By Harit Phowatthanasathien
Documented for the first time almost a century ago, herd immunity has been the source of controversy in the 2020 world filled with turmoil and disarray. Early research of the herd immunity phenomenon cropped up in the 1930s in communities ravaged by measles. This subtle naturally occurring phenomenon was noticed as a consequence of mass measles vaccinations, leading to increased research in epidemiology (Jones and Helmreich, 2020). Both worldwide vaccinations and herd immunity being the leading forces in the fight towards smallpox eradication, herd immunity is synonymous with pandemics and Covid-19 is no different. So, what is herd immunity?
Herd immunity is defined as the resistance to the spread of a contagion within a population with a high proportion of individuals that are sufficiently immune to the disease. The interpretation of the word “high” in this definition is tied to the disease in question. The Rubella outbreaks of the mid-1960s required herd immunity thresholds of 83% to 93%, while the Measles outbreaks required a higher threshold range of 90%-95% immunity. But what determines the required threshold for these different diseases? The relationship between the basic reproduction number (R-Nought) has been a simple consistent factor in setting a herd immunity threshold. The relationship takes the shape of a logarithmic graph with higher R-Nought values requiring higher herd immunity thresholds (Fine, Eames, and Heymann, 2011). This follows the logic that with increasingly infectious diseases, a large portion of the population must be immune before the herd immunity can prove effective for those who are unable to protect themselves. R-Nought however must be viewed as a rough indicator of an effective threshold because other factors including, seroconversion, imperfect immunity, heterogeneous demographics, and geopolitical factors can heavily influence threshold values (Fine, Eames, and Heymann, 2011).
In addition to the multitude of factors in determining threshold levels, herd immunity can only be effectively practiced under specific conditions; the pandemic must originate from a single host species that spreads via direct contact and infection induces a solid immunity (Fox, 1998). The former implies herd immunity’s ineffectiveness when dealing with a disease with high mutation rates because antibody specificity to fight off one variant of a disease does not extend to its other variants. Diseases in which herd immunity is practical are those which are spread via direct contact, as opposed to those via animals and common surfaces. The latter point that infection builds an adequate immunity is a vital distinction to ensure that outbreaks are caused by contact rather than reinfection. Without this assumption, herd immunity is unreliable, and its implementation could lead to unnecessary lives to be lost.
One’s immunity may be taken for granted; however, it is the only system protecting you from a fatal blow by the common cold. Unfortunately for a portion of the population, they do not have the luxury of an effective immune system and thus heavily depend on herd immunity as a barrier. Newborn babies, the elderly, people with HIV, patients on chemotherapy, or the extremely ill at hospitals, all rely on herd immunity to continue a healthy life (Herd immunity Vaccine Knowledge, 2019). According to the 2011 US census, an estimated 24.16% of individuals are categorized as vulnerable, leaving the remaining 71.84% of the population to serve as the herd immunity shield (Delaney, 2020). It is scary to think that this 71% maximum immune proportion of the population is far from some of history’s deadliest diseases which required 80% to 90% threshold levels, Covid-19 potentially being one of them (Rogers and Health, 2020).
This begs the question of how herd immunity in 2020 can be achieved. Since the Sars-Cov-2 (Covid-19) pandemic aligns with the aforementioned herd immunity requirements – originating from a single host species and spreading via direct contact – herd immunity becomes a viable long-term solution. Mass vaccinations and natural infections are two methodologies to achieve herd immunity (Herd immunity and COVID-19 (coronavirus): What you need to know, 2020). Mass vaccinations are the ideal approach, considering the rapid rate of infection and the scope of affected individuals. Vaccinations allow more people to build immunity without getting sick, allowing for herd immunity to be built in a faster period. However, the major drawback of the vaccination method is the lasting effect of immunity. Its protection can subside overtime, requiring revaccinations and booster shots, which some individuals might fail to get.
Additionally, religions, skeptical, and fearful beliefs may cause groups of people to object to vaccinations, stalling herd immunity. The latter option to achieve herd immunity is natural infection, which is waiting for the natural rate of infection to continue unimpeded until enough recovered individuals can provide herd immunity for the entire population. It goes without saying that the natural selection approach will cause an extraordinary amount of lives to be lost. At the current state of the pandemic, it is difficult to deduce an exact death rate percentage, however, collected data suggests it to be 10 times higher than the flu, illustrating the sheer risk in pursuing herd immunity via natural infection (Herd immunity and COVID-19 (coronavirus): What you need to know, 2020). Furthermore, the concept does not fully guarantee that enough of the population will recover to reach herd immunity thresholds.
It is safe to say there is much more to learn about Covid-19, but herd immunity might be the long-term goal to effectively reduce its global effect. In these turbulent times, herd immunity could be perceived as a saving grace, however, it must be viewed from an objective lens including its risks, drawbacks, and viability, in order to properly plan and progress towards it.
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