Vaccine side effects: what is normal and what is not
Every effective vaccine causes some reaction, because the point is to provoke the immune system. Knowing the normal range means you can tell it apart from something that needs attention.
Common and expected (no action needed beyond comfort)
- Redness, swelling, warmth or tenderness where the injection was given, for one to three days.
- A mild fever, usually under 38.5C, in the first 24 to 48 hours.
- Being more irritable, clingy or sleepy than usual for a day.
- Reduced appetite for a feed or two.
- For the MMR vaccine: a mild fever and sometimes a faint measles-like rash about 7 to 10 days later, as the vaccine 'takes'. This is not infectious.
- For MenB: fever is common enough that infant paracetamol is recommended routinely around the appointment.
Less common but not dangerous
- A small hard lump at the injection site that can last a few weeks.
- A larger area of swelling of the whole upper arm or thigh after a booster dose, which settles on its own.
- Brief fainting in older children and teenagers, usually from the needle rather than the vaccine - which is why they are asked to sit for a few minutes afterwards.
Get medical advice the same day if:
- A fever goes above 39C and does not come down with the correct dose of paracetamol or ibuprofen.
- A fever lasts more than 48 hours, or a child seems to be getting more unwell rather than better.
- A child has a seizure (fit).
- A baby is floppy, unresponsive or unusually pale for a period ('hypotonic-hyporesponsive episode') - alarming to see but recovery is complete.
- There is persistent inconsolable crying for more than three hours.
Call emergency services if:
Within minutes to an hour of the vaccine there is swelling of the lips, tongue or throat, a widespread hives rash, wheezing or difficulty breathing, or collapse. This is anaphylaxis. It is very rare - roughly one in a million doses - and clinics carry adrenaline and are trained to treat it immediately, which is why the few minutes of waiting after a vaccine matters.
Reporting a reaction
Every country has a system for reporting suspected vaccine reactions (the Yellow Card scheme in the UK, VAERS in the US, and equivalents elsewhere). Reporting anything unexpected, even if you are not sure it was the vaccine, is how safety monitoring works - you do not need to be certain.