Active or inactive it triggers a small immune response, and once the body recognises the virus it knows how to respond if there is a future full blown exposure. That recognition (immunity) decreases over time, so it makes sense that repeating sooner would keep the immunity higher. I've heard of nasal sprays for humans, the guys at work get it and all say they get sick from it.I was interested (I know the human flu vaccine isn't live) and found an article on a USA vet site which said that intramuscular injections are usually inactivated, intranasal vaccines are usually live. The interesting thing was that it went on to talk about a live, intranasal, version of the flu vaccine, implying that it offered better protection than the inactive injection. Has anyone heard of this? I've only ever been offered an injection by my vet.