The one exception is the polio vaccine, which is swallowed instead of injected. In general, vaccines should not be given to children if they are suffering from an acute illness, although if they have a minor infection and show no signs of a fever, it may be okay if the child's GP is happy to go ahead. Any child who has had a severe reaction to a previous dose should not be given any more. This is because the system may not be able to cope with the influx of a live virus, even a weakened version of one. It seems to be the nature of the immune response.
Immunity has a memory so when it sees something that it has seen before it makes a bigger response.
Why booster doses are sometimes needed Because vaccines are generally not as effective at providing immunity as exposure to natural infection, their protective effect can wane over time. Instead, they are given to children aged around four because, experts say, around one in ten children vaccinated at a year old will not respond properly to the first vaccine. In addition, the pre-school booster is designed to pick up those children who slipped through the net the first time. But it varies according to the way the body reacts to each individual vaccine. For example, says Dr Finn, tests have shown that giving the MMR jab before one year old seems to have little effect on the immune system.