Flu vaccines are completely ineffective on everyone. A year ahead of time, they start planning the flu vaccine, what strains of the flu virus they will produce a vaccine for. So a year ahead of time, they GUESS what strain or strains of flu will be around a year later.
Even if they guess right, in a years time, the flu virus has mutated so manny times as to make any vaccine from several mouths back about as useless as tits on a bore hog. Antibodies work in a key and lock fashion. One little point on the key has changed, it will not open the lock.