Fever after a flu shot: what should you know?

Flu vaccination prepares immune defences using vaccine components; it does not cause influenza infection. Some people feel warm, tired, achy, or sore at the injection site while the immune system responds. A fever that begins later may instead be a separate viral illness, especially with cough, sore throat, or exposure to someone sick. The vaccine also does not protect against every cause of fever.

Take the temperature, rest, drink fluids, and wear comfortable layers. Keep the arm moving gently if it is sore. Fever medicine may be appropriate for discomfort, but check the label and ask a pharmacist or clinician if the person is a child, pregnant, has liver or kidney disease, or takes other medicines. Do not use leftover antibiotics, and do not take more than the recommended amount of any product. Read about the vaccine material in [Inactivated Vaccine](/materials/inactivated-vaccine/) if you want background on how this type of vaccine is made.

Call emergency services for difficulty breathing, swelling of the face or throat, widespread hives with faintness, collapse, or severe confusion. Contact a clinician if fever is high, lasts longer than expected, returns after improving, or comes with chest pain, severe weakness, or dehydration. A person with a serious long-term condition or reduced immunity should ask their usual healthcare team when fever begins.

Ask whether the symptoms fit a vaccine reaction, whether another infection is possible, which medicine is safe, and whether future vaccination needs a modified plan. Share the vaccine date, previous reactions, allergies, and all current medicines.