It depends on the type of vaccine used:
I'm waiting for an answer from the breeder. This may ruin my plans to import those Dorkings from Canada that I've been wanting so badly. I'm supposed to go get them next week.
I spoke to a vet at UCD and he said that he would recommend vaccinating my entire flock. The breeder was surprised that I am able to show without vaccinating. I don't understand that - if vaccinated birds revert to virulence, wouldn't they be able to infect other birds at a show?