Mild side effects usually begin soon after you get the vaccine and last one to two days. Possible mild side effects of the flu shot include: Soreness, redness, and swelling at the injection site Fainting, mainly in adolescents Headaches Fever Nausea Possible mild side effects of the nasal spray include: Runny nose Wheezing Headache Vomiting Muscle aches Fever Serious side effects usually begin within a few minutes to a few hours after receiving the shot. Possible serious side effects of vaccination include: Difficulty breathing Hoarseness Swelling around the eyes or lips Hives Paleness Weakness Racing heart Dizziness Behavior changes High fever If you experience any of these reactions, seek medical attention immediately.