What you should really consider is how much of a chance is there of your cat getting exposed to other animals.

Ours are "indoors only", but they have a cat door out into a screen porch out back. Which butts up against the woods, separated from the start of the trees by a 3 foot wide gravel path.

Stray and outdoor cats can and do come through the yard and path, and the more obnoxious toms try to attack right through the screen. (we have to patch all the pulls and tears periodically.)

Raccoons come right up to the screen also. In our area, there is a big rabies problem among raccoons especially. Once, a raccoon got through the screening (pulled up a corner on one of the doors) and actually made it into the house, as we had the door from the house into the porch open for fresh air (why not, the porch is entirely closed in, right? Ha!). Also, skunks, oppossums, squirrels, various other rodents...... And rabies is so contagious that a cat could indeed catch it through a screen. If a rabid animal ended up spitting some saliva through the screen onto our cats, that could be enough, as well as to transmit it to us, also.

So we vaccinate. Seen several of my father's cats die of the leukemia, it is not fun. (Mom used to take in cats for the local animal welfare, especially the non-adoptable ones, they always had about 6 with them at a time.)

Our first two, we got as kittens and they lived to 19 years old, each. The one who passed last year, we adopted at least 9 years old, when she was a rescue cat who had been a stray for at least 6 months, then ended up passed around a bit for a year before we took her in, weighing 18 pounds and looking like a fur-covered beach ball. We slimmed her down a bit to 14 lbs, and she lived another 6 years with us. Now we have two 4 year olds. Who, thank you for reminding me, are about due for their shots. But I will ask the vet about the better versions.