From BMJ May 2019: Minerva BMJ 2019;365:1993
A review in Science indicates that vaccines for mumps, whooping cough and yellow fever lose their effectiveness more quickly than those for measles, diptheria, tetanus and flu.
The flu vaccine at best only protects about 60% of the people given it in any given year. Its effectiveness also declines after just a few months. If you are first in the queue to get it towards the end of September, much of its effects will be lost by January and February which are the peak months for flu infection.
My comment: Maybe you should plan to get the jag any time from mid November to mid December if you are very keen on getting maximum effectiveness to prevent flu?