Not sure what you mean by adding up.
mRNA stuff is different than DNA
it codes faster with different mutations, which leads to new mutations, it is a living organism.
if the host provides a mechanism to prevent it from spreading, that strain sort of dies out.
at the same time there are other mutated strains that are not affected by that mechanism that go on infecting and probably more successful
The growth models that are used in previous epidemic models are usually sometype of non-linear model or if more current a logistic curve approach.
no doubt some of the folks in math modeling in biological systems are working with 3-5-7th order catastrophe models, with the big computers
