VETSCOPE

I get my cat vaccinated every year, but this year she suddenly became ill about 10 days after the vaccine. I took her to the vet and she had a very high fever. The vet told us the vaccine might've had some effect but the true cause wasn't clear. The fever subsided after about a week of antibiotics, but I'm afraid of the same thing or maybe even worse side effects happening next year. She had a serious roundworm infection when she was young, and since then she's been unable to digest certain proteins (She will have diarhhea after eating food such as chicken, salmon, bonito). It's commonly said that you should get your pet vaccinated every year, but is that really necessary?? I've heard of some clinics who only vaccinate once in three years. By the way, I have three cats in total, and they're always kept indoors.