Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help. Please give me a moment to review your concerns.
I understand that you are curious about proper vaccine protocol for indoor cats.
Some of this will depend upon whether you have one or more cats, whether any of the kitties go outside, and whether your kitties ever try tom sneak outside.
I highly recommend all kittens start out with the same protocol to get a baseline of protection established.
All kittens or new adults coming into your home should be tested for the immunosuppressive viruses feline leukemia and feline immunodeficiency virus. These are more common in stray cats due to their exposure to other cats, but any queen can carry these viruses and pass them to her kittens in utero or after birth when nursing. If a kitty is positive for these viruses he/she could infect your other cat(s), and they will suppress their immune system making secondary infections more likely and increasing the odds of developing anemia, kidney disease and some types of cancer.All cats should also have a stool sample checked for gastrointestinal parasites, very common in young cats.
Cats that go outdoors should be placed on a worming schedule and receive heartworm prevention in areas with mosquitoes.If the kitty is negative for immunosuppressive viruses the kitty should a series of two leukemia vaccines 3 to 4 weeks apart and at least two distemper/respiratory complex vaccines (panleukopenia, calici virus, and rhinotracheitis) 3 to 4 weeks apart with the last leukemia and distemper vaccines being given when the kitty is at least 12 weeks, as well as a 1 year Rabies vaccines. These vaccines should all be boostered one year later. After that vaccines given and their frequency will depend upon whether he goes outdoors and is exposed to many other kitties or not.In most instances in cats that go outside leukemia is given yearly and the distemper complex every 3 years.
If a kitty will only ever live indoors and is not exposed to other cats then the kitten series of leukemia and booster one year later should be sufficient.
I personally recommend even indoor kitties get distemper vaccines every 3 years until about 10 years of age. By then immunity should be established in most cats and risk of exposure being indoors only is such that the benefits aren't enough to continue vaccines.
Of course if later a kitty becomes an outdoor kitty too vaccines can and should be updated.
Rabies vaccine frequency depends upon state law and may be yearly or every 3 years, but Rabies vaccines should be given as directed by the state for life, whether your kitty is indoor only or outside. This protects your kitty and you should an accidental exposure ever occur (bat gets indoors, kitty accidentally gets outside).
Best of luck with your kitties, please let me know if you have any further questions.
I understand that in the UK Rabies is not an issue, but if your kitty travels to endemic areas then it becomes essential. So if you travel with your kitty even if your little one is indoors only he or she will need a Rabies vaccine at that point.
By now both should have fairly good immunity built up.
My concern is that although they don't leave the yard since they sit in your garden, so accidental exposure is possible. Strays are attracted to the smells and sounds of a home/yard with kitties. So perhaps every 3 year boosters for the distemper vaccines would be best for at least one more set.
That's good news. You would be amazed to see some feral cats climb walls to get in! They live a pretty desperate existence.