Dr Vinodh Kumar,O.RDivision of Epidemiology

ICAR-Indian Veterinary Research Institute

Izatnagar, Uttar Pradesh, India

Introduction Alternate Names: Herd effect, community

immunity, Population immunity, or Social immunity

Protection from disease in a group, due to a large enough proportion of the population having immunity to prevent the disease from spreading from person to person

HistoryCoined in 1923Threshold theorem- Smith, 1970 and Dietz,

1975 First recognized as a naturally occurring

phenomenon- A. W. Hedrich 

TerminologyBasic reproduction number ( Ro)Critical vaccination Level (Vc)Vaccine effectiveness against transmission

(E)

Basic Reproduction Number R0 is the average number of secondary cases

caused by one typical infectious individual & is measure for transmission from one generation to next generation.

 The proportion of immune individuals in a population above which a disease may no longer persist is the herd immunity threshold.

Herd immunity Reduced probability of an individual becoming infected

when it is part of a vaccinated population. The chance of becoming infected in a population

decreases with increasing density of individuals being vaccinated.

Protection by herd immunity applies to vaccinated as well as unvaccinated individuals

Proportion immune among individuals in a population Particular threshold proportion of immune individuals

that should lead to a decline in incidence of infection

Effects of Herd immunityProtection of those without immunityEvolutionary pressureSerotype replacementEradication of diseases

MechanismIndividuals who are immune to a disease act as a barrier

in the spread of diseaseHerd immunity threshold or herd immunity level

Prevention of infectious disease is an important part of modern livestock production

Infection is the invasion & replication of pathogen in the host

Infectious disease is the clinical outcome of an infection caused by that pathogen.

Complicating factorsClinical signs may appear long after initial

infection (BSE, paratuberculosis )Infections do occur sub clinically Similar clinical symptoms caused by more

than one infectious agent (mastitis)Combination of factors can cause disease

(post weaning diarrhoea in piglets)

A measure of the level of population-immunity or herd-immunity is the proportion who are thus immune from further infection.

For many infections, the level of herd immunity may have an effect on the transmission of the infection within the population and, in particular, may affect the risk of an uninfected becoming infected.

For such infections, increasing the level of herd immunity will decrease the risk of an uninfected person becoming infected.

If the herd effect reduces the risk of infection among the uninfected sufficiently then the infection may no longer be sustainable within the population and the infection may be eliminated.

This concept is important in disease elimination or eradication programmes. It means, for example, that elimination can be achieved without necessarily vaccinating the entire population.

Types of Herd ImmunityInnate (Inherent) Herd Immunity: It is

genetically determined physiological changes with respect to antibody production or other defence mechanism in a herd. It does not depend on the previous exposure of herd with infection or it may arise in a herd through prolonged exposure to an infection or natural selection.

Acquired Herd Immunity: It is a type of herd immunity where a sufficient number of its members have actually been exposed naturally or artificially to infectious agents during their lifespan. This kind of exposure may be made very early in life.

Some population of domestic fowl have innate resistance to pullorum disease due to an inherited difference in lymphocyte numbers immediately after hatching.(Robert & Card,1926)

Inheritance of resistance to influenza virus in mice is probably due to a single dominant autosomal allele. (Lindermann, 1964).

R smaller or larger than 1 marks whether or not an epidemic outbreak will occur when an infection is introduced in a susceptible population.

R<1- minor outbreakR>1- major& minor outbreakR predicts the probability of a major

outbreak; if R<1, P=0; if R>1,P>0.

R0 VALUE OF SOME IMPORTANT DISEASES

HIV/ AIDS in human beings is 2-5.SARS Infection in India is 2-5.Chicken Pox is 16-18.FMD is Cattle in UK Farms is 3.5 to 4.5.IBR in cattle 7.TB in cattle is 2.6.Rabies in dogs is 2.44 and WHO

recommends for 70% Vaccination of dogs in a population

FACTORS AFFECTING R0

It can be explained by the epidemiological triad

Host Factor: Mixed Population, different age group of animals, difference in nutritional status, inbred population, parasitic load and mobility of host.

Environment Factor: Seasonal Variation e.g FMD(autumn and spring) and Malaria(hot and humid climate).

Agent Factor: The agent may not spread at the same rate in all the countries. Genetic changes in the host factors like Genetic drift and genetic shift.

HOW TO REDUCE R0 VALUE

Reducing or eliminating the shedding of the agent by the infected host. e.g by antibiotics and segregation and quarantine.

Reducing the duration of environmental survival of the agent. e.g sunlight.

Reducing or eliminating vehicle contamination and fomite transmission.

Controlling the Vector Population for biological transmission.

Reducing the exposure of susceptible host. E.g density reduction.

Complete eradication of certain agents by mass vaccination.

IMPORTANCE OF RO

For an infectious disease with average infectious period 1/γ and transmission rate β, Ro = β/γ:

For a closed population, an infectious disease can only invade if there is a threshold fraction of susceptibles greater than 1/Ro .

Vaccination policy: if proportion of susceptibles is reduced to below 1/Ro ,we can eradicate the disease.

Disease Elimination If the herd effect reduces the risk of infection

among the uninfected sufficiently then the infection may no longer be sustainable within the population and the infection may be eliminated.

The “effective reproduction number” (R) has to be reduced below 1.

If a proportion P of the population are immune then R = (1- P) R0

So, to get R down to about 1, P must be up to 1-1/ R0

Thus if R0 = 5 then vaccine coverage will have to be in excess of 80%

Methods to increase herd immunity

Active immunisationPassive immunisation

vaccinationClinical protectionReduced susceptibilityReduced infectivity

Manipulation Herd immunity can be manipulated byChoice of vaccineInterval between vaccination and possible

exposureProportion to be vaccinated in the population

Advantages & Disadvantages of Herd Immunity

 Potential for infection elimination.Reduced risk of infection for those refusing

vaccination (“free riders”).Reduced risk of infection for those for whom

vaccination is contraindicated.

DisadvantagesHerd immunity generally applies only to

diseases that are contagious. Raise the average age of infection among

those who are infected (e.g. polio, rubella, varicella etc., )

References‘Herd Immunity’’: A Rough Guide. VACCINES CID 2011:52

Sheriff et al (2012). The Role of Herd Immunity in Parents’ Decision to Vaccinate Children: A Systematic Review .PEDIATRICS. 130( 3) :522-530