INTRODUCTION:

Herd immunity occurs when a significant portion of a population becomes immune to an infectious disease and the risk of spread from person to person decreases; those who are not immune are indirectly protected because ongoing disease spread is very small. The proportion of a population who must be immune to achieve herd immunity varies by disease. For example, a disease that is very contagious, such as measles, requires more than 95% of the population to be immune to stop sustained disease transmission and achieve herd immunity. [1]

For many infectious agents, the number of individuals in a population (herd) who are (relatively) immune to infection with an infectious agent depends on the proportion who have previously been infected with the infectious agent and the proportion of the remainder who have been immunised with an efficacious vaccine against the agent.

A measure of the level of population immunity, or herd-immunity, is the proportion who are thus immune from further infection. For many infections, the level of herd immunity in a population may have an effect on the amount of transmission of the infection within the population and, in particular, may affect the risk of an uninfected becoming infected. For such infections, increasing the level of herd immunity will decrease the risk of an uninfected person becoming infected. [2-3]

Herd protection may be very important for disease control through vaccination. If the herd effect reduces the risk of infection among the uninfected sufficiently, then the infection may no longer be sustainable within the population and the infection may be eliminated. This concept is important in disease elimination or eradication programmes. It means, for example, that elimination of the infectious agent in a population can be achieved without necessarily vaccinating the entire population. [4]

SYNONYMS:

· Herd effect

· Community immunity

· Population immunity

· Social immunity

· Herd protection

· Group protection

MEANING:

· The term ‘‘herd immunity’’ is widely used but to describe the proportion immune among individuals in a population. Others use it with reference to a particular threshold proportion of immune individuals that should lead to a decline in incidence of infection. Still others use it to refer to a pattern of immunity that should protect a population from invasion of a new infection.

· A common implication of the term is that the risk of infection among susceptible individuals in a population is reduced by the presence and proximity of immune individuals (this is sometimes referred to as ‘‘indirect protection’’ or a ‘‘herd effect’’) [5].

HISTORY:

· In 1923- The term "herd immunity" was coined. [6]

· In 1930s- Herd immunity was first recognized as a naturally occurring phenomenon when A. W. Hedrich published research on the epidemiology of measles in Baltimore, and took notice that after many children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children. [7,8]

· In the 1960s- In spite of this knowledge, efforts to control and eliminate measles were unsuccessful until mass vaccination using the measles vaccines were began. [8] Mass vaccination, discussions of disease eradication, and cost–benefit analyses of vaccination subsequently prompted more widespread use of the term herd immunity [9].

· In the 1970s- The theorem used to calculate a disease's herd immunity threshold was developed [9].

· In the 1960s and 1970s- During the smallpox eradication campaign, the practice of ring vaccination, to which herd immunity is integral, began as a way to immunize every person in a "ring" around an infected individual to prevent outbreaks from spreading [10]. Since the adoption of mass and ring vaccination, complexities and challenges to herd immunity have arisen.

· In recent decades-

o It has been recognized that the dominant strain of a microorganism in circulation may change due to herd immunity, either because of herd immunity acting as an evolutionary pressure or because herd immunity against one strain allowed another already-existing strain to spread. [11,12]

o Emerging or ongoing fears and controversies about vaccination have reduced or eliminated herd immunity in certain communities, allowing preventable diseases to persist in or return to these communities.

BENEFICIAL EFFECTS OF HERD IMMUNITY:

A major beneficial effect of increasing the level of population immunity through vaccination is that it may result in the elimination of the infectious agent from the population.

· Protection of those without immunity:

Some individuals either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated. Even if the increase in population immunity is not sufficient to achieve infection elimination (because, for example, the efficacy of the vaccine is poor or it is not possible to achieve sufficiently high vaccine coverage), the risk of infection among unvaccinated person may still be reduced. This may be particularly important for those for whom vaccination is contraindicated

· Serotype replacement:

It may occur if the prevalence of a specific serotype declines due to high levels of immunity, allowing other serotypes to replace it. Initial vaccines against Streptococcus pneumoniae significantly reduced nasopharyngeal carriage of vaccine serotypes (VTs), including antibiotic-resistant types, only to be entirely offset by increased carriage of non-vaccine serotypes (NVTs).

· Eradication of diseases:

It has been established and maintained in a population for a sufficient time, the disease is inevitably eliminated and no more endemic transmissions will occur. If elimination is achieved worldwide and the number of cases is permanently reduced to zero, then a disease can be declared as eradicated.

DELETERIOUS EFFECTS OF HERD IMMUNITY:

Although reducing the risk of infection among susceptible is generally a benefit, for some infections it may also result in deleterious effects. For example, for common childhood infections, a consequence of reducing the risk of infection among susceptible is that for those who are infected the average age at infection will increase. This may be of consequence when the clinical consequences of infection are greater for those infected at older ages (e.g. polio, rubella, varicella, measles, and hepatitis A)

COVID-19 AND HERD IMMUNITY:

Proper handwashing, wearing a mask and social distancing and are currently the well-known methods that help to prevent all around the world from contracting and potentially infectious SARS-CoV-2, the virus that causes COVID-19.

There are numerous reasons why herd immunity is not the answer to stop the spread of the new coronavirus:

1. There is no recommended vaccine for SARS-CoV-2.

2. The research studies on antivirals and other effective medications to treat COVID-19 are ongoing.

3. Researchers couldn’t find whether we can contract SARS-CoV-2 and develop COVID-19 more than once.

4. Individuals who contract SARS-CoV-2 and develop COVID-19 can experience serious side effects which can lead to death.

5. It is exactly unknown that why some people who contract SARS-CoV-2 develop severe COVID-19, while others do not.

6. Morbidity and mortality rates are high among the high-risk groups including old age, immunocompromised and children.

Herd immunity for covid-19 in the future:

· Scientists are currently working on a vaccine for SARS-CoV-2. If we have a vaccine, we may be able to develop herd immunity against this virus in the future. This would mean getting the SARS-CoV-2 in specific doses and making sure the majority of the world’s population is vaccinated.

· Almost all healthy adults, teens, and older children would need to be vaccinated to provide herd immunity for people who can’t get the vaccine or who are too ill to become naturally immune to it.

· Herd immunity is not the answer to stopping the spread of SARS-CoV-2, the new coronavirus that causes COVID-19. Once a vaccine is developed for this virus, establishing herd immunity is one way to help protect people in the community who are vulnerable or have low functioning immune systems.

CONCLUSION:

Herd immunity to go into effect and stop the disease. The main goal of herd immunity is to prevent others from catching or spreading an infectious disease. It can help to stop or slow down the spread of an infectious disease like measles, swine flu or COVID-19. Herd immunity may not completely stop the spread of SARS-CoV-2, the new coronavirus that causes COVID-19. Once a vaccine is developed for this novel corona virus, establishing herd immunity is one of the effective ways to protect both the immunocompromised and the vulnerable people in the community. This immunity may not always protect every individual in the community, but it could help prevent widespread disease.

REFERENCE:

1. https://jamanetwork.com/ on 10/29/2020

2. John TJ. Samuel R (2000). “Herd immunity and herd effect: new insights and definitions”. Eur J Epidemiol. 16(7):601-6.

3. Fox JP, Elveback L, Scott W, Gatewood L, Ackerman E (1995). Herd immunity: basic concept and relevance to public health immunization practices. Am J Epidemiol. 141(3):187-97.

4. Peter GS (2010). “Concepts of herd protection and immunity”. Procedia in Vaccinology 2. 134–139. Available online at: www.sciencedirect.com.

5. https://academic.oup.com/cid/article/52/7/911/299077 by guest on 29 October 2020

6. Topley, W. W. C.; Wilson, G. S. (May 1923). "The Spread of Bacterial Infection. The Problem of Herd-Immunity". The Journal of Hygiene. 21 (3): 243–49. doi:10.1017/s0022172400031478. PMC 2167341. PMID 20474777.

7. Hedrich, A. W. (1933). Monthly Estimates of the Child Population Susceptible to Measles, 1900–1931, Baltimore, Md. American Journal of Epidemiology, 17(3), 613–636.

8. Hinman, A. R.; Orenstein, W. A.; Papania, M. J. (1 May 2004). "Evolution of measles elimination strategies in the United States". The Journal of Infectious Diseases. 189 (Suppl 1): S17–22. doi:10.1086/377694. PMID 15106084. Sencer, D. J.; Dull, H. B.; Langmuir, A. D. (March 1967). "Epidemiologic basis for eradication of measles in 1967". Public Health Reports. 82 (3): 253–56. doi:10.2307/4592985. JSTOR 4592985. PMC 1919891. PMID 4960501.

9. Fine, P.; Eames, K.; Heymann, D. L. (1 April 2011). "'Herd immunity': A rough guide". Clinical Infectious Diseases. 52 (7): 911–16. doi:10.1093/cid/cir007. PMID 21427399.

10. Strassburg, M. A. (1982). "The global eradication of smallpox". American Journal of Infection Control. 10 (2): 53–59. doi:10.1016/0196-6553(82)90003-7. PMID 7044193

11. Bull, R. A.; White, P. A. (2011). "Mechanisms of GII.4 norovirus evolution". Trends in Microbiology. 19 (5): 233–40. doi:10.1016/j.tim.2011.01.002. PMID 21310617

12. McEllistrem, M. C.; Nahm, M. H. (2012). "Novel pneumococcal serotypes 6C and 6D: Anomaly or harbinger". Clinical Infectious Diseases. 55 (10): 1379–86. doi:10.1093/cid/cis691. PMC 3478140. PMID 22903767

Received on 06.11.2020 Modified on 01.12.2020

Int. J. Nur. Edu. and Research. 2021; 9(1):125-127.

DOI: 10.5958/2454-2660.2021.00032.6