Macie, my Sheltie is 3 years old. She had her first shot at around 5 months. (August 2007) She had her 2nd shot in July 2009- They wouldn't give her the 3 yr shot because she had skipped a year. I was told when I came back in July of this year, she would get the 3 year vaccine. What is the difference between the annual vaccine and the 3 yr vaccine, if any?