Herd Immunity

HERD IMMUNITY:

Also known as Herd Effect, community Immunity, Population immunity, Social immunity

Herd immunity refers to group protection beyond what is afforded by the protection of immunized individuals

PROCESS:

Herd immunity can be achieved by:
- Immunizing a population with a vaccine that interrupts transmission, such as the live, attenuated polio vaccine.
- Natural infection as well as vaccines (e.g., measles).
- 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.
For herd immunity to occur, the vaccine must prevent transmission of the virus as well as prevent disease.

ELEMENTS:

Occurrence of clinical and subclinical infection in the herd -4 Clinical or subclinical infection may cause development immunity to that particular disease.
Immunization of herd –>
- By vaccination of herd through on-going immunization programme.
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.

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

TYPES:

ADVANTAGES:

Helf full in persons having weak immune systems
Potential for infection elimination
Reduced risk of Infection for those who refuse vaccination or for whom vaccination is contraindicated.

DISADVANTAGES:

Only applicable to the diseases which are contagious.
Raises the average age of infection among those who are infected e.g, Rubella, varicella and polio etc

Role of HI in elimination of diseases.

Till date, 2 diseases namely Rinderpest and smallpox have been eradicated using the concept of HI and is underway for poliomyelitis.

Basic reproduction rate – (R0)

The basic reproduction rate (R0) is used to measure the transmission potential of a disease.
It is thought of as the number of secondary infections produced by a typical case of an infection in a population that is totally susceptible.

Herd Immunity threshold –

This number represents how many people in an unprotected population one infected person could pass the disease along to. For example, R0 for measles is between 12 and 18, while for polio, it is between five and seven. The higher this number is, the higher the immunity threshold must be to protect the community.
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%.

Estimated Herd Immunity thresholds for vaccine preventable diseases
Disease	Transmission	R0	Herd Immunity threshold
Diptheria	Saliva	6-7	85%
Measles	Airborne	12-18	92-94%
Mumps	AirborneDroplet	4-7	75-86%
Pertussis	AirborneDroplet	12-17	92-94%
Polio	Faecal – oral	5-7	80-86%
Rubella	AirborneDroplet	5-7	80-85%
Small pox	Social contact	6-7	83-85%

Spread of epidemic is influenced by herd immunity
Herd immunity Can be acquired by immunisation
Herd immunity is mostly due to sub-clinical infection
Herd immunity can be achieved by immunizing a population with a vaccine that interrupts transmission
In herd immunity live virus in the vaccine can replicate in the immunized person and spread to other members of the population
The vaccine must prevent transmission of the virus as well as prevent disease.
Herd immunity provides protection to Non immunized persons
Threshold level of herd immunity for Pertussis is 90%

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