Threshold+level+of+immunogenicity+required

I think immunogenicity can be measured through antibody titres and percent lysis from CTL assays. The values necessary to provide protection or provide immunogenicity that is high enough above background will probably vary for each disease.

here is some more info from a quick google search http://www.plosone.org/article/info:doi/10.1371/journal.pone.0025398#pone.0025398-Lin2 q As shown by clinical results with the Vi-rEPA vaccine in Vietnam [13]–[15], a conjugate Vi vaccine appears to meet these criteria and, in addition, clinical data generated in Vietnam have allowed the establishment of a serological threshold for clinical protection [13]. ref 13 THE EFFICACY OF A SALMONELLA TYPHI Vi CONJUGATE VACCINE IN TWO-TO-FIVE-YEAR-OLD CHILDREN

Hib-MenCY-TT Safety and Immunogenicity http://www.medscape.com/viewarticle/778660_3 q The proportions of children who, after dose 3, had hSBA titers ≥1:8 (the clinical threshold defined as protective) to serogroups C and Y were 99% and 96%, respectively.[5]