Doctors debunk 15 common misconceptions about the flu and the flu shot - Business Insider

... A couple of reasons why you may still get the flu? 1. You may have been exposed to the virus right before you got the vaccine, so the vaccine didn't have time to kick in. 2. You might be exposed to other strains of the flu that weren't in the vaccine. 3. Your immune system may not have mounted a strong enough response to the vaccine. Or 4, you have some other virus and it's not actually the flu. Myth 5 the flu vaccine is effective for a year. Well, it's typically effective for the entire flu season, which is about 6 months, but then the immunity starts to wane, and also the types of viruses or the strains of viruses that cause infections could mutate...
https://www.businessinsider.com/doctors-debunk-flu-myths-vaccine-shot-symptoms-fever-cough-2020-10