I have read from a few sources that is is not necessary to keep giving boosters, they only work against the dogs own immune system. Mine have had the first lot and then one booster each.

It is worth reading up on. I confronted my old vet with the facts from the net and he could not deny what I was saying all he kept saying was you really should have them.

But then again I have never had to leave my dogs in kennels. I go to agility with my dogs and they don't check them and I know my agility teacher is against booster because she is into alternative treatments. I have also heard of some kennels now that don't require boosters.