R0 VALUE & HERD IMMUNITY

(HERD EFFECT/ COMMUNITY IMMUNITY/

POPULATION IMMUNITY/ SOCIAL

IMMUNITY)

DR. BHOJ R SINGH, PRINCIPAL SCIENTIST (VM)

HEAD DIVISION OF EPIDEMIOLOGY

INDIAN VETERINARY RESEARCH INSTITUTE, IZATNAGAR-243122, BAREILLY, UP, INDIA.

TELEFAX +91-581-2302188

THE PROPORTION OF IMMUNE INDIVIDUALS IN A POPULATION

ABOVE WHICH A DISEASE MAY NO LONGER PERSIST IS THE

HERD IMMUNITY THRESHOLD.

R0 VALUES

The average number of secondary cases arising from an

average primary case in an entirely susceptible population.

The basic reproduction number (basic reproductive rate, basic

reproductive ratio R0) of a contagious disease is the number of

cases than a case of the disease generates (on an average) over

the course of its infectious period in a susceptible population.

FACTORS DETERMINING THE R0

R0 will vary from agent to agent depending on the

infectiousness of the agent.

R0 may also vary from population to population

depending on population density.

Course of infectiousness of the disease. Incubation

period, latent periods and period of infectiousness).

Mode of transmission and contagiousness.

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. Evolution of

new antigenic variant strains.

COURSE OF SOME INFECTIOUS

DISEASES IN DAYS

Infectious disease Incubation period Latent period Infectious period

Measles 8-13 6-9 6-7

Mumps 12-26 12-18 4-8

Pertussis 6-10 21-23 7-10

Rubella 14-21 7-14 11-12

Diphtheria 2-5 14-21 2-5

Chickenpox 13-17 8-12 10-11

Hepatitis B 30-80 13-17 19-22

Poliomyelitis 7-12 1-3 14-20

Influenza 1-3 1-3 1-3

Smallpox 10-15 8-11 2-3

Scarlet fever 2-3 1-2 14-21

Infectious disease Host R0

Measles Humans (UK) 12-18

Pertussis (whooping cough) Humans (UK) 12-18

Chickenpox (varicella) Humans (UK) 10-12 (16-18 in India)

Rubella Humans (UK) 5-7

Smallpox Humans 3.5-7Feline immunodeficiency virus

(FIV) Domestic Cats 1.1-1.5

Rabies Dogs (Kenya) 2.44

Phocine distemper Seals 2-3

Tuberculosis Cattle 2.6

Influenza (Pandemic) Humans 2-4

Foot-and-mouth disease Livestock farms (UK) 3.5-4.5

Mumps Humans 4-12

Poliomyelitis (polio) Humans 5

HIV/AIDS Hetro 2-5

HIV Male homosexuals UK 4

HIV Female prostitutes in Kenya 11

Malaria Humans ≈ 100

SARS Human 2-5

IBR Cattle (UK) 7

TB Cattle 2.6

R0 of Some Diseases

R0 IS AFFECTED BY MODE OF

TRANSMISSION

Disease Transmission R0

Measles Airborne 12–18

Pertussis Airborne droplet 12–17

Diphtheria Saliva 6–7

Smallpox Social contact 5–7

Polio Fecal-oral route 5–7

Rubella Airborne droplet 5–7

Mumps Airborne droplet 4–7

HIV/AIDS Sexual contact 2–5

SARS Airborne droplet 2–5

Influenza (1918

pandemic strain)Airborne droplet 2–3

CALCULATION OF R0 R0= β/ γ

γ= 1/ average infectious period

β= Transmission rate (Number contacts by infective case indefined time, contact rate)

If susceptible fraction of a population is >1/R0 then only disease can progress. We can get is by vaccination, preventive therapy or control measures.

When initial fraction of susceptible population is less than γ/β

or 1/R0 then infection can not progress and dies out, it is called the threshold fraction.

R0 is also defined as inverse of relative removal rate (the already got infected during the period).

1- 1/R0 is also defined as fraction of the population to be vaccinated for getting herd immunity.

HOW TO REDUCE R0 VALUE?

R0 can be reduced through intervention at any point in the

transmission cycle by the following methods:

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, fumigation, aeration etc.

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, provision of protective gears as masks, goggles,

aprons, gloves, gumboots etc.

Increasing the resistance of susceptible host by vaccination,

passive immunization etc.

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 susceptible individuals greater than 1/Ro .

If R0 is 2.5 then 1/R0 is 0.4, i.e., for control of the disease less than 0.4 fraction of the population be susceptible or more than 60% be non-susceptible or immune.

Vaccination policy: if proportion of susceptible individuals is reduced to below 1/Ro the disease can be eradicated.

LIMITATIONS

When calculated from mathematical models, particularly usingordinary differential equations, R0 is, in fact, simply athreshold, not the average number of secondary infections.

There are many methods used to derive such a threshold from amathematical model, but many of them often give anhypothetical value sometimes far away from the the true valueof R0. This is particularly problematic if there are intermediatevectors between hosts, such as malaria.

Methods include the survival function, rearranging thelargest value from the Jacobian matrix, the next-generationmethod, calculations from the intrinsic growth rate, existence ofthe endemic equilibrium, the number of susceptibles at theendemic equilibrium, the average age of infection and the finalsize equation.

Few of these methods agree with one another, even whenstarting with the same system of differential equations. Evenfewer actually calculate the average number of secondaryinfections. Since R0 is rarely observed in the field and is usuallycalculated via a mathematical model, this severely limits itsusefulness

HERD IMMUNITY

The term herd immunity was first used in 1923.

It was an integral part During the Small Pox eradication in the 1960s and 1970s.

The practice of Ring Vaccination, of which herd immunity is integral to, began as a way to immunize every person in a "ring" around an infected individual to prevent outbreaks from spreading.

Vaccination controversies and opposing of vaccination are mainly due to failed herd immunity, either it was not be established or disappeared in certain communities, allowing preventable diseases to persist in or return to these communities.

Topley, W. W. C.; Wilson, G. S. (May 1923). "The Spread of Bacterial Infection. The Problem of Herd-Immunity". The Journal of Hygiene (London). 21 (3): 243–249. PMC 2167341 . PMID 20474777. doi:10.1017/s0022172400031478.

Strassburg, M. A. (1982). "The global eradication of smallpox". American journal of infection control. 10 (2): 53–9. PMID 7044193. doi:10.1016/0196-6553(82)90003-7.

DEFINITION OF HERD IMMUNITY

As per John TJ, Samuel R. European Journal of Epidemiology

2000;16, Herd Immunity can be defined as follows:

1. The resistance of a group for attack by a disease because of the

immunity of a large proportion of the members and the

consequent lessening of the likelihood of an affected individual

coming into contact with a susceptible individual.

2. The prevalence of immunity in a population above which it

becomes difficult for the organism to circulate and reach new

susceptible is called herd immunity.

3. It is well known that not everyone in a population needs to be

immunised to eliminate disease.

HERD IMMUNITY

The indirect protection from infection of

susceptible livestock in a herd, and the

protection of the herd as a whole, which is

brought about by the presence of immune

individuals.

The number of individuals in a population

(herd) who are (relatively) immune to

infection with an infectious agent may

depend on the proportion who have

previously been infected with the agent

and the proportion who have been

vaccinated with an efficacious vaccine.

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 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.

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)

Cameroon et al have shown that

resistance to brucellosis in swine may

be genetically determined.

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.

Polio in paralytic form are rare in countries with poor hygiene

and sanitation where exposure to the virus occurs in early part

of life but in countries where the hygiene is better and exposure

is delayed till school age then paralytic manifestations are

higher.

ADVANTAGES OF HERD IMMUNITY

Potential for infection elimination.

Reduced risk of infection for those refusing vaccination (“freeriders”).

Vaccination against sexually transmitted diseases (STIs)targeted at one sex result in significant declines in sexualdisease in both sexes.

Reduced risk of infection for those for whom vaccination iscontraindicated (e.g., immune-suppressed) or who cannot bevaccinated e.g., cancer patients, too young animals andpregnant animals.

Prioritization of vaccination towards target groups or HighRisk groups in the community may lead to protection of thewhole community e.g. prioritization of vaccinating childrenagainst pneumococcus and rotavirus, school-age children forseasonal flu immunization reduces of the disease burden inthe whole community.

Limitations Herd immunity generally applies only to diseases that are contagious. It

does not apply to diseases such as tetanus, botulism food borne infectionsand intoxications.

Raise the average age of infection among those who are infected.

Particular problem for those infections where the severity of infectionincreases with age (e.g. polio, rubella, varicella, measles, hepatitis A).

It is not a permanent attribute, depending on the duration of the immunityconferred after vaccination the structure of herd for susceptible versusimmune rapidly changes.

Herd immunity might be associated with emergence of variants ofpathogens more dangerous than the existent due to Evolution Pressure onthe pathogen or Selection Pressure on the antigen variant.

Herd immunity may lead to antigenic variation among pathogens at muchfaster rate than it would have been in the absence of herd immunity.Leading to Serotype Replacement.

Herd immunity not work for many of the infectious diseases like Tetanus,Botulism, and similar toxico-infections.

BACTERIAL DISEASES OF LIVESTOCK

Sl.

No

Name of the

Disease

Host Range Type of Vaccine

Used

Duration of

Immunity

1 Haemorrhagic

Septicemia

Cattle, Sheep & Goat,

Pig

Inactivated alum

adjuvant vaccine

6 months

2 Black Quarter Cattle, Sheep & Goat Inactivated alum

adjuvant vaccine

6 months

3 Anthrax Cattle, Sheep & Goat Sterne-avirulent spore

vaccine

1 year

4 Brucellosis Cattle, Sheep & Goat Live freeze dried

vaccine

Life Long

5 Enterotoxemia Sheep Inactivated alum

adjuvant vaccine

6 months

6 Leptospirosis Canine Killed Mixed Vaccine 1 year

DISEASE ELIMINATION & HERD

IMMUNITY 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 more than 1-1/ R0.

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

80%.

QUIZ

What are the factors affecting reproduction ratio (R0

) of a disease?

Give R0 values for important animal diseases.

How is associated with herd immunity?

Give herd immunity values required for prevention of FMD, HS, BQ, Enterotoxemia, Goat Pox, Sheep Pox, PPR, Brucellosis, Classical swine fever.

What are different types of herd immunity in animals?

An useful link https://www.historyofvaccines.org/content/herd-immunity-0