You shouldn't compare mice models to humans on lethality of a virus.

In this previous study out of Boston University, they had transgenic mice that had human ACE2 receptors, so that they'd be susceptible to SARS-COV-2.

In the study, all the mice infected with the original wild type coronavirus died.

Obviously in humans the mortality rate of the coronavirus is not 100%.

These mice still have mouse immune systems- they just have human ACE2 expression. The mice had viral infection in their brains.

So don't start panicking when you see headlines like this. It is a mortality rate specific to that mice model and viral dose.