My guess is that the first one primes the immune system to be ready for the attack, and the second one is where the most antibodies are made since the body now kinda knows what it's going after after receiving the first shot. That's super informal, MJ probably could say things better and more completely.
I should add, that some viral vaccines only need one successful vax to provide long term immunity. I am wondering those might be highly adjuvated and maybe the flu Vax not so much. No clue, not much info out about it since most dog owners haven't been using it since it's only for dogs in high dog exposure areas.
Edited by MaxaLisa (02/04/18 12:39 PM)