Small, smaller, smallest: is aerosol transmission possible for SARS-CoV-2?

The outbreak of pneumonia caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has resulted in over three million confirmed cases with more than two hundred thousand deaths globally as of April 28, 2020. It’s known that SARS-CoV-2 can be transmitted human-to-human, predominantly through close contact with infected people, contact with surfaces contaminated with SARS-CoV-2, and inhalation of virus laden liquid droplets. These droplets are generated when we cough, sneeze, speak or even breathe and can cause infection in other people when they are inhaled. The devil is in the details: the droplets are categorized as “droplet” or “aerosol” based on their size, which is important to determine their transmission characteristics. A commonly accepted cut-off size between “droplet” and “aerosol” is 5 µm [1]. Droplets are larger (>5 µm), too heavy to remain suspended in air and therefore travel less than 1 meter. The understanding that SARS-CoV-2 is mainly transmitted by droplets supports the CDC recommendation for “social distancing” of 6 feet. The recommended face masks are designed to protect the wearer and others from droplets and sprays. Different from the droplets, aerosols are smaller (≤ 5 µm) thus travel slowly and further in the air. These particles are then more dangerous. N95 respirators are masks designed to filter and protect the wearer from aerosols as they can filter out 95% of droplets smaller than 0.3 um. However, these masks are in very short supply. 

Aerosol transmission causes the viral spread in other diseases such as tuberculosis, measles and chickenpox. It has been suggested as another potential spread mode of SARS-CoV-2, but very little is known on the aerodynamic characteristics, infectivity and transmission pathways of SARS-CoV-2 in aerosols. Viruses are detected by looking for their genetic material with different assays in the laboratory. Because aerosols are so small, they likely contain very little genetic material and thus have been difficult to study. A recent study by Liu et al quantified the SARS-CoV-2 in aerosol samples at 30 sites in two designated hospitals and public areas in Wuhan [2]. They succeeded by using a new technique called ddPCR (digital droplet polymerase chain reaction), which can detect really small amounts of genetic material. The results showed that very low or no virus genomes were found in most well ventilated patient areas in the hospitals, however, a high concentration was observed in a single patient’s toilet room without ventilation. Another study investigated a cluster of COVID-19 cases associated with a shopping mall and concluded that the virus may be spread via aerosolization in a confined public space (e.g., restrooms or elevators) [3]. Although the virus can be detected, a major remaining issue is whether that small amount of virus can cause infection. To answer this question, van Doremalen et al reported that virus from experimentally generated aerosol particles with SARS-CoV-2 can remain alive and infectious in aerosols for hours and on surfaces up to days [4]. No evidence has shown SARS-CoV-2 from human exhaled aerosols can survive outside the body for long, but the Middle East respiratory syndrome coronavirus has a strong capability to survive and remain infectious for up to 60 minutes after aerosolization [5]. Unfortunately, these very small but mighty SARS-CoV-2 laden aerosols are probably still infectious. 

In addition to supporting the social distancing and sanitization measures currently recommended by the CDC, these studies of SARS-CoV-2 aerosols reinforce the following important recommendations:

·     Avoid poorly ventilated indoor areas (e.g. restrooms or elevators) 

·     Avoid crowded areas if at all possible, and otherwise use face protection against aerosols like an N95 mask 

·     An effective sanitization of high risk areas in the hospital is required

·     Surface sanitization of the clothing of medical staff before changing may help reduce their potential infection risk.

See primary paper at https://www.nature.com/articles/s41586-020-2271-3

By Xiaoyuan Zhou

Can SARS-CoV-2 be detected in the smallest particles, called aerosols? Yes says a new paper from Ke Lan’s lab at Wuhan University, which has implications for even more rigorous cleaning protocols.