Introduction

Vaccines activate the immune system so that it is hoped that the response of the host to subsequent infections will be less harmful for the host. Leaving aside the mechanisms by which vaccination works, one can observe three changes in the host-pathogen interaction, i.e. compared to a non-vaccinated individual, a vaccinated individual has: (1) a lower probability of becoming infected when exposed (reduced susceptibility), (2) fewer clinical signs when it is infected (clinical protection), and (3) less infectivity when it is infected (reduced infectivity).

For a first vaccine evaluation, reduced infectivity would not be considered as a positive effect of the vaccine, because it is not of direct benefit to the individual receiving the vaccine. It is important to take reduced infectivity into account, when estimating the combined effect of reduced susceptibility and clinical protection, i.e. vaccine efficacy, because reduced infectivity lessens the exposure of the other individuals in the population. Thus for the estimation of vaccine efficacy one should compensate for differential exposure, and reduced infectivity is a nuisance parameter.

It is especially important to consider how vaccines differ in their effect on infectivity whenever several vaccines are available, that all have similar efficacy. It is known for some vaccines, for example those against measles and polio, that although in vaccinated individuals clinical signs are either absent or very mild, vaccinated individuals are susceptible and they can replicate and excrete virus (infectivity). This implies that virus could circulate in vaccinated populations and therefore it is important to quantify and compare the amount of virus circulation in groups of individuals vaccinated with various vaccines.