Pet owners all know that a rabies vaccine for their pet(s) is mandatory. As in people the subject of vaccination can be a bit controversial, but the controversy isn't over whether to vaccinate or not, it's over how often to vaccinate.
I just learned that SC state law doesn't specifically require annual rabies vaccinations (thanks to the people behind the Rabies Challenge Fund for the tip). It's left up to the vet to determine the appropriate vaccination frequency.
From SC Code Title 47 Chapter 5 (Rabies Control Act) (emphasis mine),
SECTION 47-5-60. Inoculation of pets; certificates and tags.
A pet owner must have his pet inoculated against rabies at a frequency to provide continuous protection of the pet from rabies using a vaccine approved by the department and licensed by the United States Department of Agriculture. Evidence of rabies inoculation is a certificate signed by a licensed veterinarian. The rabies vaccination certificate forms may be provided by the licensed veterinarian or by the department or its designee. The veterinarian may stamp or write his name and address on the certificate. The certificate must include information recommended by the National Association of State Public Health Veterinarians. The licensed veterinarian administering the vaccine shall provide one copy of the certificate to the owner of the pet and must retain one copy in his files for not less than three years. With the issuance of the certificate, the licensed veterinarian shall furnish a serially numbered metal license tag bearing the same number and year as the certificate with the name and telephone number of the veterinarian, veterinary hospital, or practice. The metal license tag at all times must be attached to a collar or harness worn by the pet for which the certificate and tag have been issued. Annually before February first, the veterinarian shall report to the department the number of animals inoculated against rabies during the preceding year. The department, in conjunction with licensed veterinarians, shall promote annual rabies clinics. The fee for rabies inoculation at these clinics may not exceed three dollars, including the cost of the vaccine, and this charge must be paid by the pet owner. Fees collected by veterinarians at these clinics are their compensation.
For most vets, I'm sure that "at a frequency to provide continuous protection of the pet" means annually because that's the protocol they grew up with. There seems to be a growing amount of evidence showing that current rabies vaccines can provide protection up to 3 years, and efforts are under way to test rabies vaccine protection out to 5 and 7 years.
For the state's official stance, contact the SC State Veterinarian (I think DHEC is probably the best place to start). The email I received from the Rabies Challenge Fund lists Dr. Stephanie Cox as the State Veterinarian, although I was unable to confirm this. The searches I performed listed Dr. John A. Caver from Clemson as the State Vet, although that appointment was made in 2005 and I have no idea if that information still current.
When your pet is due for the next rabies shot, check with your vet to see if a 3 year vaccination protocol is appropriate. Your vet may want to perform blood tests to check antibody levels to ensure an adequate level of protection.