The USDA allows vaccine manufacturers to relabel their 3 year rabies vaccines as 1 year products. This link http://www.calmanimalcare.com/vaccine.htm , you will be taken to the Calm Animal Care website, which has posted Colorado State University's Small Animal Vaccination Protocol for its veterinary teaching hospital, which states:"Even with rabies vaccines, the label may be misleading in that a three year duration of immunity product may also be labeled and sold as a one year duration of immunity product."
"In the case of Defensor 1 and Defensor 3 vaccines made by Pfizer, testing is the only difference between the products. 'The formulations are the same, but regulatory requirements for the one- and three-year vaccines are different, requiring distinct and separate studies for each label,' said Pfizer spokesman Richard Chambers."
States Consider Controlling Rabies Vaccination Intervals, by Edie Lau The Veterinary Information Network News Service 8/12/11 http://news.vin.com/VINNews.aspx?articleId=19501
According to Dr. Ronald Schultz of the University of Wisconsin School of Veterinary Medicine, "There is no benefit from annual rabies vaccination and most one year rabies products are similar or identical to the 3-year products with regard to duration of immunity and effectiveness. However, if they are 1 year rabies vaccines, they must be legally given annually!" from What Everyone Needs to Know about Canine Vaccines
In an April 1, 2008 DVM360 article entitled, Canine Vaccine Update (Proceedings) http://veterinarycalendar.dvm360.com/avhc/Medicine/Canine-vaccine-update-Proceedings/ArticleStandard/Article/detail/562405 by Dr. Craig Datz states that, "..some brands of rabies vaccine are identical whether labeled as 1- or 3-year…"
2006 Canine & Feline Vaccination Guidelines, A Forum on Issues and Controversies by Dr. Richard B. Ford, DVM, Professor of Medicine, North Carolina State University College of Veterinary Medicine http://www.hcvma.org/notes/SpeakerNotesRichardFord.pdf Table 2 on Page 4 states: Vaccine Type: Rabies, 1-year: Minimum Duration of Immunity: 3 Years (must be administered annually)