The one important exception to the way these numbers work out is pertussis. Due to diagnostic bias and the general ease of misdiagnosis, there is no way to know if unvaxed kids get pertussis more or less often than vaxed kids. On top of everything else, it is apparently possible to have pertussis with no symptoms at all. Herd immunity doesn't exist for this illness, with or without vaccination...because it is possible to get it more than once.
6/22/10 at 10:25pm