During the current pandemic, we often hear the term herd immunity. What is Herd Immunity, you ask? Herd immunity is “resistance against the contagious/infectious disease in a mass population, which can be achieved if a large number of people have achieved the individual immunity for that specific disease.”
It has been reported that, in infectious diseases, if a certain percentage of people in a population acquire immunity to resist the disease in their body, herd immunity is developed within a period of time that leads to a fall in number of cases. Usually, this is achieved by vaccination. However, the science of herd immunity depends on one important factor, i.e., “Basic Reproduction Number R0” which is, expected number of cases generated by one case, where every individual from that population is susceptible to infection. Using this number R0, we can determine how herd immunity could be reached.
(R0-1)/R0 is the fraction of individuals that need to be infected to achieve herd immunity.
Let us suppose R0 = 2 which means, one affected person would infect two other people. In that case, if we vaccinate half of the population (one out of every two people), then the number of cases is expected to decrease gradually. Similarly, for R0 = 3, we need to vaccinate 2/3rd of the population. But the Basic Reproduction Number for a highly infectious disease is very high. For example, in case of measles, R0 = 18. In that case, we need to vaccinate every 17 out of 18 people or 94% of the population.
This works, since a greater fraction of people from a population are immune to disease, viruses can’t find new hosts to infect them and the spread tends to minimize. And this, in turn, gradually leads to eradication of the disease.
Vaccination is not the only way to achieve herd immunity; it can also be achieved when a sufficient number of people in the population have recovered from a disease and have developed antibodies against any future infection. A survey showed that those who survived 1918 Influenza pandemic were later immune to Influenza A – subtype of Influenza. These evidences lead to hypothesize that for a highly infectious disease, a large number of people need to be infected and recovered from disease to achieve herd immunity this way. For the case of COVID-19, it is not known for sure if this virus makes a person immune to future infection.
Evolution also plays an important role here. Herd immunity in a community leads to evolutionary development in some viruses. When herd immunity is achieved, some viruses survive from it by antigenic drift. So, what happens in antigenic drift? To understand this, we need to know about how human immune system works. Human immune system has different kinds of cells, known as “antibodies” which resist “antigens”. Antigen is the surface structure of the body of virus which connects with human body cells. When a new virus enters our body, antibodies interact with virus antigens, read it and resist the virus from infecting the whole body. And hence, the human body achieves immunity against that specific disease.
But viral genomes are constantly mutating. Genome contains all the information about an organism. So, a genome mutation leads to change in structure of antigens. It is as a result of this change in structure of antigens, the human immune system no longer remembers that antigen, and hence, has to read it again.
Another evolutionary change in viral genome is by antigenic shift. In this process, viral genome completely changes. This leads to complete shift in the structure of that antigen.
Because of antigenic drift and antigenic shift, our immune system, which previously read the antigen and resisted its infection, can no longer remember the antigen structure. And by the time our antibodies read antigen structure, virus may already have started reproduction in our body. If this occurs, it is hard to achieve herd immunity. It is because of this reason that the second waves have been reported during pandemics in the past, and this makes it more difficult to control pandemics.
- Microbiology Society video:
- Wikipedia article – Herd Immunity