Moreover, there is a perfectly good reason why there is not nor will there be double-blind placebo controlled trials for vaccines. It's simply unethical. Anti-vaccine nuts love to point to the lack of placebo controlled trials for vaccines in an attempt to explain away the lack of any good evidence for their own favorite CAM modalities.
I don't think that conducting a double-blind placebo controlled trial for flu vaccine would be unethical in the least. Placebo controlled studies are unethical only if the participants' health would be seriously jeopardized when receiving the placebo treatment (i.e. death or serious/irreversible health problems). I wouldn't count having a three day flu as a very serious problem.
Moreover, to state that flu vaccines work because, in general, vaccines are a good thing blatantly violates how medical science should work. If there is no well designed study to show that flu vaccines are effective then we cannot state that as such.
As Ben Goldacre would put it, getting the flu vaccination is a medical intervention and as such will always carry a risk of some magnitude. Without conducting a careful risk-benefit study, you cannot give a clear recommendation.