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Herd immunity is a significant epidemiologic phenomenon that becomes effective when a large majority of people in a population are vaccinated against an infectious disease and hence restrict its general spread and protect unvaccinated persons indirectly.
Herd immunity can be achieved naturally, where individuals produce antibodies after the recovery from disease, or more effectively through mass vaccination campaigns. When a high rate of the population is immunized, the pathogen of the disease has no hosts to infect therefore is reduced in terms of transmission levels, ultimately destroying the disease in some cases. This has played a very important role in the prevention and control of disease as well as the eradication of diseases such as poliomyelitis, measles, and smallpox.
Immunisation is also very effective in generating herd immunity because it transmits immunity with reduced risk to people as opposed to natural infection. The percentage, nonetheless, for herd immunity varies depending on the infectivity of the disease, typically in basic reproduction number (R0). The proportion to be immunised must be larger for more infectious diseases to bring the transmission to an end. Although herd immunity is a protection for the non-immunizable by health status or immunocompromised, it must have sustained efforts in immunization programs and public health interventions to be able to achieve high enough levels of immunity in the population.
Understanding herd immunity
Herd immunity refers to when the majority of people in the population have been vaccinated against a disease that can spread. Under such conditions, the people that have had the illness previously or received vaccination so the illness cannot transmit itself to other persons around them.
Therefore, when many people are vaccinated against certain diseases, the chain of infection is interrupted and can also protect people who cannot be vaccinated due to health conditions such as allergies, compromised immune systems, or age restrictions. The population size over which it will be effective in achieving herd immunity will usually depend on the disease and basic reproduction number (R0), the number of people infected, and on average how many it will infect. The larger the R0, the higher the percentage of the population that will have to be immune to terminate the transmission chain.
For example: Measles has an R0 of 12-18 and would therefore require about 95% immunity. Polio has an R0 of 5-7 and would therefore require about 80-85% immunity. Covid-19 has provided a range of R0 estimates, which have required shifting levels of immunity in response to shifting variants.
Role of vaccination in herd immunity
Immunisation is the best method of attaining herd immunity without risking the population by natural infection. Vaccines do this by inducing the immune system to identify and eliminate pathogens and thus acquire immunity without disease.
Prevention of disease outbreaks: After the immunisation of the majority of the population, the epidemics cease. Before the measles vaccine, epidemic-sized measles was an expected feature. Mass immunisation throughout the world diminishes epidemic-sized measles.
Immune protection of vulnerable groups: Herd immunity covers groups of people who are unable to be immunised with certain vaccines, i.e., neonates, elderly, immunocompromised, and cancer patients. Therefore, herd immunity decreases the number of cases of diseases in the population.
Decreasing health care burden: Immunisation decreases hospitalisation and healthcare expenses. Immunisation against the flu during the flu season, for instance, decreases hospitalisation and healthcare expenses for influenza complications.
Eradication of Smallpox
Smallpox was a very dangerous and very contagious disease that affected people from all over the world. Due to a very successful international campaign for immunisation, the disease was completely eradicated from the world in 1980.
Eradication of Polio in most countries
Polio was used to vaccinate thousands of children annually but is now eliminated from most of the world. It's only heard of in a few places nowadays, and this is a testament to how significant it is that immunisation has to be ongoing.
Measles and Rubella control
Measles continues as a persistent threat in chosen areas of low immunisation, but countries that once had mass vaccination campaigns are declining quickly. Rubella, the second most important cause of devastating birth defects, has been eradicated in most countries by herd immunity.
Challenges in reaching herd immunity
Although being an ideal factor, immunisation to reach herd immunity is not without hurdles.
Vaccine hesitancy: Misinformation, fear, and vaccine resistance have resulted in low very immunisation coverage in a few groups, such that preventable disease comes back.
Immunisation coverage variation: Certain regions, especially in the developing world, are confronted with the availability of vaccines amidst economic and logistic challenges. Securing global vaccine supply is paramount in a bid to attain herd immunity.
Pathogen evolution and variants: Influenza and SARS-CoV-2 viruses evolve and make the vaccine ineffective. Continuous research and updating of vaccine policy are required.
Vaccine technology like mRNA changing our capability to fight and ward off extremely infectious diseases in the most effective way possible. Public health education in global health, international cooperation, and funding immunisation programmes will provide herd immunity and global health security.
Conclusion
Herd immunity is the basis of public health on the ethical ground of group protection against infection by disease. It relies upon the knowledge that, if a large proportion of the group are immunised or have already had contact with the disease, the disease spread is prevented or reduced. It not only immunises but also indirectly safeguards those who cannot be immunised, like infants, the elderly, and immunocompromised individuals.
Immunisation through vaccination is the ideal and safest method of developing immunity in large numbers without subjecting individuals to the risk of exposure to the caprices of natural disease.
It is a rational, science-match-based process of developing immunity at low risk instead of depending upon the spread of disease with the risk of developing severe disease, long sequels, and death. Their ability to bring or cut down to the barest minimum the instances of diseases like smallpox, polio, and measles is evidence enough. But it takes a concerted effort to build and sustain herd immunity. Reducing vaccine levels can permit suppressed past diseases to flare up, as recent measles epidemics in districts with low levels of vaccines have done. Misinformation, vaccine fear, along with logistical complexity can jeopardise decades of achievement made by progress in public health. This can cause a great setback in situations like this.
It's not just an individual choice but a matter of civic responsibility. By becoming vaccinated, one gets the overall society healthier, avoids the risk of an epidemic, and protects society's most vulnerable. Doctors, politicians, and governments owe it to themselves and society to increase awareness about vaccines, fight against rumours, and vaccinate all segments of society.
Herd immunity can be our best tool against infectious disease. If we can optimise vaccination and remind ourselves again of the value of immunity in entire populations, we can save people from a lot of unnecessary pain, including helping public health and making a future where preventable disease is no longer a danger to society.
(Author is with Internal Medicine, Apollo Spectra Hospital, Chennai)
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