This document discusses herd immunity, including its history, terminology, mechanisms, effects, and importance. Some key points: - Herd immunity occurs when a large proportion of a population is immune to an infectious disease, providing indirect protection to susceptible people. This can occur via vaccination or previous exposure. - Important terminology includes basic reproduction number (R0), which is the number of secondary infections from one case, and critical vaccination level (Vc), which is the threshold of vaccination needed to induce herd immunity. - Herd immunity reduces the probability that susceptible individuals will be exposed to an infectious disease. The critical threshold is when the proportion of immune individuals in a population exceeds 1/R0.

1. Dr Vinodh Kumar,O.R
Division of Epidemiology
ICAR-Indian Veterinary Research Institute
Izatnagar, Uttar Pradesh, India

2. 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

3. History
Coined in 1923
Threshold theorem- Smith, 1970 and Dietz, 1975
 First recognized as a naturally occurring
phenomenon- A. W. Hedrich

4. Terminology
Basic reproduction number ( Ro)
Critical vaccination Level (Vc)
Vaccine effectiveness against transmission (E)

5. 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.

6. 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

7. Effects of Herd immunity
Protection of those without immunity
Evolutionary pressure
Serotype replacement
Eradication of diseases

8. Mechanism
Individuals who are immune to a disease act as a barrier
in the spread of disease
Herd immunity threshold or herd immunity level

9. 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.

10. Complicating factors
Clinical 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)

11.  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.

12.  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.

13. Types of Herd Immunity
Innate (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.

14.  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).

15. 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 outbreak
R>1- major& minor outbreak
R predicts the probability of a major outbreak; if R<1,
P=0; if R>1,P>0.

16. 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

17. 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.

18. 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.

19. 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.

20. 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%

21. Methods to increase herd
immunity
Active immunisation
Passive immunisation

22. vaccination
Clinical protection
Reduced susceptibility
Reduced infectivity

23. Manipulation
Herd immunity can be manipulated by
Choice of vaccine
Interval between vaccination and possible exposure
Proportion to be vaccinated in the population

24. 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.

25. Disadvantages
Herd 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., )

26. 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