If most of the population is immune to an infectious disease, this provides indirect protection for those who are not immune to the disease, or herd immunity (also called herd protection).
For eg, if 80% of a population is immune to an infection, four out of five people will not get infected (and will not transmit the disease further) if they meet anyone with the disease. The transmission of infectious diseases is kept under control in this manner. Usually, 50 percent to 90 percent of a population needs immunity to gain herd immunity, based on how infectious an illness is.
Herd immunity, also referred to as 'population immunity,' is the indirect defense against an infectious disease that exists when either vaccine or immunity acquired by prior infection is resistant to a population. That encourages the achievement of 'herd immunity' by vaccines, not by encouraging some portion of the population to transmit a disease, since this will result in needless cases and deaths.
How have we achieved herd immunity for other infectious diseases?
Examples of contagious disorders that were once very prevalent but are still uncommon in the U.S. are measles, mumps, polio, and chickenpox, as vaccinations helped to create herd immunity. In populations with reduced vaccination coverage, we often see outbreaks of vaccine-preventable diseases because they do not have protection from herds. (An example is the 2019 measles outbreak at Disneyland.
Other viruses (such as the flu) mutate over time, but antibodies from a previous infection only provide protection for a limited period of time. This is less than a year because of the flu. If, like other coronaviruses that actually infect humans, SARS-CoV-2, the virus that causes COVID-19, we should assume that people who get infected will be immune for months to years, but certainly not their whole lives.
Herd Immunity and COVID - 19
Attempts to achieve 'herd immunity' are technically dubious and immoral by introducing individuals to a virus. It can lead to needless diseases, pain and death if COVID-19 travels across communities of any age or health status.
In most nations, the vast majority of individuals remain vulnerable to this infection. Seroprevalence studies show that COVID-19 has affected fewer than 10% of the population of most countries.
We are also talking about COVID-19 immunity. Most people who are infected with COVID-19 produce an immune response within the first few weeks, but we don't know how good or permanent the immune response is, or how it varies with different people.
It would not be possible to determine how much of a population is immune until we properly understand COVID-19 immunity and how long the immunity lasts, let alone making potential predictions.