[Shethink13]

Influenza vaccines are made using three or four different strains of influenza, but are effective against a number of different clades, as long as they are still fairly close to the original strain.

But of course there does exist naturally acquired immunity for Influenza Type A and B, which would be protective against the four strains of influenza that are present in the vaccine (generally 2 Flu A strains (H1N1,H3N2) and 2 Flu B strains dependent on the season. But no health professionals are ever interested in testing individuals for the presence of these antibodies before recommending the flu vaccine. What other pharmaceutical do you know of is recommended without first knowing whether it is necessary?

[exDemMom]

Influenza is a very special case where vaccines are involved.

For most diseases, antibody mediated immunity decreases over time. This is the case with influenza, as well. In addition, influenza mutates a lot through various mechanisms, so that even if you are immune to a particular strain of H1N1, you are not necessarily completely immune to the strain that circulates the following season. While there is, as you noted, some cross-immunity, that does not substitute for a fresh dose of (hopefully more targeted) immunity conferred by vaccine.