Herd Immunity

Herd Immunity

Q. 1

All of the following about ‘herd immunity’ are true, except:

 A

Spread of epidemic is influenced by it

 B

Can be acquired by immunisation

 C

It is mostly due to sub-clinical infection

 D

Herd structure is constant

Q. 1

All of the following about ‘herd immunity’ are true, except:

 A

Spread of epidemic is influenced by it

 B

Can be acquired by immunisation

 C

It is mostly due to sub-clinical infection

 D

Herd structure is constant

Ans. D

Explanation:

A herd structure includes the population belong to the herd species, the presence and distribution of altered animal hosts and possible insect vector as well as those environmental and social factors that favour or inhibit the spread of infection from host to host.

This herd structure can never be constant as it is subjected to new variations.

These new variations occur due to new births, new deaths and population mobility.

RefPark’s Textbook of Preventive and Social Medicine, 19th Edition; Pages 96 – 97


Q. 2

All of the following are true about herd immunity, EXCEPT:

 A

It can be achieved by immunizing a population with a vaccine that interrupts transmission

 B

The live virus in the vaccine can replicate in the immunized person and spread to other members of the population

 C

Herd immunity can be achieved only by vaccines.

 D

The vaccine must prevent transmission of the virus as well as prevent disease.

Q. 2

All of the following are true about herd immunity, EXCEPT:

 A

It can be achieved by immunizing a population with a vaccine that interrupts transmission

 B

The live virus in the vaccine can replicate in the immunized person and spread to other members of the population

 C

Herd immunity can be achieved only by vaccines.

 D

The vaccine must prevent transmission of the virus as well as prevent disease.

Ans. C

Explanation:

Herd immunity can be achieved by immunizing a population with a vaccine that interrupts transmission, such as the live, attenuated polio vaccine, but not with a vaccine that does not interrupt transmission, such as the killed polio vaccine (even though it protects the immunized individual against disease).

In addition, the live virus in the vaccine can replicate in the immunized person and spread to other members of the population, thereby increasing the number of people protected.

Herd immunity can be achieved by natural infection as well as vaccines (e.g., measles).

For herd immunity to occur, the vaccine must prevent transmission of the virus as well as prevent disease. 

Ref: Levinson W. (2012). Chapter 33. Host Defenses. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.

Q. 3

Herd immunity provides protection to –

 A

Immunized persons

 B

Non immunized persons

 C

Both

 D

None

Q. 3

Herd immunity provides protection to –

 A

Immunized persons

 B

Non immunized persons

 C

Both

 D

None

Ans. B

Explanation:

Ans. is ‘b’ i.e., Non-immunized person 

Herd immunity

  • It is the level of resistance of a community or group of people to a particular disease.
  • It occurs when the vaccination of a portion of the population (or herd) provides protection to unprotected (non-vaccinated) individuals.

How does herd immunity provide protection to nonimmunized person ?

o Infection is spread from person to person.

o If a large number of population are immune, it is more difficult to maintain a chain of infection.

o The more immune individuals present in a population, the lower the likelihood that a susceptible person will come into contact with an infected individual.

o For example, if person ‘A’ had a disease and exposed person ‘B’ who was immune because of vaccination, person `B’ would not get ill and could not pass on the disease to person ‘C’ when he comes into contact with him. So, even if person ‘C’ is not vaccinated, He indirectly gets protection from the disease.

o Hence herd immunity may be used to reduce spread of an infection and to protect a vulnerable, un-vaccinated subgroup.

Quiz In Between


Q. 4

About herd immunity, all are true except – 

 A

depends on clinical and subclinical cases

 B

Influenced by immunization

 C

depends on the presence of alternative host

 D

Herd immunity is constant

Q. 4

About herd immunity, all are true except – 

 A

depends on clinical and subclinical cases

 B

Influenced by immunization

 C

depends on the presence of alternative host

 D

Herd immunity is constant

Ans. D

Explanation:

Ans. is ‘d’ i.e., Herd immunity is constant 

Elements which contribute to herd immunity :‑

a)  Occurence of clinical and subclinical infection in the herd -4 Clinical or subclinical infection may cause development immunity to that particular disease.

b)  Immunization of herd –> By vaccination of herd through on-going immunization programme.

c)  Herd structure —› Herd structure is never constant. It is subjected to constant variation because of new births (addition of unimmunized person to herd), new deaths (removal of immunized person from the herd) and population mobility (addition or removal of immunized or unimmunized person to herd depending on the population which is mobilizing). The herd structure includes not only the hosts (population) belonging to the herd species but also the presence and distribution of alternative animal host and insect as well as those environmental and social factors that favour or inhibit the spread of infection from host to host.

o So, herd immunity is not constant because of continous changes in these elements.


Q. 5

All of the following are true about the Herd immunity for infectious diseases except –

 A

It refers to group protection beyond what is aforded by the protection of immunized individuals

 B

It is likely to be more for infections that do not have a sub-clinical phase

 C

It is affected by the presence and distribution of alternative animal hosts

 D

In the case of tetanus it does not protect the individual

Q. 5

All of the following are true about the Herd immunity for infectious diseases except –

 A

It refers to group protection beyond what is aforded by the protection of immunized individuals

 B

It is likely to be more for infections that do not have a sub-clinical phase

 C

It is affected by the presence and distribution of alternative animal hosts

 D

In the case of tetanus it does not protect the individual

Ans. B

Explanation:

Ans. is ‘b’ i.e., It is likely to be more for infections that donot have a sub-clinical phase 

o Herd immunity is more for infections that have a subclinical phase, because by subclinical infection the individual develops immunity against that particular infection and buildup herd immunity.

o Herd immunity implies group protection beyond that afforded by the protection of immunized individuals.

Significance of herd immunity

Herd immunity predicts occurance of epidemics.

o If herd immunity is sufficiently high the occurance of epidemics is highly unlikely. o Herd immunity may lead to elimination of the diseae : –

i)   Diphtheria and poliomyelitis may be eliminated by stepping up herd immunity.

ii) Small pox was eliminated by elimination of source of infection and not by herd immunity.


Q. 6

Which vaccine gives herd immunity- 

 A

OPV

 B

Typhoid

 C

Cholera

 D

None

Q. 6

Which vaccine gives herd immunity- 

 A

OPV

 B

Typhoid

 C

Cholera

 D

None

Ans. A

Explanation:

Ans. is ‘a’ i.e., OPV 

o The vaccine progeny of OPV is excreted in the faeces and secondary spread occurs to household contacts and susceptible contacts in the community.

o Non-immunized persons may therefore, be immunized.

o Thus widspread “herd immunity”results, even if only approximately 66% of the community is immunized.

Quiz In Between


Q. 7

A herd immunity of over – % is considered necessary to prevent epidemic spread of diphtheria – 

 A

50%

 B

55%

 C

60%

 D

70%

Q. 7

A herd immunity of over – % is considered necessary to prevent epidemic spread of diphtheria – 

 A

50%

 B

55%

 C

60%

 D

70%

Ans. D

Explanation:

Ans. is d i.e., 70%

o A herd immunity of over 70% is considered necessary to prevent epidemic spread, but some believe that the critical level may be as high as 90%.


Q. 8

Herd Immunity is a feature of all of the following diseases except:       

September 2008

 A

Diphtheria

 B

Polio

 C

Tetanus

 D

Measles

Q. 8

Herd Immunity is a feature of all of the following diseases except:       

September 2008

 A

Diphtheria

 B

Polio

 C

Tetanus

 D

Measles

Ans. C

Explanation:

Ans. C: Tetanus

Herd immunity (or community immunity) describes a type of immunity that occurs when the vaccination of a portion of the population (or herd) provides protection to unprotected individuals.

Herd immunity theory proposes that, in diseases passed from person to person, it is more difficult to maintain a chain of infection when large numbers of a population are immune.

Vaccination acts as a sort of firebreak or firewall in the spread of the disease, slowing or preventing further transmission of the disease to others.

Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter will not contract and transmit the disease between infected and susceptible individuals.

Hence, a public health policy of herd immunity may be used to reduce spread of an illness and provide a level of protection to a vulnerable, unvaccinated subgroup.

Since only a small fraction of the population (or herd) can be left unvaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines because of a medical condition such as an immune disorder or for organ transplant recipients.

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

Its value varies with the virulence of the disease, the efficacy of the vaccine, and the contact parameter for the population. No vaccine offers complete protection, but the spread of disease from person to person is much higher in those who remain unvaccinated.

Herd immunity only applies to diseases that are contagious. It does not apply to diseases such as tetanus (which is infectious, but is not contagious), where the vaccine protects only the vaccinated person from disease.

Herd immunity should not be confused with contact immunity, a related concept wherein a vaccinated individual can ‘pass on’ the vaccine to another individual through contact


Q. 9

Threshold level of herd immunity for Pertussis is‑

 A

80%

 B

70%

 C

90%

 D

50%

Q. 9

Threshold level of herd immunity for Pertussis is‑

 A

80%

 B

70%

 C

90%

 D

50%

Ans. C

Explanation:

Ans. is ‘c’ i.e., 90% 

Herd immunity

  • It is the level of resistance of a community or group of people to a particular disease.
  • It occurs when the vaccination of a portion of the population (or herd) provides protection to unprotected (non‑ vaccinated) individuals.
  • Advantage of herd immunity
  • It is not necessary to achieve 100% immunization to control a disease by providing herd immunity.
  • When a certain percentage of population, is vaccinated, the spread of disease is effectively stopped.
  • This critical percentage is referred to as herd immunity threshold.

Disease                Herd immunity threshod

Diphtheria      →      85%

Measles          →      83-94%

Mumps           →      75-86%

Pertussis        →      92-94%

Polio              →      80-86%

Rubella          →      80-85%

Small pox      →      83-85%

Quiz In Between



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