Usually, the reaction will occur within about 10 days of having the vaccination, but they can appear any time up to around 28 days later.
The spots or rash that your child gets will be similar to measles. It will start as a rash of small red dots on the chest and neck, and they may spread to other parts of the body. They will typically disappear after a few days, although your child may have a mild fever while they are there.
If the spots are very bright red, or the spots don’t blanch when they are gently pressed, you should definitely call your doctor, as they may be a sign of another disease. Likewise, if your child develops a reaction immediately after the vaccine, then he or she may be having an allergic reaction, and will require emergency medical treatment. Symptoms of an allergic reaction like this could include a rash, difficulty breathing, or swelling of the lips, as well as extreme lethargy or weakness.