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.
Saunders WB. Dorland's Illustrated Medical Dictionary. 28th Edn. Philadelphia: 1994; p 812.
Keusch GT, Bart KJ. Immunization and Vaccine Use. In: Fauci AS, Brawnwald E, Isselbacher KJ, et al. (eds), Harrison's Practice of Internal Medicine. 14th Edn, New York: McGraw Hill, 1998; pp 758-771.
Isaacs D, Moxon ER. Immunisation. In: Weatherall DJ, Ledingham JGG, Warrel DA (eds), Oxford Text Book of Medicine. 3rd Edn, Vol. 1. Oxford: Oxford University Press, 1996; p 315-321.
Nathanson N. Eradication of poliomyelitis in the United States. Rev Infect Dis 1982; 4: 940-945.
De Quadros CA, Andrus JK, Olive JM, Carrasco P. Strategies for poliomyelitis eradication in developing countries. Public Health Rev 1993/94; 21: 65-81.
Anderson RM. The concept of herd immunity and the design of community-based immunisation programmes. Vaccine 1992; 10: 928-935.
Anderson RM, Donnelly CA, Gupta S. Vaccine design, evaluation and community-based use for antigenically variable infectious agents. Lancet 1997; 350: 1466-1470.
Fine PEM. Herd Immunity: History, theory, practise. Epidemiol Rev 1993; 15: 265-302.
Ooi PL, Goh KT, Doraisingham S, Ling AE. Prevalence of varicella-zoster virus infection in Singapore. South East Asian J Trop Med Public Health 1992; 23: 22-25.
Ohga S, Okada K, Miyazaki C, Akazawa K, Ueda K. The measles outbreak in Chikuhau district, Fukuoka, Japan, 1990: Correlation between herd immunity level and outbreak size. Acta Paediatr Japan 1992; 4: 447-453.
Rooney PJ, Coyle PV. The role of herd immunity in an epidemic cycle of hepatitis A. J Infect 1992; 24: 327-331.
John TJ. Immunisation against polioviruses in developing countries. Rev Med Virology 1993; 3: 149-160.
John TJ. Can we eradicate poliomyelitis? In: Sachdev HPS, Choudhury P (eds), Frontiers in Paediatrics, New Delhi: Jaypee Brothers, 1996; p 76-90.
Trollfors B, Taranger J, Lagergard T, Sundh V, Bryla DA, Schneerson R, Robbins JB. Immunisation of children with pertussis toxoid decreases spread of per-tussis within the family. Pediatr Infect Dis J 1998; 17: 196-199.
John TJ, Pandian R, Gadomski A, et al. Control of poliomyelitis by pulse immunisation in Vellore, India. Brit Med J 1983; 286: 31-32.
John TJ, Ray M, Steinhoff MC. Control of measles by annual pulse immunization. Amer J Dis Child 1984; 138: 299-300.
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John, T.J., Samuel, R. Herd immunity and herd effect: new insights and definitions. Eur J Epidemiol 16, 601–606 (2000). https://doi.org/10.1023/A:1007626510002
- Public Health
- Infectious Disease
- Health Intervention
- Geographic Variation
- Clear Understanding