We were originally told most mutations are less lethal than the original. Now, which is it?
That’s how they usually work...unless you engineer them to become more lethal.
There are many, many mutations constantly occurring. The overwhelming percentage of them are less dangerous. The tiny percent that are more contagious do what contagious things do: they spread. So we hear about them.
The bioweapon creators have always known that contagiousness is the most important characteristic, outside of the psyching out of the enemy mindset.