What should you know about flu shot fatigue?

Your immune system begins responding to the vaccine after it is given. That response may leave you feeling sleepy, low in energy, or mildly achy. The injection itself does not cause influenza. Fatigue may occur with soreness at the injection site, a headache, or a mild temperature. These effects should settle rather than steadily intensify.

Plan a quieter day if possible, drink regularly, eat as usual, and sleep when your body needs it. Gentle movement is reasonable if you feel able, but postpone strenuous exercise while you are unwell. Check with a pharmacist or clinician before taking a medicine for discomfort, especially if you are pregnant, have liver or kidney disease, or take regular medicines. Keep the vaccination record and note when symptoms began.

Contact a clinician if fatigue is severe, continues without improvement, or stops you managing basic activities. Seek urgent help for trouble breathing, swelling of the face or throat, widespread hives, fainting, confusion, chest pain, or marked weakness. Those symptoms can signal a serious reaction or another illness and should not be attributed to the vaccine without assessment.

Ask whether your symptoms fit an expected vaccine reaction, whether another infection could explain them, and which pain or fever medicines are suitable with your health conditions. You can also ask when to return to exercise and whether a future flu vaccine should use a different formulation.