If a vaccine is given when a baby still has antibodies to the disease, the antibodies can stop the vaccine working. This is why routine childhood immunisations do not start until a baby is two months old, before the antibodies a baby gets from its mother have stopped working. This is also why it is important for parents to stick to the immunisation schedule, as a delay can leave a baby unprotected. A delay can increase the chance of adverse reactions to some vaccines, such as pertussis (whooping cough).
There are some excellent websites that will answer all your questions and queries about immunisation and vaccination. If you are worried about giving the MMR vaccine, you should access the MMR site.
NHS Choices - Immunisation Information
The most comprehensive, up-to-date and accurate source of information on vaccines, disease and immunisation in the UK.
NHS Choices - MMR Information
This website has been put together to answer any questions you might have about MMR. You can look for information and resources in the MMR library, ask an expert panel a question, and read up on the latest news stories relating to MMR.