A cat isn't guaranteed not to get cat flu because of being vaccinated. You were unlucky, and it was a coincidence that it happened one month later. If a vet can get money from you every year for booster shots, then they will. The animals are yours, it's up to you to decide what to do. Educate yourself. I know someone whose cat died within a week of their annual booster, the cat suffered fits and went blind, apparently that is not uncommon, and it was caused by the vaccine the vet said. I stopped vaccinating my last cat after hearing that and he lived to the age of 19, and wasn't vaccinated after the age of 8.