1) Most important is parasites; must not have any ascarids; (roundworms), as these severely change the ability to produce cytokines, also interfere with the ability to make antibodies against distemper, which interferes with vaccine also.
2) Look for skin parasites. (All these things cause a change in the way the inducer works).
3) Full blood count
4) Full urine test
5) Thyroid test
Some breeds are incapable of creating cytokines. See the following post below.