Feeling sick after a flu shot

Feeling mildly unwell after a flu shot can happen as your immune system responds, but the vaccine cannot cause influenza. A sore arm, tiredness, headache, muscle aches, or a low fever may begin soon after vaccination and usually settle without special treatment.

The injection trains your immune system to recognise influenza. That activity can leave you tired or achy for a short time. The vaccine is made from inactivated material, so it cannot give you flu; you may instead have picked up another infection around the same time. A severe allergic reaction is uncommon but can develop soon after vaccination, with facial swelling, wheezing, widespread hives, or trouble breathing.

Rest, drink fluids, and use a cool compress on a tender arm. Gentle movement can reduce stiffness. If you normally can take it, ask a pharmacist about an appropriate pain or fever medicine and follow the label. Avoid strenuous activity while you feel faint or feverish. Keep a note of when symptoms started and whether they are improving.

Seek emergency help for trouble breathing, swelling of the lips or throat, collapse, or rapidly spreading hives. Contact a clinician if fever is high or persistent, symptoms are worsening rather than easing, or you are concerned about a reaction. A severe headache with weakness, confusion, or a stiff neck also needs urgent assessment.

Ask which symptoms are expected after your vaccine, which pain medicines fit your health history, and whether a future dose needs any special planning if you reacted before.