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No vaccines are 100% effective, so it's possible that patient A catches Covid-19 even after receiving two doses of vaccine. I understand that the risk is low, and the symptoms will generally be mild.
Now, patient A infects patient B who is not vaccinated.
Will patient B experience mild symptoms, or will their chances of hospitalization, ICU and death be the same as they were at the beginning of the pandemic?
There's no way to know what effect passage through one vaccinated individual would have on an unvaccinated individual. The relationship between a virus's fitness in a particular host and the severity of disease it causes in that host is not straightforward, but generally we would expect a virus variant that can infect a vaccinated host well enough to pass on to another to be MORE aggressive and therefore cause more severe illness. With SARS-2 specifically, there's a chance a vaccine-selected variant could be milder, if the way it evades immune response is by more selectively limiting infection to those cells in the lungs that are involved in forming respiratory droplets and aerosols. (here is a reference if you want to read about mucosal vs. systemic immunity in COVID) My gut feeling is that such a mutation is much more complicated and therefore less likely than mutations that increase immune evasion and replication speed; which would lead to more severe disease.
Passage through animals leads to attenuation because the cellular machinery and immune response are different. A virus optimized for cows is no longer optimized for humans. A vaccinated human is still a human though, the mutations that work in one will work in the other. Here is a paper discussing how HIV adapts to the immune system of individual infected humans, and how those strains are 'pre-adapted' to closely related hosts. You may find it relevant.
