I work alot in those flu clinics you see around town during the months of october and november. I recommend anyone to build up their immune system prior to getting the shot with extra Vit C sprinked throughout the day, getting a high quality immune booster for week before, avoiding sugars in diet,and eating good vitamin riched foods. Any vaccine will depress the immune system some and those on the border line basis might feel little sick, arm hurting,etc. We recommend after the shot to also do the above immune boosting plus move the arm alot for 48 hrs. When your immune system is good, one doesn't usually have side effects. Be sure to NOT get the shot if you are allergic to eggs,chicken,chicken feathers or other vaccine preservatives. Make sure you feel healthy the day you get the shot. Notify the nurse if this is your first flu shot as you need to stay close to them for 15 minutes after getting the shot in case you have a reaction(is rare).Sometimes the arm will be sore,red,swollen and heavy feeling that night. You can take a tylenol,put an ice pack on, but the best thing to do is to move it alot. Hope this helps you all, Azpam