I would guess because he got the first shot at 6 weeks before the immune system can properly process it.....they are giving a fourth. It would have been easier on the pup to have given the rabies alone and later but if he did not have a reaction - he will need a booster for that one in a year.
NAPWDA Certified Cadaver Dog
Waiting at the Bridge (italics
=GSDs) (hemangiosarcoma=blue):Grim , Cyra, Toby
, Rainbow, Linus, Oscar
, Arlo & Waggles