In this work, different transmission modes of the SARS-CoV-2 virus  and their role in determining the evolution of the Covid-19 pandemic are analyzed  . Probability of infection caused by inhaling infectious droplets (initial, ejection diameters between 0.5-750\mu m) and probability of infection by the corresponding desiccated nuclei that mostly encapsulate the virions post droplet evaporation, are  calculated. At typical, air-conditioned yet quiescent , large indoor space, for the average viral loading, and at early times, cough droplets of initial diameter between 10 \mu m and 50 \mu m have the highest infection probability. However, by the time they are to be inhaled, the diameters are most likely 5-6 times smaller with respect to their initial diameters. While the initially near unity infection probability due to droplets (airborne/ballistic) rapidly decays within the first 25s, the small yet persistent infection probability of airborne desiccated nuclei decays appreciably only by  1000s  . Combined with molecular collision theory adapted to calculate frequency of contact between the susceptible population and the droplet/nuclei cloud, infection probabilities are used to define infection rate constants , ab-initio, leading to a SEIR model . Assuming the virus sustains equally well within the dried droplet nuclei as in the droplets, the floating nuclei leads to a stronger contribution to the corresponding rate constants with respect to the droplets, in the above-mentioned conditions. Combining both pathways, the basic reproduction number \mathcal{R Viral load, minimum infectious dose, sensitivity of the virus half-life to the phase of its vector , extent of dilution of the respiratory jet/puff by the entraining air are the important factors that determine specific physical modes of transmission and the pandemic evolution .
Identifying the relative importance of the different transmission routes of the SARS-CoV-2 virus is an urgent research priority. To that end, the different transmission routes, and their role in determining the evolution of the Covid-19 pandemic are analyzed in this work . Probability of infection caused by inhaling virus-laden droplets (initial, ejection diameters between 0.5-750\mu m) and the corresponding desiccated nuclei that mostly encapsulate the virions post droplet evaporation, are individually calculated. At typical, air-conditioned yet quiescent indoor space, for average viral loading, cough droplets of initial diameter between 10-50 \mu m have the highest infection probability. However, by the time they are inhaled, the diameters reduce to about 1/6^{th their initial diameters. While the initially near unity infection probability due to droplets rapidly decays within the first 25s, the small yet persistent infection probability of desiccated nuclei decays appreciably only by \mathcal{O 1000s ), assuming the virus sustains equally well within the dried droplet nuclei as in the droplets . Combined with molecular collision theory adapted to calculate frequency of contact between the susceptible population and the droplet/nuclei cloud, infection rate constants are derived ab-initio, leading to a SEIR model applicable for any respiratory event - vector combination. Viral load, minimum infectious dose, sensitivity of the virus half-life to the phase of its vector and dilution of the respiratory jet/puff by the entraining air are shown to mechanistically determine specific physical modes of transmission and variation in the basic reproduction number \mathcal{R .
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), assuming the virus sustains equally well within the dried droplet nuclei as in the droplets
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probabilities are used to define infection rate constants ,
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. Assuming the virus sustains equally well within the dried droplet nuclei as in the droplets, the floating nuclei leads to a stronger contribution to the corresponding rate constants with respect to the droplets, in the above-mentioned conditions. Combining both pathways, the basic reproduction number \mathcal{R
applicable for any respiratory event - vector combination.
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, extent of
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the important factors that
shown to mechanistically
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the pandemic evolution
variation in the basic reproduction number \mathcal{R