Let s not overlook an important fact:
There is no way you can have 900+ mice and not have some of them escape.
You can bet money that one or more, and my bet is on a lot more, have escaped and whatever they are carrying is now out there.
It might be a few weeks before a human shows signs of infection, but it is coming.
Who’s to say the mice didn’t escape year/s ago?
Because of the way mice are kept in research facilities, they are unlikely to escape. This was a clandestine lab, but still probably used the same kind of cages. Even if a mouse did escape, I doubt there would be much of an impact.
Mice are engineered to be infected with human Covid by replacing the mouse ACE-2 receptor gene with the human ACE-2 receptor gene. They are sold to researchers by biosupply companies.
The primary consequence of such mice escaping would be that they might interbreed with wild mice, resulting in a small number of wild mice that have a human ACE-2 receptor gene. I doubt that the human form of the gene would persist for very many generations.