My two went to the new vet the other day and got the 3 year vaccination - my understanding was the main difference is that the label rated it for 3 years (as opposed to it containing 3 times the product), while the annual one only has a label rating it for one year. So vets are reluctant to operate "off label" (liability). My dogs showed no effects at all from the 3 year vaccination. One of the links I had in my earlier post said that it is fine to use the annual vaccination just every 3 years, you just need to give your vet "informed consent" so they can use it "off label". For kennels that require kennel cough, you can get a vaccination that is effective within 72 hours apparently.