 Well, it is believed by the experts, scientists and researchers in the field of infectious diseases that if herd immunity gets established and retained in a population for enough duration of time, the disease is predictably reduced if not completely eradicated. In other words, herd immunity puts brakes on the community transmissions. Today, I, Hemant Batra, the public policy speaker, shall be talking about the herd immunity. So keep watching this video ahead. We have been hearing a lot about herd immunity as the lockdown is loosening up in India and elsewhere in the world and literally coming to an end. What is herd immunity? You must be pondering. Let me take you through this concept, which is also known as community immunity, population immunity or social immunity. It is a kind of incidental shield or protection from infectious diseases, including epidemics or pandemics, which happen when a huge proportion of a population becomes immune to an infection. That is resistant and protected to an infection. This immunity happens not by some miracle or intervention of some supernatural forces. It happens naturally, biologically, either through immunization, that is vaccination or earlier infections, which build a kind of shield around you, around those people who are not immune. In a population in which a substantial sizeable percentage of people acquire immunity, those people obviously are no more the cause of disease transmission. That's what herd immunity is all about. This leads to the breaking of chains of infection, leading to the slowing down of the spread of disease. The greater the number of immune persons in a population, the lesser the likelihood that non-immune or vulnerable individuals will come into contact with an infectious individual. This helps in protecting non-immune individuals from disease and infections. Herd immunity was documented as a biologically-happening phenomenon in the 1930s, when it was noticed that after a substantial number of children had become immune to measles. The number of new infections provisionally declined, including among susceptible children. As I mentioned earlier in my talk, that people can become immune by recovering from a previous infection or through vaccination. However, there are certain exceptions to herd immunity, to this principle of herd immunity or this biological model of herd immunity. Some people have serious medical conditions, for instance, and their immunity is low, so they may become sacrificial victims in this process. Well, as soon as a particular level or threshold goal gets achieved, that number gets achieved, herd immunity steadily reduces and even eradicates disease from a population. As stated in the Oxford textbook of infectious disease control, this elimination, beg your pardon, if achieved worldwide may result in the permanent reduction in the number of infections to zero, called eradication. For instance, 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 as well. Now the illustration which is in front of you now shows that an eruption of infection in a community in which a few people are infected, shown in red, as you can see, and the rest are healthy, but unimmunized. They are shown in blue, as you will notice. The illness spreads freely through the population in this illustration which is before you. The second image now which you see illustrates a population where a small number have been immunized. This is shown in yellow, as you will notice. Those not immunized become infected while those immunized do not. Now see the third image. A large proportion of the population has been immunized, as you will see. This prevents the illness from spreading significantly, including to unimmunized people. Now you will notice that in the first two examples or illustrations or images or graphics, most healthy unimmunized people become infected. Whereas in the bottom example, which is the third example, only one fourth of the healthy unimmunized people become infected. The model of science of herd immunity does not apply to all viruses and infections. Basically it applies to only those that are communicable and contagious, meaning that they can be transmitted from one individual to another. Tetanus, for instance, is infectious but not contagious. So herd immunity does not apply in such cases. The herd immunity threshold for this COVID-19 is still unclear. But many epidemiologists consider it will be achieved when between 60% and 80% of the population has been infected and develops resistance. A lower level of immunity in the population can slow down the spread of a disease slightly. But the herd immunity number represents the point where infections are significantly less likely to turn into large occurrences of infections. That's all for today. See you next time.