ANIMALS DIDN'T DIE OF THE VACCINE AUGH This asshole needs someone to pound the concept of euthanizing a test animal to check for changes in various tissues and organs. Also, it's not like they can be re-used for a different research project afterwards, so yeah, they get killed and examined.
I think they're required to euthanize? The Animal Freedom from Testing, Experiments & Research Act was introduced a few years back (it would allow healthy animals to be adopted), but I don't know if it ever actually got voted on or went into effect. And it specifically excluded rats and mice.