My two eldest cats are birman brothers and life expectancy is about 15 yrs for birmans.
Franta sufferes with seizures on and off and was very bad two/three years ago and two vets from different practices recommended he was not vaccinated any more because he had some kind of brain infection and as it was not known what caused the seizures they thougyht the vacs would be too much for his immune system. He also only has one working kidney discovered when he was ill after a dental just before the seizures started. Sadly the current opinion is that the infection in his brain proberly came from the dental that was done and the infection got into his blood stream and into brain.
His brother Ducha, I stopped him having vacs a year ago just due to his age, the vet agreed it was a good idea, turns out that it was maybe not a good idea.......sigh
It interesting cos I have always disagreed with vacs every year but the drug companies always insist the vacs do not last for more than about 6 weeks over a year and Ducha was about 5 months over the year when he caught cat Flu from Franta. As my two younger cats 9 and 11yrs at the time did not catch it and were vacced to date , unfortunately it seems that certainly the cat flu part does need to be done every year.
My first cat Kocka was also not vaccinated from about 16 years as even the dead vaccine made her very ill but she never caught cat flu from all the neighbouring cats, maybe she was just lucky! She lived until 20yrs old.