I try to get a flu shot every year.
They are free for everyone where I live and I'm happy to take advantage of something which has the potential to protect my health and the health of others.
Needles don't bother me.
Most flu vaccines use dead flu virus or only bits of dead flu virus, and cannot cause influenza infection -- they can cause mild symptoms of infection like fever and aches, though, because most of the symptoms of infection are generic ones caused by your own body; Fever, inflammation, extra mucus production in your nose/sinuses/lungs -- all of those symptoms are caused by your own body trying to expel/destroy pathogens (like bacteria and viruses), not by the pathogens themselves.
Some vaccines use live attenuated virus --meaning virus that has been altered and weakened so that it cannot cause full-blown infection (there is only one in use in Canada, it is a nasal spray) -- it can only cause serious illness in severely immunocompromised people.
When you get sick after getting a flu shot, it can be a few different things happening (these are just the ones I can think of):
1. If it's mild and very brief (and happens immediately after you get the jab/shot) it may just be your body mounting an immune response to the vaccine-- which is exactly what it's supposed to do....Vaccines work like a sort of "practice run" for your immune system -- they basically teach your body how to respond to flu by giving it a dead or weakened version of the virus to practice on, so if you are ever infected with a normal, live version of the virus your body will be able to fight it off before you get sick.
2. It may be that you have caught a strain of flu that the vaccine didn't protect you against. (Flu vaccines protect against only 3 or 4 strains of flu virus each year..... there are hundreds of possible strains.)
3. It may be that you have caught a different type of viral illness that causes similar symptoms. (e.g. a really bad cold caused by a rhinovirus or a coronavirus)
4. It may be that your body didn't respond to the vaccine, or that the response wasn't strong enough (didn't mount that "practice run" immune response, or the "practice run" response wasn't strong enough) -- so you ended up with partial immunity or no immunity at all.
5. If it happens immediately after you get the jab/shot, you could be having some kind of adverse reaction to the vaccine.
6. If it happens within the first week or two of receiving the vaccine, maybe you could have been infected with the flu before your body had time to build an immunity to the virus.