Key Factors Influencing Herd Immunity in Populations
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.
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 on What Is Herd Immunity and Why Is It Important?
1. What is herd immunity and how does it actually work?
Herd immunity, also known as community immunity, is a form of indirect protection from an infectious disease. It occurs when a large percentage of a population becomes immune to a disease, either through vaccination or previous infection. This makes the spread of the disease from person to person unlikely, thereby protecting the entire community, including those who are not immune themselves.
2. What is the main benefit of achieving herd immunity in a population?
The single most important benefit of herd immunity is that it protects the most vulnerable individuals in a community. This includes people who cannot be vaccinated due to medical reasons (like infants, pregnant women, or immunocompromised patients). By reducing the overall spread of the disease, the risk of them getting infected is significantly lowered.
3. Can you provide some real-world examples of diseases controlled by herd immunity?
Yes, several diseases have been successfully controlled largely due to herd immunity achieved through widespread vaccination. Key examples include:
- Measles: High vaccination rates have made measles outbreaks rare in many countries.
- Polio: Global vaccination efforts have brought the world close to eradicating polio by establishing herd immunity.
- Mumps and Rubella: The MMR vaccine has drastically reduced the circulation of these viruses in the population.
4. How is the required percentage for herd immunity calculated for a specific disease?
The percentage, or threshold, for herd immunity is not the same for every disease. It is calculated based on the disease's contagiousness, measured by the basic reproduction number (R₀). R₀ is the average number of people one sick person will infect in a non-immune population. The more contagious a disease (higher R₀), the higher the percentage of people who need to be immune to achieve herd immunity.
5. Why is achieving herd immunity through vaccination considered better than through natural infection?
Achieving herd immunity through vaccination is much safer because it provides protection without causing the actual illness. Relying on natural infection would require a massive number of people to get sick. This would lead to severe complications, long-term health issues, and a significant number of preventable deaths, which is a dangerous and unethical public health strategy.
6. If a community has herd immunity, does it mean unvaccinated individuals are completely safe?
No, not completely. Herd immunity works on probabilities. It dramatically reduces the chance of an unvaccinated person encountering the virus because the chain of transmission is frequently broken. However, if they do come into contact with an infected person, they are still highly susceptible to the disease. The risk is lowered, but it is not zero.
7. What are the major challenges that can prevent a community from reaching herd immunity?
Several factors can make it difficult to achieve herd immunity. Some of the most common challenges include:
- Vaccine Hesitancy: When a significant number of people refuse to get vaccinated.
- Virus Variants: New strains of a virus that can evade the immunity provided by vaccines or past infections.
- Uneven Vaccination: When some areas have high vaccination rates while others have very low rates, creating pockets where the disease can spread easily.
- Waning Immunity: When the protection from a vaccine or infection decreases over time, requiring booster shots.
