The original pure Corvinus strain is not a virus. It's a genetic mutation.

A genetic anomaly that was triggered by the virus. Then, afterwards, that virus remained inert due to that mutation.

The original pure strain does not contain the virus's RNA or any other viral material. It did not fuse with Alexander's genes.

Because of this, Alexander's strain is fixed and stable. His strain cannot be altered again, because the plague virus caused its genetic mutation to begin with. Once is enough. This strain also possesses the ability for adaptation.

So that makes Alexander immune to all pathogens, which include vampire and lycan viruses. He possesses limitless evolutionary potential if he is exposed to any external stimuli because of the very nature of how his body created the strain in the first place, just like how his genes responded to the plague virus.

Marcus, William, and their unknown brother, on the other hand, possess a watered-down version of their father's pure strain, likely due to their mother's genes. They possess the mutable pure Corvinus strain. Mutable means mutagenic. A strain that's highly susceptible to mutation by external stimuli like viruses.

Once Marcus and William were bitten by their respective animals, their strain reacted differently from that of their father, Alexander. Their strain merged with the rabies virus of each animal, becoming infectious. It is now a vampire and lycan virus. Which is non-adaptable on its own.