There are so many incidences of sudden bald spots appearing in otherwise healthy well cared for cats these days that some homeopathic vets suspect that these are cuased by the overwhelming of a cat’s system with chemicals and toxins. In other words, we over-medicate our feline friends: flea medications and vaccine shots. Then we take them home to a house we clean with all sorts of powerful household detergents… you follow my drift. Sometimes I think this is true. But this is not helping you right now. Since the bald spot on your cat is in the neck region and seems to be growing, are you certain that it is not ringworm? When you go back to the vet, have a skin scraping done to rule out mites or heaven forbid – mange. It may also be the case that your cat has suddenly developed an allergic reaction to hs collar due to his immune system being temporarily depressed becuase of the recent rabies shot and the stress of the Vet visit.

Oh… my cat has a tiny tiny bald spot on his left snout from what I suspect was a bee sting from years and years ago. Hair never grew back on that spot which is about the size of a mung bean. Its so small that it does not detract from his general handsomeness

diet

The vaccination may have caused a slight reaction by the sounds of it.The only other reasons for sudden hair loss in cats is fleas or allergys.Sorry i have not been able to give you any other advice.Hope it is of some help though.Good luck and hope all goes fine with him at the vets.