there are so many different types of flu, although many of the symptoms are the same, the virus differs in so many ways, no one person can suffer with the same flu virus twice.
this is because the white blood cells and antibodies within the blood will instantly recognize a virus that has been had before.
many inoculations work on the idea of not providing/injecting a cure for a specific virus, but rather to inject a small controlled amount of a specific virus so that the body can easily combat it and book it (so to speak) for later reference if the virus were ever to be encountered again, the bodies immune system can recognize it and thwart the virus without undue suffering or need for further medication.
i think there is a specific name for this type of vaccination and inoculation, but can't recall it