Vaccine Side Effects
The expected reactions after vaccination, and the rare ones that need attention.
Last revised
Key takeaways
- Common and expected: sore arm, tiredness, headache, mild fever for a day or two
- Timing: usually within 48 hours, settling on their own
- Most useful single change: Use paracetamol for discomfort and keep the arm moving.
- Commonest mistake: Assuming a mild fever means the vaccine gave you the disease.
- See a doctor if any of the signs under "When to get help" below apply to you.
Why it matters
Most reactions are the immune system doing its job, and distinguishing them from the rare serious ones is what allows people to stop worrying.
The numbers, in one place
| Measure | Figure |
|---|---|
| Common and expected | sore arm, tiredness, headache, mild fever for a day or two |
| Timing | usually within 48 hours, settling on their own |
| Anaphylaxis | very rare, within minutes, and treatable on site |
| Reporting | through the MHRA Yellow Card scheme |
What helps
- Use paracetamol for discomfort and keep the arm moving
- Wait the observation period if you are asked to
- Report anything unexpected through the Yellow Card scheme
What to avoid
- Assuming a mild fever means the vaccine gave you the disease
- Routine painkillers before vaccination, which may blunt the response
Where to go next
- New to Vaccine Side Effects? Start Here
- Vaccine Side Effects: The Questions People Actually Ask
- Vaccine Hesitancy: Sorting the Evidence From the Noise
- Immunosuppression and Vaccines: What Published Guidance Supports
When to get help
Call 999 for swelling of the lips, tongue or throat, difficulty breathing, widespread rash with faintness, or collapse. Seek medical advice for a fever lasting more than 48 hours, a severe or persistent headache, or any symptom that is worsening rather than settling.
Where this comes from
How this page was put together
Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarised or generalised beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.