taken from babycentre
What will happen after my baby's been immunised?
You will be asked to stay in the surgery for about 10 minutes after the immunisation, just in case your baby reacts to the injection. Immunisations given by injection will probably cause babies and young children to be upset for a little while and they may be irritable and off colour that evening.
Side-effects are rare, but if your baby has any they will be mild. These may include:
•a mild fever
•pain, swelling or redness at the site of the injection
•feeling sick, or vomiting
•diarrhoea
•feeling a bit off-colour
The MMR vaccine can occasionally cause a measles-like rash and a mild fever six to 10 days after having the injection. Don’t worry if your baby develops a rash. It doesn't mean he has measles. He’s just having a reaction to the live but weakened virus in the MMR vaccine as his body is building immunity against the disease.
It's safe to give your baby infant paracetamol or infant ibuprofen if he has a fever after the immunisation, but only if he is three months or older. Check the packet or ask your doctor or pharmacist for advice if you’re unsure how much to give your baby.
Febrile convulsions (seizures or fits) occasionally happen in young children with a high temperature. If your baby develops a temperature of 38 degree C or higher, or has a febrile convulsion, call your doctor and treat the fever straight away.
It’s rare for this sort of reaction to happen. If you're worried that your baby may become very unwell as a result of his immunisation, you may be tempted to give him infant paracetamol just in case, even if he seems fine. However, it may help the vaccine work better if you don't.
One small study found that giving children paracetamol to prevent a fever coming on seemed to make the vaccine less effective. The children who were given paracetamol just in case produced fewer antibodies in response to the vaccine. If fewer antibodies are produced, it's possible that the vaccine may not work as well. The study recommended that unless your child has a fever that needs treating, it's better not to give him paracetamol.
http://www.babycentre.co.uk/a541968/immunisations-what-to-expect-on-the-day#ixzz2LfT3epIf