[wbill]

I'd think that you'd build up an immunity - at least to prior, existing variants of the flu - as you get a few years of shots under your belt. Obviously not to new strains and strains that aren't a part of the vaccinations, but why not to old ones? I can't imagine that your immune system "forgets" the vaccine every November.

I dunno, I'm not a doc, and I didn't even stay at a Holiday Inn last night.

[MD Expat in PA]

I'd think that you'd build up an immunity - at least to prior, existing variants of the flu - as you get a few years of shots under your belt. Obviously not to new strains and strains that aren't a part of the vaccinations, but why not to old ones? I can't imagine that your immune system "forgets" the vaccine every November.

Aside from there being several different strains of influenza, the virus which is an RNA virus) has a rather high rate of mutation. In the same way the common cold virus mutates and you can get sick from a cold (rhinovirus also a RNA virus) more than once during your lifetime.

It is thought that a flu shot can give you immunity or partial immunity for more than one year, perhaps two, but since it is a crap shoot each year which influenza strain(s) will be the most prevalent, it is recommended to get one each year.

[little jeremiah]

I studied up heavily on flu in general in 2007 on because of H5N1.

You do not build immunity except to the same variant of flu, from what I remember. And there are so many types and they change. That is the problem.