially acknowledged to date,” agrees Linsey Marr.
In reality, transmission via large-droplet spray, like transmission via aerosol, requires its own “special circumstances and settings” — for example, standing before a Covid-infected person who just sneezed.
There’s no evidence that close-range, droplet transmission is the primary driver of Covid spread and much to suggest that it’s not.
For one thing, asymptomatic and pre-symptomatic people account for at least 50% of all transmission, according to the U.S. CDC. In other words, they’re not sneezing or coughing up phlegmy, infectious gobs — the kind those plexiglass dividers are designed to contain.
What’s more, careful studies of super-spreading events have found long-range aerosol spread responsible for large numbers of infections.
For example, an analysis of the Diamond Princess cruise ship — where 712 of 3,711 passengers became infected — estimated that short-range transmission accounted for just 35% of cases. Another 35% were attributed to long-range transmission and 30% to spread via contaminated surfaces.
Then there’s the infamous American choir-practice case, in which a single infected singer transmitted Covid-19 to 53 of the choir’s 61 members. Scientists interviewed the entire choir and analysed their seating arrangements and movements throughout the 2.5-hour practice.
While it’s possible some members became infected at close range, inhalation of aerosols from shared air was “almost certainly the leading mode of transmission,” the study concluded.
No choir member sat within 3 metres in front of the infected singer, and four singers who contracted Covid sat behind the singer. For them to have become infected via large droplets, those infectious gobs would have had to travel backwards, a physics-defying scenario in a room with poor ventilation.
American chemist Jose Jimenez, who interviewed the choir, says one member contracted Covid despite remaining 44 feet (13.5 metres) from the contagious singer.
All the evidence taken together, says Jimenez, “convinced us that only airborne transmission could explain this case.”
Restaurants and shops worldwide have reduced occupancy, on the theory that spreading out patrons would control Covid spread. But without other precautions in place, such as air dis-infection, reducing capacity won’t suffice. Occupancy in sections of the choir hall ranged from 44% to 55%.
Those ubiquitous plexiglass dividers won’t help much, either.
As part of his research on Covid spread, California scientist William Ristenpart investigated transmission at a karaoke bar. After an initial outbreak, the owners installed plastic partitions in front of the singers.
“But it didn’t solve anything,” Ristenpart reported at the international Covid-transmission workshop. “Another 18 people got infected. It’s more indirect evidence for this idea of long-range aerosol transmission.”
Covid Will Fade, But Aerosol Spread Won’t
Ultimately, it doesn’t matter what percent of Covid cases are spread via large droplets or tiny aerosols. We know aerosol spread happens — often.
And those responsible for the safety of indoor spaces must take heed.
As the Diamond Princess analysis noted, the cruise outbreak underscores “the importance of implementing public health measures that target the control of inhalation of aerosols . . . not only aboard cruise ships but in other indoor environments as well.”
Which measures target aerosol spread best?
Certainly, mask mandates help. “But even with the masks, you have leakages of particles,” Lydia Bourouiba of MIT said at the Covid workshop. “The aerosol spread will be slower, but aerosols will still accumulate.”
Even universal masking wouldn’t halt transmission. “In Hong Kong, we’re very good at wearing masks, but we’ve had two community epidemics in spite of more than 99% of adults reporting wearing face masks in public,” says Ben Cowling, PhD, a University of Hong Kong epidemiologist.
At any rate, masks are not a long-term solution to aerosol spread of disease.
When the Covid pandemic fades and masks are tossed, infectious microbes will still be swirling around. These pathogens include influenza, which, like Covid-19, can be transmitted by both aerosols and large droplets.
What’s needed indoors are increased ventilation and air filtration, as well as continual, medical-grade air disinfection, such as NanoStrike technology, developed by Novaerus.
As independent laboratory tests confirm, plasma generated within Novaerus units obliterates airborne pathogens of all types: viruses, bacteria, and fungi. Instantly, virulent particles are reduced to inert debris.
Novaerus units accomplish this without emitting harmful byproducts and have proven safe for 24/7 use around even the most vulnerable patients.
Throughout the pandemic, the compact, unobtrusive devices have been running in hospital Covid wards, emergency rooms, and operating theatres, protecting staff, patients, and visitors alike.
Now that the pandemic’s end appears within sight, the same technology is being installed by pubs, restaurants, pharmacies, retail shops, offices, and schools. The goal: to fight Covid, influenza, norovirus, and other pathogens, current and emerging, that can and will spread infection.
We can’t remain 2 metres apart forever. And as science now demonstrates, standing 2 meters apart won’t stop aerosol spread, anyway.