Could airborne transmission be behind rapid spread of Covid-19?

Back in February, the World Health Organization (WHO) did not consider airborne transmission to be a major concern, saying instead that the novel coronavirus mostly spreads via virus-containing droplets emitted when an infected person coughs, sneezes or exhales.

These droplets can infect another person by falling into an eye, nose or mouth, or by getting stuck to a hand or finger and transferred from there.

However, new evidence suggests the virus may linger in the air, meaning people could actually breathe it in and get infected.

According to Joseph Allen, director of the Healthy Buildings programme at Harvard TH Chan School of Public Health, airborne transmission could be the key to understanding why this disease spreads so rapidly in certain circumstances.  

"I've been warning about airborne transmission of Covid-19 since early February," he said in a recent Op-Ed on the Washington Post. "The explosive transmission on the Diamond Princess cruise ship, as well as other coronavirus outbreaks, constituted tell-tale signs that airborne transmission was happening."

"Close contact transmission was likely happening on that cruise ship, but the disease had spread far more quickly than non-airborne diseases typically spread," he said.

Scientists have recently detected the virus in places that can be reached only by air, such as ductwork, according to a recent paper titled Re-thinking the Potential for Airborne Transmission of SARS-CoV-2.

Occurrence of asymptomatic transmission suggests people are spreading this without coughing or sneezing large droplets.

"Basic aerosol physics shows that people shed an entire continuum of particles when they cough, sneeze or talk, including large particles that settle out quickly and smaller ones that stay afloat for hours," Allen states.

Furthermore, people emit particles from their noses and mouths that are so small that instead of falling right to the ground, they can float for a time through the air. When a virus is carried by these aerosols, its odds of infecting people are higher because of the potential for them to be inhaled.

Since February, the WHO has said this method of transmission "can be envisaged" in healthcare facilities conducting procedures, such as a tracheotomy, on people with Covid-19, reports Bloomberg.

Researchers who aerosolised it intentionally found active virus can float in the air for as long as three hours.

In another study, researchers found particles of the virus in the air of rooms where patients were receiving care and in adjacent hallways. But the particles they identified were not capable of causing infection.

Researchers in another study examined air samples in two hospitals in China and raised the theoretical concern that aerosols could arise from surfaces contaminated by droplets, for instance when hospital workers shed their masks and gowns, or when floors are cleaned.

One big reason to fear airborne transmission is "super-spreader" events.

Allen suggest that such super-spreader events appear to be happening exclusively indoors, where airborne transmission is more likely.

"Consider the infamous March 10 choir practice in Skagit County, Washington, where one member of the choir infected 52 of 60 other members, leading to two deaths. Local public health departments did an investigation and concluded that all three modes of transmission were likely involved in the outbreak. But this likely under-emphasizes the role that airborne transmission played," he explains.

Neither surface nor droplet transmission is likely to have infected so many people in one event.

"But we do know that when people sing, they emit as many aerosol particles as they do when they're coughing. The practice also happened from 6:30pm to 9pm, when most buildings turn off their ventilation systems," Allen further speculates.

To protect ourselves from airborne transmission of the novel coronavirus, Allen suggests two steps.

"First, maintain physical distancing. Six feet is good, but 10 feet is better. Second, we must deploy healthy building strategies, such as refreshing stale indoor air by opening windows in our homes and cars."