Models can include viral doses (see Methods of modelling viral disease dynamics across the within- and between-host scales: the impact of virus dose on host population immunity) but you need more information about how the viral load changes with the course of the infection and how contagious people are at different stages. We don’t have that detailed information about SARS-CoV-2, so simpler models have to be used (and we don’t even have all the data we need to set the parameters of even simpler models, which is one reason the predictions have such wide ranges of possible outcomes).
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