A new flu vaccine is made every year because the strain changes and mutates to have more dangerous. Personally, around flu season there is 1 primary strain that breaks into multiple. That is why the flu shot doesn’t 100% protect you against the flu, because it can mutate and your body can’t fight against it because it doesn’t know how. The flu is a virus, which means there will never be a 100% cure, only medicines to treat the symptoms.