The transmission of SARS-CoV-2 through droplets is clear, but whether it can be transmitted through aerosols (airborne transmission) is not. This study investigated whether SARS-CoV-2 is transmitted through aerosols by measuring viral RNA in two Wuhan hospitals during the COVID-19 outbreak. The researchers found that levels of SARS-CoV-2 RNA in isolation wards and vented patient rooms was low, but higher in patients' toilet areas. SARS-CoV-2 levels were undetectable except in two areas prone to crowding. Some medical staff areas had high concentrations of SARS-CoV-2 RNA, but they decreased after strict sanitation measures were put into place. Although the infective potential of airborne SARS-CoV-2 is unclear, the researchers suggest that SARS-CoV-2 may be transmitted through aerosols. The findings suggest that room ventilation, open space, sanitation of protective wear, and disinfection of toilet areas can limit the concentration of SARS-CoV-2 RNA in aerosols. More work is necessary to understand infectivity of SARS-CoV-2 in aerosols.
The researchers wanted to understand whether SARS-CoV-2 could be transmitted through aerosols by measuring viral RNA in two wuhan hospitals.
Transmission pathways of SARS-CoV-2 include inhalation of droplets, close contact with infected people, and contact with surfaces on which SARS-CoV-2 is present. Many respiratory diseases are spread airborne, including tuberculosis, measles, and possible SARS. The potential for airborne transmission of SARS-CoV-2 is not well-understood.
In this study, the authors sampled airborne SARS-CoV-2 at 30 sites in two hospitals and public areas in Wuhan. They quantified the number of SARS-CoV-2 in these samples using droplet digital PCR-based detection (ddPCR). The two hospitals were used exclusively for COVID-19 patient treatment. Renmin Hospital was designated for treating severe symptoms of COVID-19, while Fangcang hospital was a makeshift field hospital that treated patients with mild symptoms.
There were three categories of sample locations: patient areas, coronary care units and patient ward rooms in Renmin Hospital, medical staff areas, and public areas.
In these locations, three kinds of samples were collected: aerosol samples of total suspended particles; aerodynamic size-segregated aerosol samples to determine the size distribution (a curve that determines the amount of particles present in a sample according to size) of SARS-CoV-2; and aerosol deposition samples to determine the deposition rate (the rate at which particles attach to surfaces) of SARS-CoV-2.
In general, very low or non-detectable concentrations of airborne SARS-CoV-2 were found in Renmin Hospital. This suggests that the negatively pressurized isolation units and good ventilation in the hospital were effective in limiting airborne transmission of the virus.
The highest concentration of SARS-CoV-2 observed in patient areas was in the Fangcang hospital mobile toilet room. This is a small, temporary single toilet room without ventilation. In this setting, airborne SARS-CoV-2 may come from the patient's breath or from the patient's feces or urine.
In public areas outside hospitals, a majority of the sites had little to undetectable concentrations of SARS-CoV-2 aerosol. Exceptions were a crowd-gathering site outside of a department store, and an area next to Renmin hospital where the public and outpatients passed through. These results suggest overall low risk of SARS-CoV-2 transmission in well-ventilated and open public areas, but also reinforce the importance of avoiding large gatherings of people.
Inside the Renmin Hospital ICU rooms, two aerosol deposition samples tested positive for SARS-CoV-2. One had a higher deposition rate than the other. The sample with the higher deposition rate was placed in a corner of the room without any blockage, while the sample with the lower deposition rate was blocked by medical equipment. This suggests that aerosol deposition may play a role in surface contamination and the infection of people touching those surfaces.
Medical staff areas had higher concentrations of SARS-CoV-2 aerosol compared to patient areas. In Fangcang Hospital, aerosol concentrations decreased after the implementation of rigorous sanitation procedures. This confirms the importance of sanitation in reducing airborne SARS-CoV-2.
There are two size regions of SARS-CoV-2 aerosol, the sub-micron region (smaller) and the supermicron region (larger). The sub-micron region was dominant in protective apparel removal rooms in Fangcang hospital. The researchers hypothesize that this is because the aerosol on protective apparel of medical staff is resuspended and becomes smaller while they are being removed. This aerosol may originally come from patients' respiratory droplets, airborne SARS-CoV-2, or SARS-CoV-2 deposited onto the floor.
The findings of this study provide the first investigation into the airborne properties of SARS-CoV-2 in Wuhan. They suggest that toilet use by and crowd-gathering with COVID-19-infected individuals are important sources of airborne SARS-CoV-2. They also suggest that there is a pathway of SARS-CoV-2 transmission involving the deposition and resuspension of SARS-CoV-2 aerosols during the removal of protective apparel by medical staff. The researchers call for particular public health attention on the ventilation and sterilization of toilets, personal protection measures for the general public and avoiding busy crowds, sanitation of high-risk areas in hospitals, the effectiveness of ventilation in field hospitals, the surface sanitation of protective apparel before they are removed.