Update: it wasn't a vaccine, but a wormer. They suspect a bad batch of stuff since its supposed to be non-toxic even at levels way above the normal dose. Almost half the litter died and the survivors are terribly affected with neurological symptoms.
Thankfully there was another litter born a few weeks ago and we get to pick from them. There are still lots of questions but at this point it seems to be a random event. The dead pups and suspect medication have been sent to a university for testing.
