Context:

UK’s Government hinted at a strategy that would allow the novel coronavirus to infect 60% of the country’s population so that a degree of “herd immunity” could be achieved.

Following widespread criticism, and with Imperial College London projecting a dire scenario if the pandemic remains uncontrolled, the UK has now retracted — and is looking at self-isolation for the elderly.

Definition

Herd immunity refers to preventing an infectious disease from spreading by immunizing a certain percentage of the population.

History

  1. The term herd immunity was first used in 1923 to refer to an entire population’s immunity, in reference to research examining disease mortality in mouse populations with varying degrees of immunity.
  2. Herd immunity was first recognized as a naturally occurring phenomenon in the 1930s 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.

Salient Points

  • Herd immunity is a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, whether through previous infections or vaccination, thereby providing a measure of protection for individuals who are not immune.
  • In a population in which a large proportion of individuals possess immunity, such people being unlikely to contribute to disease transmission, chains of infection are more likely to be disrupted, which either stops or slows the spread of disease.
  • The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual, helping to shield non-immune individuals from infection.
  • Some individuals cannot become immune due to medical reasons, such as an immunodeficiency or immunosuppression, and in this group herd immunity is a crucial method of protection.
  • Once a certain threshold has been reached, herd immunity gradually eliminates a disease from a population.
  • Herd immunity created via vaccination contributed to the eventual eradication of smallpox in 1977 and has contributed to the reduction of the frequencies of other diseases.
  • Mass vaccination to induce herd immunity has since become common and proved successful in preventing the spread of many infectious diseases.
Herd immunity does not apply to all diseases, just those that are contagious, meaning that they can be transmitted from one individual to another.

How does herd immunity work?

The scientific principle is that the presence of a large number of immune persons in the community, who will interrupt the transmission, provides indirect protection to those who are not immune.

  • To estimate the extent of spread and immunity, epidemiologists use a measure called the ‘basic reproductive number’ (R0). This indicates how many persons will be infected when exposed to a single case
  • An R0 of more than 1 indicates one person can spread the infection to multiple persons.
  • Scientific evidence shows that a person with measles can infect around 12-18 persons; and a person with influenza can infect around 1.2-4.5 persons, depending on the season. On the basis of the available evidence from China, and according to various experts, R0 of COVID-19 ranges between 2 and 3.

There are three ways in which an infection can spread in a community.

  1. Unvaccinated Group
    The first scenario looks at a community that is not immunized. When two infectious cases, both with an R0 value of 1, are introduced, there is a possibility of the entire community being infected, with a few exceptions.

Leads to

  1. Unsuccessful Herd Immunity
    In the second scenario, there may be some persons who have been immunised; and only these immunised persons will not be infected when at least two infectious cases are introduced in the community.
  1. Successful Herd Immunity
    The third scenario is when the majority of the community is immunised. So, when two infectious cases are introduced, the spread can take place only in exceptional cases, like in the elderly or other vulnerable persons. Even in such a situation, the immunised persons protect the non-immunised by acting as a barrier — which is herd immunity.

Leads to

Achievement of herd immunity

Individuals who are immune to a disease act as a barrier in the spread of disease, slowing or preventing the transmission of disease to others. An individual’s immunity can be acquired via a natural infection or through artificial means, such as vaccination. When a critical proportion of the population becomes immune, called the herd immunity threshold (HIT) or herd immunity level (HIL), the disease may no longer persist in the population, ceasing to be endemic.

“Herd immunity threshold”, is the number of immune individuals above which a disease may no longer circulate. The higher the R0, the higher the percentage of the population that has to be immunized to achieve herd immunity.

Polio has a threshold of 80% to 85%, while measles has 95%. With the current data for COVID-19, experts have estimated a threshold of over 60%. That means more than 60% of the population needs to develop immunity to reach the stage of herd immunity.

It depends on multiple factors:

  1. How effective the vaccine for a given disease is,
  2. How long-lasting immunity is from both vaccination and infection, and
  3. Which populations form critical links in transmission of the disease.

Effects of Herd Immunity

Protection of those without immunity

  • Some individuals either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated. Newborn infants are too young to receive many vaccines, either for safety reasons or because passive immunity renders the vaccine ineffective.
  • Individuals who are immune-deficient due to HIV/AIDS, lymphoma, leukemia, may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.
  • Vaccine contraindications may prevent certain individuals from becoming immune. In addition to not being immune, individuals in one of these groups may be at a greater risk of developing complications from infection because of their medical status, but they may still be protected if a large enough percentage of the population is immune.
  • High levels of immunity in one age group can create herd immunity for other age groups. Vaccinating adults against pertussis reduces pertussis incidence in infants too young to be vaccinated, who are at the greatest risk of complications from the disease.
  • Influenza (flu) is more severe in the elderly than in younger age groups, but influenza vaccines lack effectiveness in this demographic due to a waning of the immune system with age. The prioritization of school-age children for seasonal flu immunization, which is more effective than vaccinating the elderly, however, has been shown to create a certain degree of protection for the elderly.

Evolutionary pressure

Herd immunity itself acts as an evolutionary pressure on certain viruses, influencing viral evolution by encouraging the production of novel strains, in this case referred to as escape mutants that are able to “escape” from herd immunity and spread more easily.

  • The re-assortment of separate viral genome segments, or antigenic shift, which is more common when there are more strains in circulation, can also produce new serotypes.
  • When either of these occur, memory T cells no longer recognize the virus, so people are not immune to the dominant circulating strain.
  • For both influenza and norovirus, epidemics temporarily induce herd immunity until a new dominant strain emerges, causing successive waves of epidemics.
  • As this evolution poses a challenge to herd immunity, broadly neutralizing antibodies and “universal” vaccines that can provide protection beyond a specific serotype are in development.
Serotype
A serotype is a distinct variation within a species of bacteria or virus or among immune cells of different individuals.

Serotype replacement

Serotype replacement, or serotype shifting, 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
  • The possibility of future shifting remains, so further strategies to deal with this include expansion of VT coverage and the development of vaccines that use either killed whole-cells, which have more surface antigens, or proteins present in multiple serotypes.

Eradication of diseases

  • If herd immunity has been established and maintained in a population for a sufficient time, the disease is inevitably eliminated—no more endemic transmissions occur.
  • If elimination is achieved worldwide and the number of cases is permanently reduced to zero, then a disease can be declared eradicated.
  • The benefits of eradication include ending all morbidity and mortality caused by the disease, financial savings for individuals, health care providers, and governments, and enabling resources used to control the disease to be used elsewhere.
  • To date, two diseases have been eradicated using herd immunity and vaccination: rinderpest and smallpox.
  • Eradication efforts that rely on herd immunity are currently underway for Poliomyelitis, though civil unrest and distrust of modern medicine have made this difficult.
  • Mandatory vaccination may be beneficial to eradication efforts
Eradication can thus be considered the final effect or end-result of public health initiatives to control the spread of infectious disease.

Cost–benefit analysis

  • Herd immunity is often accounted for when conducting cost–benefit analyses of vaccination programs.
  • It is regarded as a positive externality of high levels of immunity, producing an additional benefit of disease reduction that would not occur had no herd immunity been generated in the population.
  • Therefore, herd immunity’s inclusion in cost–benefit analyses results in more favorable cost-effectiveness or cost–benefit ratios and an increase in the number of disease cases averted by vaccination.

Ways to Boost Herd Immunity

Vaccination

The primary way to boost levels of immunity in a population is through vaccination.

  • Well-developed vaccines provide protection in a far safer way than natural infections, as vaccines generally do not cause the diseases they protect against and severe adverse effects are significantly less common than complications from natural infections.
  • To achieve herd immunity through vaccination, vaccine manufacturers aim to produce vaccines with low failure rates and policy makers aim to encourage their use.
  • After the successful introduction and widespread use of a vaccine, sharp declines in the incidence of diseases it protects against can be observed, necessarily decreasing the number of hospitalizations and deaths caused by such diseases.

Passive immunity

Individual immunity can also be gained passively, in which antibodies to a pathogen are transferred from one individual to another.

  • This can occur naturally, whereby maternal antibodies, primarily immunoglobulin G antibodies, are transferred across the placenta and in colostrum to fetuses and newborns, or artificially, by which antibodies from the serum or plasma of an immune individual are injected into a susceptible person.
  • Protection generated from passive immunity is immediate but wanes over the course of weeks to months, so any contribution to herd immunity is temporary.

Issues with achieving Herd Immunity for Covid 19

It is very risky to seek herd immunity by allowing a large proportion of the population to get infected. Such a strategy at this stage, experts have underlined, would be based on many unknowns and variables.

  1. Much about the behavior of the pathogen is still unclear. There isn’t enough statistically significant data to estimate conclusively how many persons can get the virus from a single infected person.
  2. It can take months, or even longer, to build group immunity to COVID-19. During that time, the need is to protect people who are at greater risk; the numbers so far indicate that people above 55, especially those with co-morbidities like cardiovascular disease and hypertension, are the most vulnerable.
  3. While herd immunity may come about from a pandemic because the people who survive may develop immunity — they also may not — it is important to note that for COVID-19, we still don’t know whether one can become immune to the virus.
  4. Strategy to achieve herd immunity would put a huge burden on the healthcare system. Allowing the virus to pass through the population means a surge of patients, putting pressure on existing ICU and emergency beds.
  5. Epidemiologists stress “flattening the curve” — slowing the spread of an infection over a large population — and this cannot be achieved by allowing the virus to pass through the entire population.
  6. 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.
  7. Emerging or ongoing vaccine controversies and various reasons for opposing vaccination.

Free riding

Herd immunity is vulnerable to the free rider problem.

  • Individuals who lack immunity, particularly those who choose not to vaccinate, free ride off the herd immunity created by those who are immune.
  • As the number of free riders in a population increases, outbreaks of preventable diseases become more common and more severe due to loss of herd immunity.
  • Individuals may choose to free ride for a variety of reasons, including
    • Perceived ineffectiveness of a vaccine,
    • Believing that the risks associated with vaccines are greater than those associated with infection,
    • Mistrust of vaccines or public health officials,
    • Social norms or peer pressure and
    • Religious beliefs.
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