Herd immunity and herd effect: new insights and definitions

The term herd immunity has been used by various authors to conform to different definitions. Earlier this situation had been identified but not corrected. We propose that it should have precise meaning for which purpose a new definition is offered: "the proportion of subjects with immunity in a given population". This definition dissociates herd immunity from the indirect protection observed in the unimmunised segment of a population in which a large proportion is immunised, for which the term 'herd effect' is proposed. It is defined as: "the reduction of infection or disease in the unimmunised segment as a result of immunising a proportion of the population". Herd immunity can be measured by testing a sample of the population for the presence of the chosen immune parameter. Herd effect can be measured by quantifying the decline in incidence in the unimmunised segment of a population in which an immunisation programme is instituted. Herd immunity applies to immunisation or infection, human to human transmitted or otherwise. On the other hand, herd effect applies to immunisation or other health interventions which reduce the probability of transmission, confined to infections transmitted human to human, directly or via vector. The induced herd immunity of a given vaccine exhibits geographic variation as it depends upon coverage and efficacy of the vaccine, both of which can vary geographically. Herd effect is determined by herd immunity as well as the force of transmission of the corresponding infection. Clear understanding of these phenomena and their relationships will help improve the design of effective and efficient immunisation programmes aimed at control, elimination or eradication of vaccine preventable infectious diseases.