PAUL A. OFFIT, MD: We've been using combination vaccines really since the 1940s. The first combination vaccine was diphtheria/pertussis/tetanus. Now we have combinations with measles, mumps, rubella (MMR) and other larger combinations.

Frankly we've worked very hard to try and put those vaccines together so that one can have fewer and fewer shots. It's not easy. I mean sometimes the buffering and stabilizing agents in those vaccines conflict and so it hasn't been an easy task.

But there's certainly no reason not to get combination vaccines. Often it means fewer shots and there's certainly no benefit really to separating those vaccines out.