There is a lot of resources available (http://www.webmd.com/cold-and-flu/news/20111025/how-effectiv...). Numbers like 40%, 50%, 60%, 70% appear in research - a lot of bias.

"While the vaccine does work, and we still recommend that it be used, it does not demonstrate the kind of efficacy that has often been reported," says study researcher Michael T. Osterholm, MD, of the University of Minnesota Center for Infectious Disease Research and Policy." - from "Lancet"

In case of flu it is difficult to measure vaccine effectivness since it is often hard to figure out if someone got flu or was attacked by some other virus that has similar symptoms.

It is usually not feasible to do indepth virus (genetical?) analysis to check which kind of virus attacted given sick person.

Symptoms are so common to many other virsu infections, so the data is very noisy and it can work both againts and for vaccines.

It would be cool if some HN community member figure out to measure that better :)

The strain of influenza each year is different. In order to create enough of the vaccine companies need to start making it before it's known which strain will be active. They make an educated guess with varying degrees of accuracy. Some years they get it spot on, some years they don't.