The efficacy of the flu shot varies every year. I think that last year, it was only about 40% effective against the most widely circulating strain, the H3N2 strain. In some years, the efficacy can be 80-90%. The reason the efficacy varies so much is because the people who determine which flu strains to include in the vaccine are looking at currently circulating strains and using that data to make a prediction about which strains will be most prevalent several months from now. In other words, they are much like weather forecasters. Sometimes, they are very close in their predictions. Other times, the circulating strain shifts, so that the dominant strain is not what they predicted.
In any case, some protection is better than none. If you have been vaccinated against a strain that is not a circulating strain but is similar to it, your immune system will still partially recognize the virus. If you get sick, the resulting disease will not be as severe.
Link? I don't believe this.
Additionally do you understand how they determine efficacy?