To date, scientists have established three ways in which severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) — the virus that causes coronavirus disease 19 (COVID-19) — can be transmitted:
- inhalation of liquid droplets from the coughing or sneezing of a person with the infection
- close contact with a person who has the infection
- contact with surfaces that contain the virus
According to a recent study in China, aerosols may offer a fourth transmission route for the virus.
Aerosols are airborne particles that measure around 1 micrometer (one-thousandth of a millimeter) in diameter at most. Because they are much smaller than droplets, they remain suspended in the air for longer and can travel farther.
In their study, the researchers detected genetic material, RNA, of the virus in aerosols sampled from two hospitals and various public places in the city of Wuhan, where the outbreak started.
Because of the relevance of their findings to the ongoing public health emergency, the researchers posted a full, unedited preprint of their study paper on the server bioRxiv on March 10, 2020. An unedited abstract of the paper was also recently published in the journal Nature.
The scientists used air filtration devices called aerosol traps to collect a total of 35 samples from 31 locations between February 17 and March 2, 2020.
The number of samples was relatively low because access to hospitals became tightly restricted at the peak of the outbreak.
In addition, the researchers emphasize that their study did not establish whether the virus-laden aerosols were capable of infecting people.
In confined spaces, however, aerosols are known to transmit other pathogens, including those that cause tuberculosis, measles, and chickenpox.
Aerosol transmission may also have played a role in the spread of the coronavirus that caused an outbreak of the respiratory infection SARS in Hong Kong in 2003.
The senior author of the present study was Dr. Ke Lan, of Wuhan University’s Modern Virology Research Center, and the team also featured public health experts from Shanghai and Hong Kong.
They collected aerosol samples from two hospitals where COVID-19 patients were being treated.
One hospital was for patients with severe illness. The other, a temporary field hospital in a converted sports stadium, was for the quarantine and treatment of patients with mild symptoms.
The concentration of virus-containing aerosols was generally very low, or even undetectable, in patient wards, which the authors attribute to effective isolation procedures and good ventilation.
However, there were high concentrations in unventilated portable toilets at the field hospital.
In their preprint, the scientists write:
“Our finding has confirmed the aerosol transmission as an important pathway for surface contamination. We call for extra care and attention on the proper design, use, and disinfection of the toilets in hospitals and in communities to minimize the potential source of the virus-laden aerosol.”
The scientists also detected high levels of airborne viral RNA at the field hospital in areas used by medical staff to take off personal protective equipment.
This suggests that virus particles in droplets or aerosols were deposited on the equipment during long hours spent close to patients. The particles were then resuspended in the air when the equipment was removed, the authors propose.
However, after the introduction of more frequent and thorough sanitation procedures, they report, levels of virus-laden aerosols in these areas became undetectable.
The team recommends disinfecting protective apparel before removing it to avoid the propagation of virus-laden aerosols.
The scientists also placed aerosol traps in public spaces, including a supermarket and residential building.
Most of these had undetectable or very low concentrations of SARS-CoV-2 aerosols. Two exceptions were sites where people came together in relatively large numbers: one close to the entrance of a department store and another in a public area near one of the hospitals.
This suggests that people who had the infection but experienced no symptoms were nonetheless breathing out virus-laden aerosols.
The team concludes:
“The results showed overall low risks in the public venues but do reinforce the importance of avoiding crowded gatherings and implementing early identification and diagnosis of asymptomatic carriers for early quarantine or treatment. Personal protection equipment, such as wearing masks in public places or while in transit, may reduce aerosol exposure and transmission.”
Taken together, the scientists’ findings indicate that thorough cleaning of surfaces, good ventilation, and avoiding crowds could reduce the risk of exposure to airborne virus particles.
However, commenting on the methods employed in this study, some experts have pointed out that — even when the researchers were able to detect viral particles — the particles may no longer have been infectious.
Lawrence Young, a professor at the University of Warwick, in the United Kingdom, for instance, explains that “Detecting the virus’ genetic material (RNA) with a very sensitive test (RT-PCR) is not the same as detecting [an] infectious virus. [A] dead or degraded virus would still be detected with this technique.”
“In fact,” Prof. Young adds, “this method is so sensitive that there needs to be stringent control measures to prevent sample cross-contamination.”
“The sampling of aerosols is subject to enormous variation, and the small sample size in this study also means that the data needs to be interpreted with caution,” he notes.
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