Yes, there are a few who react to distemper. Since you don't know the vaccine history, they one given a month ago could have been his second or third or eighth or... Immediate and serious reactions are not common after DHPP alone. You can talk to the vet about premedicating with Benadryl if you want. some will say go for it. Others don't like to. Just watch him close for the rest of the day, keep Benadryl and the ER number on hand, and stay near the vet's office for the first 1/2 hour or so (this goes for all vaccines given to all dogs, imo). It should be fine.
