The concept of vaccination failure.

Despite remarkable success of immunization programmes on a global perspective, vaccines are neither 100% efficacious nor 100% effective. Therefore, vaccination failure, i.e. occurrence of a specific disease in an individual despite previous vaccination, may occur. Vaccination failure may be due to actual vaccine failure or failure to vaccinate appropriately. Universally accepted concepts and definitions of vaccination failure are required to assess and compare the benefit of vaccines used in populations. Here we propose general definitions for types of vaccination failure. In the future, these should be complemented by specific definitions for specific vaccines as needed depending on public health considerations.