Wed July 6th 2022
16:00 – 17:00
Seminar Quantifying the risk of airborne transmission of SARS-CoV-2 by a cross-approach between epidemiology and biology
Bruno Andreotti


Two approaches have been proposed in the scientific literature to determine the risk of virus transmission in different environments. The epidemiological approach, based on contact tracing and testing, provides precise quantitative information but requires very large statistics and has a blind spot: remote contamination in anonymous public places such as public transport is difficult to assess. The "ab initio" approach, based on knowledge of virology, immunology, and molecular biology, as well as on the laws of dispersion of contaminants in the air, is complementary: it allows one to determine the risk in any type of environment but can present important biases of parameterization and calibration. I will compare these two approaches to identify their current limitations, to link the epidemiological and biological characteristics associated with each viral strain and to deduce modalities for the construction of sanitary protocols from this crossed set of knowledge. I will then revisit the quantitative assessment of airborne transmission risk, considering asymptotic limits that considerably simplify its expression. I will show that the aerosol transmission risk is the product of three factors: a biological factor that depends on the viral strain, a hydrodynamical factor defined as the ratio of concentration in viral particles between inhaled and exhaled air, and a face mask filtering factor. The short range contribution to the risk, both present indoors and outdoors, is related to the turbulent dispersion of exhaled aerosols by air drafts and by convection (indoor), or by the wind (outdoors). I will present experimental results showing that airborne droplets and CO2 molecules present the same dispersion. As a consequence, the dilution factor, and therefore the risk, can be measured quantitatively using the CO2 concentration, regardless of the room volume, the flow rate of fresh air and the occupancy. I will show that the dispersion cone leads to a concentration in viral particles, and therefore a short range transmission risk, inversely proportional to the squared distance to an infected person and to the flow velocity.
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The 10th Complex Motion in Fluids 2022
Max Planck Gesellschaft
Centre for Scientific Computing