SARS-CoV-2 may spread through droplets suspended in the air

Anand Vaidya, TIFR Hyderabad

Wearing a mask has become a way of life. It is now as common as wearing footwear or carrying a cell phone. Along with social distancing and washing hands, wearing a mask is key to slowing down the spread of COVID-19. While washing hands reduces infections via accidentally touching our face with contaminated hands, social distancing and masks mitigate the spread via air while breathing, talking or coughing. But this spread through air or aerosols has been hotly debated, mainly due to the lack of controlled scientific experiments and the size of aerosol drops. The following papers take important strides to confirm the spread of COVID-19 through air.

Mathilde Richard and colleagues, infected ferrets with SARS-CoV-2 virus in a laboratory environment leading to human like-COVID-19 infection [1]. They then placed an infected ferret and an uninfected one next to each other, in two separate cages. The cages were 10 cm away from each other to prevent physical contact between the ferrets and they were under a constant airflow from the infected animal toward the uninfected. The uninfected ferret got infected with the virus in 3 to 7 days. Keeping an infected and an uninfected ferret together in the same cage transmitted the virus in 1 to 3 days. Similar experiments with golden hamsters (a species of rodent) also showed the spread of SARS-CoV-2 through air [2]. This could be either due to the respiratory droplets, which are larger in size or the aerosols that are smaller. But none of these studies clarify the size of the respiratory drops that are mainly responsible for the spread through air. A different group of scientists studied precisely this aspect.

Joshua Santarpi and colleagues collected air around six human COVID-19 patients and separated three different sized drops - smaller than 1 micron, between 1 and 4 micron and larger than 4.1 micron (thickness of human hair is about 50 micron) [3]. They then tested to see if these three samples contained SARS-CoV-2 virus that could infect human cells and found that the drops smaller than 1 micron successfully infected human cells. These smaller drops are easily airborne and could play a key role in spreading the infection through air. Though other similar studies, based on a couple of patients, showed mixed results, it is important to study these drops using larger numbers of patients in different hospitals. 

1. Richard M., et al SARS-CoV-2 is transmitted via contact and via the air between ferrets, Nature Communications (2020)
2. Sia S. F., et al Pathogenesis and transmission of SARS-CoV-2 in golden hamsters, Nature (2020)
3. Santarpia, J., et al, The Infectious Nature of Patient-Generated SARS-CoV-2 Aerosol, medRxiv (2020)
4. Ong S. W. X. et al, Air, Surface Environmental, and Personal Protective Equipment Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) From a Symptomatic Patient, Journal of the American Medical Association (2020)

[Last updated 10 September 2020]