Side effects are always possible with the flu vaccine. These include localized redness at the injection site, headache, nausea, muscle aches, and fever. But they are generally mild and will go away in a few days. "Very rarely, people can have a severe allergic reaction to another ingredient aside from egg that's used in the vaccine (such as gelatin), which can be life-threatening," Eiras says.
If you have a severe allergy to a vaccine component or you are unsure, talk to your doctor — you may not be able to get the flu vaccine or need to get it under medical supervision. And remember, most reactions to the flu shot are not evidence of an allergic reaction, says Eiras. They are just normal side effects.
There is also some evidence that there might be a small association between the flu vaccine and Guillain-Barré syndrome (GBS), says Eiras, which is a rare disorder that causes the immune system to attack nerve cells. "This is also very rare, and GBS is associated with a variety of other infections as well, so we aren't 100% sure how this happens in relation to the vaccine." If you’ve had GBS, talk to your doctor before getting a shot. If you haven’t, don’t worry. Compared to the known benefits of the flu vaccine, these risks are rare enough that they shouldn’t deter any healthy person from getting the flu shot.