The duplication of usually large-scale efficacy trials to generalise the clinical database of a new vaccine to other populations can be avoided by bridging studies. Immunogenicity data can easily be used to extrapolate efficacy results when the immune response correlates with vaccine induced immunity. In the absence of such a correlate of protection, the bridging process will be more controversial. Previous experiences should be valuable in identifying the factors, either ethnic or vaccine-related that may have an influence on vaccine efficacy. The probability that such a factor is associated with a clinically significant difference in efficacy is critical to the decision to conduct a bridging study. Bridging studies are designed to demonstrate equivalent immunogenicity i.e. exclude a clinically significant difference in the immune response between the population in whom efficacy was shown and the population to whom those efficacy results are extrapolated.