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

In comparison to unvaccinated individuals, vaccinated individuals have fewe r clinical symptoms, reduced susceptibility and reduced infectivity. The fi rst 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 analysi s 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 e ffect on the population is due to the effects of vaccination on susceptibil ity 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 vacci nated. Note that the protection by herd immunity applies to vaccinated indi viduals as well as to unvaccinated individuals. Thus, protection against di sease can be achieved not only by vaccinating the individuals that have to be protected but also by vaccinating other individuals in the same populati on. Such an application of herd immunity is especially important in protect ing farm animals. To plan and evaluate vaccination at the population level, the herd immunity needs to be quantified. It will be illustrated that it i s possible, not only theoretically but also practically. to quantify herd i mmunity among farm animals with data from small-scale experiments as well a s with data from field trials, (C) 2001 Elsevier Science Ltd. All rights re served.