(Yes, putting aside most of my crafts projects has left me with a lot of time to read and think and exposit.)
Somewhere I once read a statement by a doctor (I wish I had the reference, but I don't) saying that vaccination could only ever be an effective strategy against a limited number of slowly-mutating micro-organisms. Limited, because there is a physical limit to how many vaccinations a human body can receive and produce immunity from. Slowly-mutating, because the antibodies that are created to identify and destroy a particular threat won't be effective if it has changed so much that they can't recognize it.
Influenza is not slowly-mutating, and has a variety of strains. So, for the flu shot, each year "they" try to guess what the most prevalent strains will be (say, strains A, B, and C), and the manufacturers produce a vaccine that targets these strains. A fairly high percentage of the population dutifully get their flu shots, and for most of them it is effective: they don't fall ill with strain A, strain B, or strain C, and so those strains don't spread through the population. Instead, the flu strains that go around are strains D, E, and F. Health officials issue a statement: "Oops, we guessed wrong. We promise to do better next year, so be sure to get your flu shots then!" Rinse, and repeat the whole sequence the following year.
So, the strains that are in your flu shot are never the strains that are actually in circulation around you that year. Probably widespread flu shots do help somewhat to reduce the total number of flu cases, by blocking the spread of the frontrunner strains each flu season. But they can't be counted on to protect any particular person from getting the flu, and it is bad science to claim that they can. (Public health policy operates on the assumption that People Are Idiots, and shamelessly uses bad science and propaganda to promote its various initiatives. But actually, only Most People Are Idiots.)