The airborne transmission of SARS-CoV-2 could play a greater role than previously assumed. A research team from America, together with several cooperating laboratories, found out that the virus particles in aerosols remain viable in the air for up to 16 hours.
A research team from America led by Dr. Chad Roy and Dr. Alyssa Fears from Tulane University in New Orleans, together with cooperating laboratories, has measured the potential of SARS-CoV-2 as an airborne virus and compared it with that of other known coronaviruses, namely SARS-CoV (2002) and Middle Eastern Respiratory Syndrome CoV (MERS-CoV; from 2012). Additionally, the experts investigated how long the viruses can maintain their infectivity when distributed in aerosols.
For their investigations, the researchers used jet atomizers to produce viral aerosols. Comparative experiments were conducted in four separate aerobiology laboratories. The aerosols were generated in exposure chambers. Generators distributed the aerosols in orders of magnitude from 1–3 μm, meaning they are respirable particle sizes. Inside the chambers, aerosols were exposed to a total flow of half to one air exchange per minute – depending on the test laboratory.
By using the chambers and the corresponding flow rates, the research team was able to determine the dynamic potential for airborne transmission. Aerosol samples were collected during the initiation of the aerosols into the chamber by the atomizers and continuously for 10–30 minutes. To determine the aerosol efficiency, the researchers defined a so-called spray factor. The spray factor is the result of two values: The initial titer, which determines the number of viruses in the respective liter dispensed via the atomizer (PFU (plaque forming units)/liter in liquid supply) and the number of viruses in the aerosol (PFU/litre aerosol). Both values form the quantitative indicator with which the potential for airborne transmission of the viruses was assessed.
Further studies with SARS-CoV-2 in one of the cooperating laboratories determined the long-term stability of the airborne virus. A rotating (Goldberg) drum was used to create an environment in which the settling speed of the 2–3 μm particles is overcome by the rotational speed of the drum. This produces a static aerosol suspension (aerosol suspension stability experiment). The collected aerosol samples were subjected to various methods to determine the concentration of infectious virus particles (cell culture, RNA determination) The scanning electron microscope was also used to investigate whether the structure of the virus in the aerosol suspension changed.
Measured by the so-called spray factor, SARS-CoV-2 showed the highest concentration of virus particles compared to SARS-CoV and MERS-CoV. The aerosol suspension stability experiment showed that SARS-Cov-2 maintained its ability to replicate and therefore retained its infectivity at all times – even when sampling was performed at 16 hours aerosol suspension. Using scanning electron microscopy, the researchers were also able to show that the airborne SARS-CoV-2 maintained its structure as well as its size and aspect ratios for up to 16 hours. This means that after 16 hours and 10 minutes, the virus particles still resembled the shape and structure of the virus particles examined before aerosol generation, which leads the research team to conclude that infectivity is still present.
According to the scientists, these data suggest that SARS-CoV-2 generally retains infectivity during airborne transmission over short distances and that this novel coronavirus is more viable over longer periods of time than would be expected when produced as a highly respirable particle (2 μm).
Some of the aerosols naturally produced by coughing and breathing fall within the size distribution used in the experimental studies carried out by the American scientists (<5 μm). The authors conclude that individuals infected with SARS-CoV-2 have the ability to produce viral bioaerosols that can remain infectious over long periods after produced through human excretion and airborne transport.
4. Fears AC et al. Comparative dynamic aerosol efficiencies of three emergent coronaviruses and the unusual persistence of SARS-CoV-2 in aerosol suspensions. medRxiv preprint doi: doi: https://doi.org/10.1101/2020.04.13.20063784