If I remember correctly, the Lepto vaccine can kill a dog, but it is usually immediate.
I think it depends on whether they use a killed vaccine, etc, on whether symptoms or the virus might activate in a dog. Lepto is bacterial, so I am not sure how that works. The parvo-distemper and rabies (didn't read that was given) is viral, and the vaccine injects some killed-virus so that the body will build up antibodies. Yes, dogs can develop a fever.
And with an animal that is already ill, vaccines can be very dangerous.
It is possible that the parvo-distemper booster caused an issue in your dog? Yes, I think it is. But I worry more about the coronavirus vaccine as I have heard that can cause issues too. I don't give that one because coronavirus can be dealt with if it develops. It is treatable. I have heard years ago that the vaccine caused seizures in dogs. Maybe the ironed that out, but maybe it is causing other problems now. It is really hard to say.
I am really sorry this has happened to your puppy. You did everything you should have. You worked with a vet. You kept taking him back. Could it be the vaccine? Could it be the wormer? Could it be the dog food? Could it be some insidious tick disease?
Have you considered a necropsy. I don't know if they can tell you for sure what killed your puppy but if they could figure it out, then if there is something in the environement, vaccines, wormer, or food, another puppy will not share the same fate.
Again, terribly sorry.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC