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Herd Immunity and Herd Effect: New Insights and Definitions

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Herd Immunity and Herd Effect: New Insights and Definitions European Journal of Epidemiology 16: 601±606, 2000. Ó 2000 Kluwer Academic Publishers. Printed in the Netherlands. Herd immunity and herd eect: new insights and de®nitions T. Jacob John1 & Reuben Samuel1,2 1Department of Clinical Virology, Christian Medical College Hospital; 2Department of Community Health, Christian Medical College, Vellore, Tamil Nadu, India Accepted in revised form 21 April 2000 ``Words are not only vehicles to convey ideas but also their drivers.'' Anon. Abstract. The term herd immunity has been used by a population in which an immunisation programme is various authors to conform to dierent de®nitions. instituted. Herd immunity applies to immunisation or Earlier this situation had been identi®ed but not cor- infection, human to human transmitted or otherwise. rected. We propose that it should have precise meaning On the other hand, herd eect applies to immunisation for which purpose a new de®nition is oered: ``the or other health interventions which reduce the proba- proportion of subjects with immunity in a given pop- bility of transmission, con®ned to infections trans- ulation''. This de®nition dissociates herd immunity mitted human to human, directly or via vector. The from the indirect protection observed in the unim- induced herd immunity of a given vaccine exhibits munised segment of a population in which a large geographic variation as it depends upon coverage and proportion is immunised, for which the term `herd ecacy of the vaccine, both of which can vary geo- eect' is proposed. It is de®ned as: ``the reduction of graphically. Herd eect is determined by herd immu- infection or disease in the unimmunised segment as a nity as well as the force of transmission of the result of immunising a proportion of the population''. corresponding infection. Clear understanding of these Herd immunity can be measured by testing a sample of phenomena and their relationships will help improve the population for the presence of the chosen immune the design of eective and ecient immunisation parameter. Herd eect can be measured by quantifying programmes aimed at control, elimination or eradi- the decline in incidence in the unimmunised segment of cation of vaccine preventable infectious diseases. Introduction (1) ``Herd immunity. The resistance of a group to attack by a disease because of the immunity of a large The term `herd immunity' is increasingly frequently proportion of the members and the consequent less- seen in recent literature on the epidemiology of in- ening of the likelihood of an aected individual fectious diseases and on their prevention and control coming into contact with a susceptible individual'' [1]. by immunisation. This was found by Medline search (2) ``Herd immunity. It is not necessary to immu- (1992±1998) using the key words `herd immunity' and nise every person in order to stop transmission of an `herd eect'. While a large number of papers were infectious agent through a population. For those found with the key word `herd immunity', none was organisms dependent upon person-to-person trans- found with the key word `herd eect'. A review of mission, there may be a de®nable prevalence of im- several papers has shown that `herd immunity' is used munity in the population above which it becomes by dierent authors for dierent ideas, such as a dicult for the organism to circulate and reach new concept, a phenomenon or a measurable parameter. susceptibles. This prevalence is called herd immunity'' In this paper we oer new insights and suggest a new [2]. de®nition of the term `herd immunity' and dissociate (3) ``Herd immunity. It is well known that not it from the indirect eect of immunisation of part of a everyone in a population needs to be immunised to population on the incidence of infection or disease in eliminate disease ± often referred to as herd immu- the unimmunised remainder, for which the term `herd nity. This is because successful immunisation re- eect' is proposed and de®ned. duces the number of susceptibles in the population and this eectively reduces the eciency with which De®nitions currently available for herd immunity the microbe is transmitted from one person to the other. This has the same eect on the incidence of Three de®nitions of herd immunity given in recent infection as a reduction in the number of individuals text or reference books are quoted below. The em- in a population. The microbe cannot sustain itself phasis of phrases is ours. and disease disappears at some level of vaccine 602 coverage that is less than 100%. On the other hand, and additional annual repetitive vaccination of the coverage below that needed to prevent disease may same cohorts of susceptible children were necessary have little impact on the total number of suscepti- for 8±9 years to interrupt transmission in Brazil, bles ± as has been predicted using mathematical showing that `herd immunity' was not evident for models and veri®ed, in the case of rubella, by ob- the same vaccine against the same disease, but in a servation. The implementation of immunisation dierent region [5]. This clearly shows that herd programmes needs to be accompanied by case sur- immunity (by the third de®nition) is a function of veillance, in conjunction with analysis of appropri- not only immunisation but also the force of trans- ate serological samples both before and after the mission of the pathogen. This de®nition is ambigu- introduction of the vaccine. If such data are not ous about one aspect of the end point: is it zero carefully scrutinised, the consequence may be dire. disease or infection? The second de®nition clearly For example, an immunisation programme may re- addresses infection while the ®rst one focuses on duce the number of cases but at the same time may infection and disease. Only the second de®nition increase the average age at which the infection oc- clari®es that we are dealing with only person-to- curs. If the severity of disease increases with age of person transmitted infectious diseases. acquisition, as in rubella (the risk of an infected A new insight in the third de®nition is that in some foetus in an infected woman) and polio (the risk of cases it might be harmful to immunise a proportion paralytic disease), an immunisation programme may and obtain only partial or incomplete `herd immu- be less useful than none at all'' [3]. nity', since it might worsen the problem by delaying The ®rst de®nition considers herd immunity con- infection and not eliminating it, as in the case of ceptually as the resistance to disease due to reduced maternal and foetal rubella [2]. This is an important risk of infection in a group of individuals as a result point. A similar situation might arise in the cases of of a large proportion among them (but not all) being immunisation against hepatitis A and varicella also. immune, not necessarily due only to immunisation Partial coverage of children may slow down virus [1]. The second de®nition clari®es that the term ap- circulation and delay infection in the unimmunised. plies to the actual proportion of immunised indi- Hepatitis A and varicella are more severe in adults viduals necessary to make it dicult for the than in children. Here a new phrase such as `partial' organism to circulate and reach new susceptibles [2]. or `incomplete' herd immunity had to be introduced By this de®nition herd immunity is a threshold value to overcome the problem caused by the very de®ni- of a measurable parameter (vaccination coverage) tion which demands disappearance of disease. resulting in retardation of person-to-person trans- A review of several recent publications with the key mission of an infection. Elimination of infection or word herd immunity showed that the term has most disease is not required but only implied in this def- often been used to mean the concept of reduced inition. How can we arrive at a de®nable level transmission due to high immunisation level, in ac- (proportion) of immunity when the end point is cordance with one or another of the three de®nitions merely diculty of transmission, which is not de- given above. For example the `concept of herd im- ®ned, hence not measurable? The third de®nition is munity' has been advocated as useful in designing closely similar to the ®rst but it develops it further to immunisation programmes. [6, 7]. Herd immunity has a phenomenon providing the means to eliminate an been quali®ed as a `key concept' in population based infectious disease from a population when a pro- immunisation programmes [7]. The author of a portion that is less than 100% is immunised [3]. Here landmark review of the history, theory and practical the phenomenon has a measurable end point ± that aspects of herd immunity chose not to prefer ``any of the disappearance or elimination of disease in a single de®nition of herd immunity, rather accepting group. In contrast to the second de®nition, here the the varied uses of the term by dierent authors'' [8]. actual input (immunisation coverage) necessary to Recognising the consequent potential for confusion, achieve this measurable endpoint can be quanti®ed. the reviewer coined the phrase `herd immunity However it seems that the term herd immunity refers threshold' to indicate the minimum prevalence of to the phenomenon of zero incidence in the unim- immune individuals necessary to interrupt trans- munised segment rather than the actual proportion mission of infection [8]. Its major purpose was to immunised to achieve it [3]. The strict application of distinguish between the desirable outcome of inter- this de®nition requires the disappearance of disease ruption of transmission from the potentially unde- due to immunisation coverage of less than 100% of sirable eect mentioned above.
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