Concepts of herd protection and immunity

Vaccines are designed primarily to protect vaccinated individuals against the target infection. In addition to this direct effect of vaccination, vaccination may increase the level of population (or herd) immunity by increasing the proportion of the population who are immune from infection. For infections that are transmitted from person-to-person, or for which humans are important reservoirs of the infectious agent, an increase in the level of herd immunity may result in a lower force of infection in the population and thus a lower risk of infection among unvaccinated persons. This is called an indirect effect of vaccination, or a herd-protective effect. The effect of vaccination in increasing the level of herd immunity is important in disease elimination programmes as, because of the indirect effect of vaccination on risk of infection, elimination may be achieved without having to vaccinate the entire population. An example is given of how the indirect effect of vaccination operates and the implications that this has for both disease control programmes and the interpretation of vaccine trials are discussed.