No, the initial shots really aren't enough protection, even for indoor cats.
I believe that rabies can be given every 3 years now, but that will probably depend on where you live and what your vet recommends. I believe everything else should still be done annually. Also, your cats really would have needed booster shots in addition to those initial shots, or you'll have to start the process all over again.
Even though your cats don't go outside, you could actually bring viruses into the house that they could contract. Viruses like panleukopenia can actually live on objects and can be transferred on clothes and shoes. So if you came into contact with an infected animal, you could unknowingly spread that potentially fatal disease to your house cats.