Herd immunity after vaccination: how to quantify it and how to use it to halt disease.

In comparison to unvaccinated individuals, vaccinated individuals have fewer clinical symptoms, reduced susceptibility and reduced infectivity. The first two effects of vaccination can mean that each vaccinated individual is protected against clinical symptoms. From experiments and field trials, the extent of individual protection can be determined by a statistical analysis of the resulting data. In addition, there is an effect of the vaccination on the populations in which one or more individuals are vaccinated. This effect on the population is due to the effects of vaccination on susceptibility and infectivity of the vaccinated individuals. The population effect is called herd immunity and is observed as a reduction in chance of becoming infected when being part of a population with some of the individuals vaccinated. Note that the protection by herd immunity applies to vaccinated individuals as well as to unvaccinated individuals. Thus, protection against disease can be achieved not only by vaccinating the individuals that have to be protected but also by vaccinating other individuals in the same population. Such an application of herd immunity is especially important in protecting farm animals. To plan and evaluate vaccination at the population level, the herd immunity needs to be quantified. It will be illustrated that it is possible, not only theoretically but also practically, to quantify herd immunity among farm animals with data from small-scale experiments as well as with data from field trials.