This the theory of the flu virus, on which the flu shot is based every year.
There are gazillions of flu virus particles in the world. Every year one of those (and only one) mutates, somewhere in Asia (why in Asia?). Viruses are so small that they can’t be seen under a microscope, but still scientists are able to identify this one mutated virus.
Once this virus has mutated it starts to multiply. For that it needs hosts, but nobody gets sick. Then it starts to travel. It jumps from healthy host to healthy host and travels by foot, bus, car, train, plane or ship to all areas of the world. It does this remarkably fast, so it arrives everywhere in time for the “flu season”. And the virus also knows where to travel to, for it first goes to the Northern Hemisphere and then six months later to the Southern Hemisphere.
And when the virus has arrived… it waits. Where does it wait? Considering that viruses can’t exist outside a host and nobody has the flu, the virus has nowhere safe to wait. But nevertheless, when winter arrives at once everywhere the “dormant virus” comes to life and starts infecting people everywhere at the same time. And then when winter is over it disappears. It doesn’t stay dormant till next year. For next year the whole cycle start over again with a different “mutation”.
It’s no surprise that the flu shot is 0% effective.
Common sense: 0