When I first got my cat, I had him vaccinated and a big, ugly lesion
opened up between his shoulder blades.  It looked horrible.  But, it
didn't seem to affect his general health, and after awhile it healed up.

I studied up on the regulations for animal vaccines and discovered it is
a largely unregulated business, which relies on after-the-fact reports of
side effects/problems rather than systematic testing before a particular
vaccine is marketed.  I also discovered that manufacturers of animal
vaccines make extensive use of "adjutivants," which are chemical
irritants added to the vaccine to heighten the immune system reaction and
thereby reduce the required dosage of the more expensive biological
cultures that are the basis of the vaccine.

I keep my cat indoors, and after what I read, I decided I will only get
him vaccinated in the future if it is necessary for travel.

Charlie