Vaccination allergy

Has anyone's kitty experienced a vaccination allergy? My Tabby, August, developed an allergy to yeast contained in vaccination serum at the age of six and used up a couple of nine lives to survive the reaction. Fortunately he was an indoor cat and so could be protected from many possible afflications that can occur with roaming kitties. My curiosity continues about this allergy because although we were able to treat August successfully and he did live to be 16, I never was able to get a statisfactory explanation about the reaction he had to vaccination. I would appreciate your thoughts on this one.