Sometimes, your doctor will advise waiting if your child is on certain medications – those that contain steroids for instance, or if your child is ill. The latter is not because the vaccine is any less effective though – it’s merely to avoid your child having the side effects of vaccination on top of their illness.
For children with compromised immune systems – those with HIV or AIDS and certain cancers for instance, no live vaccines (like chicken pox and MMR for example) will be given. That’s to prevent the risk of them contracting the disease from the vaccine. Otherwise healthy children can, however, receive their vaccinations if their illnesses are mild to moderate.
If your child is sick when due to have an immunisation and you’re in doubt, check with your doctor first.