Rach - i believe if you have them immunised within the first week or so of being exposed their reaction to it is a lot less than normal.

We thought DS had been exposed, and had him immunised a week later (he had a temp, but was too early for spots), this was at 12 months, but he never got it in the end.

You can have the immunisation done early, the only reason why they don't do the CP at 12 months is there is already 3 other needles, thats why they space them.