The World Health Organization has expanded its coronavirus guidance to include the possibility in certain circumstances of airborne transmission, in which the virus could be spread through tiny droplets that linger in the air.
The update came Thursday after an open letter signed by more than 200 scientists pressed the agency this week to acknowledge the potential role that tiny droplets, or aerosols, play in airborne transmissions among people in crowded, indoor settings for prolonged periods of time.
"There have been reported outbreaks of COVID-19 in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people may be shouting, talking, or singing," the WHO said in its updated review of the evidence. "In these outbreaks, aerosol transmission, particularly in these indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods of time with others, cannot be ruled out."
The agency said more research is "urgently needed to investigate such instances and assess their significance for transmission of COVID-19."
The WHO's expanded guidance on airborne transmission is notable, but experts maintain that aerosols are likely to be just a small part of how the coronavirus spreads and that close contact with an infected person is still the most common source of transmission.
"You can put all these definitions in place, but we've always been concerned about spread when people are in small spaces, indoors, for long periods of time," said Cindy Prins, an epidemiologist at the University of Florida in Gainesville.
For respiratory illnesses like COVID-19, the disease caused by the coronavirus, the medical community focuses on two primary ways that the virus can spread: airborne transmission and what's known as droplet transmission.
With droplet transmission, it's thought that virus-filled particles can be ejected from the mouth or the nose when a person speaks, coughs or sneezes. The droplets can be flung through the air — up to 6 feet from the infected person — but then drop to the ground or onto other surfaces fairly quickly if they don't come into contact with other people nearby.
Airborne transmission, on the other hand, occurs when tiny aerosol particles are expelled by talking, sneezing or coughing but then remain suspended in the air. The minuscule particles can also travel away from an infected person by floating on air currents.
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Measles, chickenpox and tuberculosis are other diseases that can spread through airborne transmission.
When asked about airborne spread of coronavirus, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases said, "There's no solid evidence that that type of transmission is occurring."
But he added, "We can't rule it out completely."
While airborne and droplet transmission are different, they aren't mutually exclusive, said Dr. Isaac Bogoch, an infectious diseases physician who is an associate professor of medicine at the University of Toronto.
"We often think about these clinical definitions as silos, but that's not entirely accurate," Bogoch said. "There's a spectrum — from droplets all the way to airborne. When we think about COVID-19, there may be some airborne transmission, but it's safe to say that the majority of transmission falls toward the droplet side of the spectrum."
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Bogoch pointed to hospital protocols as a key indicator that airborne transmissions are likely to be rare. When treating coronavirus patients, most hospitals have been adhering to infection control guidelines tailored to droplet transmission, rather than the more stringent procedures to protect against airborne infections.
If COVID-19 was truly airborne, Bogoch said, infection rates among health care workers would have skyrocketed.
"Our personal protective equipment — our masks, gowns, gloves and eye protection — are chosen for droplet precautions, and the vast majority of the time when we have access to these things and we use them correctly, we're not getting this infection," he said.
Dr. Carlos del Rio, executive associate dean of the Emory University School of Medicine in Atlanta, said that aerosol transmission is likely to be a risk in certain conditions but that the WHO's updates don't represent a drastic departure from what was already known.
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"If I'm in a crowded room with a bunch of infected people, there's bound to be aerosol transmission there, but if I'm in a big room or I'm outside and someone is walking nearby, I'm not too worried about aerosols," he said.
Prins said the updated information should reinforce recommendations from public health officials that are already in place, such as practicing social distancing and avoiding congested indoor spaces.
And regardless of whether it's mandated, people should wear masks in public, del Rio said.
"Everyone should wear a mask," he said. "We have to get it across to people that this is not an option."
