Comment by dbsights

Stable across mutations? On what timeframe.. Its been stable so far.

However, create a population with narrow immunity based on a single protein, and you create a selection pressure that incentivizes mutations in that protein. Once a successful mutation exists, it has a wide open field to spread unchecked. With broad immunity (multiple proteins) it is much less likely that a variant can realize the multiple simultaneous mutations that would be required to spread effectively.