That is so true. There's actually a sign at my vet's office that says, "There's no such thing as an outdoor-only mosquito."
And just to add to the pile of troubles about vaccines, please be aware that cats in Southern states where mosquitoes are prevalent are also susceptible to heartworms. They don't seem to get them the way dogs do, but it is an issue. One of the neighbors just had to have her cat euthanized due to advanced heartworms. The cat was over 15 years old, and the vet was almost sure that the treatment would not only kill him, but make the remainder of his life a misery. I had never heard of heartworms in cats before. My vet verified it, and Hekitty is now on heartworm meds. She is indoor only, but that makes no difference to a mosquito.