The criterion of truth is that it works even if nobody is prepared to acknowledge itLudwig Von Mises
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‘Herd immunity’ is an epidemiological concept with a proper mathematical structure. During the COVID-19 pandemic, it was over-used in mainstream and social media gossiping that it has become a meaningless and unworkable term.
Herd immunity threshold is a descriptive concept
‘Herd immunity threshold’ is a description of a particular state. It is a state in which the spread of infection in the community slows down from an epidemic rate to become an endemic disease due to development of infection specific immunity among a proportion of individuals in the community.
The term ‘epidemic’ describes a phenomenon of increasing infection rate in a community. During epidemic of an infection, each infected person spreads the disease to more than 1 person. In such a situation the disease will gradually increase in number with time. Mathematically, if this is plotted against time it will have an exponential curve. However this will not continue forever. It will eventually starts to slow down and then achieve a state of equilibrium. The whole thing is like a chemical reaction between two reactants, the process continues until they reach a state of equilibrium. Knowledge about this reaction can potentially shift the final state of equilibrium in our favor.
What slows down the spread of the infection in a community?
As the pathogens spreads through the community, individuals in the community would get sick and/or develop immunity to the infection. As the number of people who have developed immunity increases, the pace of spread will start to drop. Eventually this interaction between man and virus comes to a state of equilibrium. In this state infected people on an average infects one or less than one person and such an infection is called an endemic infection. In this situation, the disease is still present in the community in few individuals but it is not spreading at an exponential rate.
This state of equilibrium in which the epidemic disease becomes endemic happens when the herd immunity threshold is achieved. A state in which a majority of the populace have developed immunity against the virus. This is the natural course of events that will happen if we do not have knowledge regarding mechanisms that influences the balance of this equilibrium. In this situation, neither the virus nor the humans are eliminated. However during this interaction, depending on the type of the virus, there is a risk for serious complications among the infected individuals. People could die if this process is allowed to progress in its course.
What are the mechanisms that could change the course of these events and influence the final equilibrium state (herd immunity)?
Antiviral – If there is an antiviral molecule then such a substance would treat the infection and prevent the pathogen from causing complications or death. Even in this situation, the medicine would not affect the rate of spread of the infection or the number of people who is going to be infected during an epidemic spread. It can only help us to treat the infection once it has happened. And also this medicine will not change the time at which the equilibrium point will happen(transition from epidemic to endemic). Thus, if we have the knowledge to make an effective antiviral when a new virus is spreading through a naive population, then this medicine would have changed the following events in the natural course of things
- It will reduce the number of deaths at the end of the epidemic
- It will reduce the number of hospitalizations at the end of the epidemic
- No significant effect on the time point at which this epidemic will end
- No significant effect on the number of people who would have gotten infected by the time the epidemic ends
Vaccine – Vaccination works by injecting a portion of the virus particles inside our body. Such particles don’t cause infection or disease but it will stimulate our immunity similar to a real infection. The body would respond as if there was an infection and mount antibodies against it. If this injection is administered to a population when a new virus enters the community, then this vaccine would have changed the following events in the natural course of things
- It will reduce the number of people who would get infected when the epidemic ends. This is because a majority of them will already have antibodies even before the virus could infect them. In this way, vaccination can shift the center point of equilibrium in favor of the humans.
- It will affect the time point when an epidemic will end. Vaccination will shorten the duration of the epidemic by making very little people available for the virus to infect.
- There will be less deaths and less hospitalizations
What is the role of social distancing, lock down, case isolation and contact tracing methods?
Pathogens like coronavirus that are shed in human oral or respiratory secretions can spread easily due to community living. If we were to live a solitary life in the forest with only rare contacts with other human beings then such a pathogen will not be able to establish an epidemic in humans. The success of these respiratory viruses is due to our social life. Community with dense population and networks of human interaction will have a high rate of transmission. In such a situation a higher proportion of the community members must carry immunity to break the spread of the virus. When interventions like social distancing, universal masking and respiratory hygiene are implemented, the transmissibility of the pathogen drops. The following changes will happen in the natural course of events
1. It will shift the equilibrium in our favor and reduce the number of people who would get infected when the epidemic ends.
2. It will slow down the rate of infection and could also end the epidemic sooner by reaching a point where an infected individual infects less than one other person due to heightened respiratory hygiene among the community.
3. However these interventions must be followed as long as the pathogen is present in the human population. Once the strategies are relaxed, the equilibrium will shift towards the original point if the reaction were to happen at a ‘natural’ rate.