The flu virus in the vaccine is killed (although I believe the nasal vaccine has a live, attenuated virus which means it is alive but cannot replicate nearly as much). To get the flu the vaccine must be live and replicating. Your body mounts an immune response to the dead virus which can cause symptoms similar to the flu although not nearly as extreme. If you are already incubating an illness getting a vaccine, which further strains your immune system (the basic principle of how a vaccine works, otherwise you wouldn't build immunity and the vaccine would be pointless) can make your illness worse. Have you ever had a cold and then caught a stomach bug? It's worse because you're dealing with two problems instead of one. That's why they won't give vaccines if you have a temperature; that indicates you may already be ill and vaccination is contraindicated. People who claim to get the flu afterward were likely already coming down with something.
Sent from my SPH-L710 using MonsterAquariaNetwork App