Correlates of protection: novel generations of influenza vaccines.

The emergence of avian influenza A/H5N1 viruses that can cross the species barrier and that cause often-fatal infections of humans is of great concern and a pandemic outbreak with these viruses is feared. The availability of effective vaccines that can protect against morbidity and mortality caused by these viruses is highly desirable and great efforts are being made to prepare these vaccines. The circulation of variants of antigenically distinct influenza H5N1 viruses belonging to different clades complicates the development of new vaccines. Preferably, vaccines induce broad protective immunity against intra-subtypic variants and ideally, also hetero-subtypic immunity. A good understanding of the correlates of immune protection may aid in the development of such vaccines. Here we reviewed potential correlates of protection against influenza and discussed some of the vaccination strategies that could result in optimal protection against epidemic and pandemic influenza.