What is Herd Immunity?

Herd Immunity is also known as community immunity. When a large part of the population gets immune to response to an infection. This what is meant by herd immunity. Through which it can limit the spreading of the disease from one person to another. Natural immunity is developed through herd immunity. This usually occurs due to the previous exposure to the vaccination or disease. But the entire population does not receive the immune to attain herd immunity. Through herd immunity not only the infected population develops immunity, the population who got contact with a susceptible individual will also receive immunity. This may control the spread of infection through human contacts.  It is possible to control the spread of disease among the population with the help of herd immunity.  


The percentage of the population acquiring immune power through herd immunity will differ from individual diseases. The high contagious disease such as measles and tuberculosis helps to generate a higher proportion of immune response for the wide population in minimum time. The development of herd immunity mainly depends on individual immunity, population-level characteristic, susceptibility, social habits, and  demographics   


Herd immunity definition is the main consideration for mass vaccination practices. Even if the vaccination is cheap, effective, and safe. Vaccination logistical, resource and social constraints will prevent the vaccination to achieve whole 100% of the population. All the vaccination can achieve some reasonable target level threshold values of herd immunity. 


The value of Herd Immunity H can calculate through the below expression. 


 Herd Immunity H > 1 - 1/R0


Here R0 represents the basic reproductive rate. This is mainly to calculate the spread level from the suspected individuals. 


The mass vaccination only gets successful through the principle of herd immunity. Through this method, the mass outbreak of disease can be controlled. Or else, the outbreak of disease will occur and does not achieve herd immunity. 


List of Diseases and Their Herd Immunity Response Rate. 

The below table shows the list of diseases, their transmission source, and their herd immunity rate. 


Disease

Transmission

R0

HIT

Measles

Airborne

12–18

92–95%

Pertussis

Airborne droplet

12–17

92–94%

Diphtheria

Saliva

6–7

83–86%

Rubella

Airborne droplet

Smallpox

5–7

80–86%

Polio

Fecal-oral route

Mumps

Airborne droplet

4–7

75–86%

COVID-19

2.5–4

60–75%

SARS

(2002–2004 SARS outbreak)

2–5

50–80%

Ebola

(Ebola virus epidemic in West Africa)

Bodily fluids

1.5–2.5

33–60%

 

History of Herd Immunity

The term herd immunity was coined in 1923. In 1930, a researcher from Baltimore A.W. Hedrich published a research paper on the epidemiology of measles and recognized that herd immunity is a naturally occurring phenomenon by taking notice of many children, who are exposed to measles and not exposed to measles. Here the number of new infection rates temporarily decreased with the susceptible children. But even getting this knowledge he cannot control or eliminate the measles disease. Later, in the 1960s vaccination for measles promotes herd immunity.  Likewise, mass vaccination helps for disease eradication. In the 1970’s the theorem for calculating disease’s herd immunity threshold was developed.  In the 1960s and 1970s, the smallpox eradication campaign started with the practice of ring vaccination. Through which the huge population acquires herd immunity and began to immunize every person in a ‘ring’ around suspected individuals. This prevents the outbreak of disease. 


Even the adoption of mass and ring vaccination also developed some complexities and challenges for acquiring herd immunity. The spreading of infectious diseases is usually made up of several assumptions. This makes many confusions among the suspected individuals. Also, according to recent researches, they found the microorganisms can adapt to the new model of development and can evaluate to attract herd immunity. Even the herd immunity among the population may not support to evolutionary pressure of the new development strains. This emerging of new strains will suppress the herd immunity of certain communities and they are returning back to those communities to invade the population. This part of the research remains untouched by many researchers. 


What are the Challenges in Creating Herd Immunity? 

For the most infectious disease,  herd immunity is important for generating immune power among the greater population. For example, Highly contagious diseases like measles required 95% of the population to get vaccinated to develop herd immunity for the large population. Though the new coronavirus has low infection rate than measles. On average, each infected person is spreading diseases to two or three peoples. This indicates that atleast 60% of vaccination or immunity should be developed among the population to generate herd immunity. 


However, the population needs a high rate of serious illness, death, and high medical treatments to develop natural herd immunity. But, this remains a big challenge for the high-income countries to treat the unchecked infection. That’s why researchers worked on developing vaccination to induce herd immunity.   


Even available vaccination cannot develop herd immunity for the long term for some viruses includes seasonal flu. This is mainly due to the virus mutation, which evades the body’s immune response. So the induced immune power cannot support for the long term. That’s why flu shot requires updating immune power every year. 


Risk of Herd Immunity

To induce herd immunity among the large population, mass vaccination plays high successful parts while treating many diseases. But this herd immunity unable to develop immunity among people with immune deficiencies or suppressed immunity due to medical reasons.  When herd immunity is establishing, some people behave as free riders and they fail to develop the immune system. When a population remains with too many free riders. It will suppress the level of herd immunity of the whole population and can put them at risk. 


Passive Immunity

Individuals can acquire immunity passively. This occurs when the antibodies of pathogens are transferred from one person to another. This can occur naturally through maternal antibodies. Here, the immunoglobulin G antibodies get transferred through the placenta and colostrum to fetuses and newborn babies from mothers. This passive immunity can acquire artificially when the antibodies are collected from the serum or plasma of the immuned person to the suspected individual.  People can get the immediate remedy through passive immunity. On the other hand, herd immunity requires weeks to months to develop antibodies in a group of people. To develop immunity in fetuses or newborns for influenza and tetanus, pregnant women can be immunized to transfer antibodies to the children.  A person with a high risk of infections can utilize passive immunity to reduce the infection rate or to reduce the severity of symptoms. 

FAQs (Frequently Asked Questions)

Q1. Why are Countries Trying to Obtain Herd Immunity to COVID-19?

Ans: The herd immunity definition states that immunity is generated for a huge population through the mass vaccination process. Covid 19 is one of the ongoing pandemics around the world. Many countries are trying to obtain herd immunity for covid 19 to save the people who cannot get vaccinated because of health conditions like an allergic reaction to the vaccine and to save other populations protected from the disease. 

Q2. What is the Definition of the Herd?

Ans: Herd generally means a social group of the same species like animals, fishes...etc. The collective animal’s behavior with its characteristics is called herding. The term herd is generally specifying the group of mammals. That too which can live in groups. This represents both domestic and wild animals.