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 Table of Contents  
PERSPECTIVE
Year : 2020  |  Volume : 9  |  Issue : 5  |  Page : 73-76

Can herd immunity be relied on as a strategy to combat COVID-19?


Department of Community Medicine, Adesh Medical College and Hospital, Shahabad(M), Haryana, India

Date of Submission07-May-2020
Date of Decision08-May-2020
Date of Acceptance11-May-2020
Date of Web Publication04-Jun-2020

Correspondence Address:
Dr. Amrit Virk
Department of Community Medicine, Adesh Medical College and Hospital, Shahabad(M), Haryana
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijhas.IJHAS_98_20

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  Abstract 


Herd immunity offers protection to a population when adequate numbers of people are immune to an infectious disease to prevent its spread. Those who recover from COVID-19 and those who receive a vaccine (as and when there is one) are expected to develop some immunity, at least for some period of time. With Severe acute Respiratory Syndrome- coronavirus-2 (SARS CoV-2), how long the immunity will last is not yet known as currently there is no vaccine. Achieving herd immunity to SARS-CoV-2 infection may be the biggest leap toward returning to a near-normal pre-COVID-19 life, as social distancing guidelines and lockdowns may carry on while the virus persists and people continue to get infected and die. This article is an attempt to throw some light on the concept of herd immunity in context to COVID-19 and the implications thereof.

Keywords: Community protection, herd effect, herd protection, vaccination


How to cite this article:
Virk A, Samdarshi N. Can herd immunity be relied on as a strategy to combat COVID-19?. Int J Health Allied Sci 2020;9, Suppl S1:73-6

How to cite this URL:
Virk A, Samdarshi N. Can herd immunity be relied on as a strategy to combat COVID-19?. Int J Health Allied Sci [serial online] 2020 [cited 2020 Jul 13];9, Suppl S1:73-6. Available from: http://www.ijhas.in/text.asp?2020/9/5/73/285979




  Introduction Top


The entire world is experiencing an unprecedented lockdown as a public health measure to check the spread of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (COVID-19). Having originated in Wuhan, the capital of China's Hubei province, coronavirus disease 2019 has spread worldwide as a pandemic never seen before in the history of humankind.[1]

Governments and their policymakers are constantly being pressed to present their rationales and strategies for breaking the lockdown for a cautious re-emergence. The changing global situation and rising COVID-19 count has made many epidemiologists come out with preventive strategies to stall the tidal wave of this disease.

One notion that has garnered immense interest is the role of herd immunity in combating COVID-19, which rests on empirically defined, quantifiable immune parameters that lay the foundation for the attainment of protection against a given pathogen.


  Understanding Herd Immunity Top


Herd immunity is a recognized epidemiological concept regarding the population-level effect of individual immunity to prevent the transmission of infectious diseases.[2] In simple words, it describes the state where majority of the people in a population are immune to an infectious disease, thus providing indirect protection so that the disease stops spreading to those not immune to the disease.

This is called “herd immunity,” “community immunity,” “herd effect,” or “herd protection,” and it gives indirect protection to unvaccinated individuals, thus preventing the circulation of infectious agents in susceptible populations such as newborns, elderly people, and those who cannot get vaccinated for a variety of reasons.

The term “Herd immunity” is also sometimes used to describe a particular threshold proportion of immune individuals that would lead to a reduction in the incidence of infection, implying that the risk of infection among susceptible individuals in a population declines due to the presence and proximity of immune individuals.[3]

Understanding herd immunity requires some contemplation on infection dynamics, modes of transmission, as well as the acquisition of immunity by individuals in the population.

Being a new virus, when the SARS-CoV-2 first started to spread, the entire human population was vulnerable as nobody was immune. In the absence of any resistance, the virus spread quickly across population worldwide and any attempt to put an end to its spread will require a significant percentage of people to be immune. But, can we achieve that level of immunity?

Herd immunity can develop in two ways:

When many people contract the disease and recover, thereby building an immune response to it (natural immunity) in due course of time

To understand how the human body develops this natural immunity, let us assume an exposure to a disease-causing organism that triggers the human body to produce antibodies which are disease-specific proteins that fight off infection. On initial exposure, it can take the body a while to develop the antibodies. As the human immune system retains knowledge of a particular infectious agent, it is able to rapidly deploy these antibodies to fight off any subsequent infection with the same agent. When an individual develops this sort of antibody-based immunity to a given infectious agent, that individual is said to be naturally “protected.” and the likelihood that they will pass it on falls significantly. This is natural immunity.

When a large group of people (the herd) is immune to an infectious agent, then an individual in the middle of this group is unlikely to become infected. The infectious agent is unable to get through the herd as the high population immunity protects even those not yet immune. Herd immunity can thus slow the spread of a contagious infectious agent.

When people are vaccinated against the disease to achieve immunity

The concept of herd protection is commonly used in relation to vaccines. In fact, herd immunity has so far been the fundamental basis of all vaccination programs.

The purpose of vaccination is to expose a person to something that induces immunity without affecting them adversely or making them sick. Vaccines are either small doses of a pathogen, a dead microorganism, or a microorganism that is similar but weaker. In order for vaccination to bring about effective herd immunity, we need upward of 70% of protective immunity in a population to prevent large-scale propagation.

The importance of herd immunity was recognized with small pox, where the initial goal was to immunize 80% of the population in order to achieve a herd effect. Although the ultimate eradication in 1977 was achieved with higher vaccine uptake rates, the herd effect contributed to the reduction of small pox by a mass vaccination program that focused on endemic countries.[4]

It is imperative to know that herd immunity comes into play only if the majority of a population have been vaccinated against an infectious disease. The percentage of people that are required to be immune (naturally or through vaccination) to prevent an epidemic varies from infection to infection.

With some extremely infectious diseases such as measles,[5] the required immunity levels are close to 90%, but with other infections, lesser proportion of immune people may suffice to counter spread of infection in the population.


  Herd Immunity in Case of Covid-19 Top


How does one conclude how much herd immunity will be required to alleviate the subsequent outbreaks of COVID-19? This estimate depends on several determinants[6] such as the reproduction number (R0) for SARS-CoV-2, which is presently believed to be 2.2.

The reproduction number, R0, defines the expected number of secondary infections arising from an infected individual during his or her entire infectious period, in a population of susceptibles.[7] Based on the calculated R0, the herd immunity estimates suggest that at least 60% of the population would need to be protected, either from natural infection or vaccination to halt the further spread of COVID-19. However, this percentage rises if R0 was underestimated.[8]

To reiterate the importance of R0, it may be worthwhile to remember the influenza pandemic of 1918 (R0 of 1.8), in which one-third of the world's population got infected and 50 million people died. Most of the COVID-19 patients are asymptomatic or show mild symptoms and recover on their own in 2–3 weeks; it is quite likely that some cases are not detected and the actual reproductive number is much higher than what has been stated so far.[9]

There are several reasons as to why herd immunity may not be the answer to stop the spread of SARS-CoV-2.

  • Unlike vaccination, herd immunity does not offer a high level of individual protection, and thus it may not be a good alternative to getting vaccinated
  • There is so far no vaccine for SARS-CoV-2. Vaccinations are till date the safest way to practice herd immunity in a population
  • Scientists do not know if re-infection with SARS-CoV-2 virus is a possibility among people who have recovered from COVID-19 once
  • Research on COVID-19 antiviral treatment is ongoing, as presently there is no finite treatment
  • Most of the elderly population is at risk for serious complications due to COVID-19, which can lead to death. However, even the young and healthy people may develop complications that can prove fatal, which raises doubts about the virulent nature of the virus
  • The reasons for selective severity of disease (COVID-19) in some people are not known. The rate of recovery and severity of the disease seem to depend on the age and health of the individuals. The younger age group without any comorbidity is expected to have a low death rate (0.2%) in comparison to older population (above 80), which have shown a much higher death rate (15%)[10]
  • There is an added possibility of a mutation in SARS CoV-2 and the emergence of a new strain of a virus which can make herd immunity ineffective[11]
  • As the details on coronavirus are not yet fully well studied, there is little information on the long-lasting nature of the antibody response.


Herd immunity is disease specific and is determined by the effortless spread of disease from person to person, or the level of infectiousness. The true nature of coronavirus and herd immunity are not yet characterized. Regardless of the specifics, achieving herd immunity by the repeated process of infection of one person, recovery and immunity will take a long time – several months or years.

History of infectious diseases worldwide proves beyond any doubt the role of herd immunity in controlling measles, mumps, polio, and chickenpox. However, it is worthwhile to mention here that immunity then was achieved through vaccination in most cases. Looking at the infectiousness of SARS-CoV-2, it is estimated that at least 60%–70% of the population will need to be immune to have herd protection.


  The Choice between Getting Infected and Getting Vaccinated Top


The prospect of developing immunity to a condition through infection, rather than through vaccination, could be harmful, as COVID-19 carries a much higher risk of severe disease and even death. The mortality rate for COVID-19 is known to vary between countries. The current estimates reveal an overall mortality rate per confirmed cases to be nearly 4.5%.[10]


  What Lies Ahead? Top


COVID-19, for now, appears far from being contained in most countries. So far, social distancing, quarantine, and other sanitizing habits such a handwashing have proved successful in flattening the curve and slowing down the pandemic. In order to achieve control over COVID-19, the following situations should be kept in mind:

  • Given the infective nature of SARS-CoV-2 and in the absence of any public health interventions, i.e., if we do not perform physical distancing or enact other measures to halt the spread of SARS-CoV-2, the virus can infect many people in a matter of a few months. This can lead to drastic overwhelming of our health facilities and lead to high death rates. Social distancing aims to “flatten the curve” and decrease strain on the health-care system. This in effect buys the scientific community time to develop treatments and vaccines, as well as build up capacity for testing and tracing
  • The current levels of infection can be maintained or reduced further by continuing physical distancing measures for an extended period of time, until a vaccine becomes available. This, however, will take concerted effort on the part of the entire population and governments
  • The most likely scenario should counterpoise somewhere in the middle, where infection rates increase and decrease, with alternate relaxations and stringent implementation of lockdowns for maintaining social distancing measures depending on the number of infections in any community.


It is laudable that there are more than 100 candidate COVID-19 vaccines in development,[12] with a few in, or soon to be in, Phase 1 trials to assess the safety and immunogenicity against a novel virus first sequenced in January, 2020.

Prolonged and consistent efforts will be needed to prevent major outbreaks until a vaccine is developed. However, it is unlikely in the long term to have the explosive spread that we are observing now because much of the population hopefully will be immune in future.


  Conclusions Top


This article brings forth the fact that usually, herd immunity does not develop in a year or two. For herd immunity to come into play, an effective, safe vaccine is a must. If we have a vaccine, we may be able to develop herd immunity against the virus in future provided majority of the population are vaccinated.

A 2–3-year timeline appears to be a more realistic though optimistic estimate for when a vaccine will be widely available to the entire population. Considering the urgency of the coronavirus pandemic, and the remote possibility that an approved, effective, safe general use vaccine is available in a year from now, it would still take many years to confer sufficient “herd immunity” to prevent the endemic spread of COVID-19.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Li Q, Guan X, Wu P, Wang X, Zhou L, Tong Y,et al. Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia. N Engl J Med 2020;382:1199-207.  Back to cited text no. 1
    
2.
Smith DR. Herd immunity. Vet Clin North Am Food Animal Pract 2019;35:593-604.  Back to cited text no. 2
    
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Kim TH, Johnstone J, Loeb M. Vaccine herd effect. Scand J Infect Dis 2011;43:683-9.  Back to cited text no. 3
    
4.
Plotkin SA. Vaccination against the major infectious diseases. CR Acad Sci III 1999;322:943-51.  Back to cited text no. 4
    
5.
Guerra FM, Bolotin S, Lim G, Heffernan J, Deeks SL, Li Y, et al. The basic reproduction number (RO) of measles: A systematic review. Lancet Infect Dis 2017;17:e420-8.  Back to cited text no. 5
    
6.
Anderson RM, May RM. Vaccination and herd immunity to infectious diseases. Nature 1985;318:323-9.  Back to cited text no. 6
    
7.
Heffernan JM, Smith RJ, Wahl LM. Perspectives on the basic reproductive ratio. J R Soc Interface 2005;2:281-93.  Back to cited text no. 7
    
8.
Altmann DM, Douek DC, Boyton RJ. What policy makers need to know about COVID-19 protective immunity. Lancet. 2020;395:1527-9.  Back to cited text no. 8
    
9.
Biggerstaff M, Cauchemez S, Reed C, Gambhir M, Finelli L. Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: A systematic review of the literature. BMC Infect Dis 2014;14:480.  Back to cited text no. 9
    
10.
Wu JT, Leung K, Bushman M, Kishore N, Niehus R, de Salazar PM, et al. Estimating clinical severity of COVID-19 from the transmission dynamics in Wuhan, China. Nat Med 2020;26: 506-10.  Back to cited text no. 10
    
11.
Fine P, Eames K, Heymann DL. “Herd immunity”: A rough guide. Clin Infect Dis 2011;52:911-6.  Back to cited text no. 11
    
12.
Accessed from: https://www.jhsph.edu/covid-19/articles/achie ving-herd-immunity-with-co vid19.html. [Last accessed on 2020 May 04].  Back to cited text no. 12
    




 

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Abstract
Introduction
Understanding He...
Herd Immunity in...
The Choice betwe...
What Lies Ahead?
Conclusions
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