Herd immunity, also known as community immunity, is a concept that has often been misunderstood during the COVID epidemic. Herd immunity occurs when a large part of a population is immune to a specific disease, such that the incidence of the disease is very low. As such, even non-immunized people have a much lower chance of getting the disease. The percentage of the population is required to have herd immunity is dependent largely on the specific pathogen. For highly transmissible pathogens such as measles or pertussis, around 95% of the population must be vaccinated to prevent disease outbreaks. However, for other less transmissible pathogens, it may only be 80%. For some diseases such as influenza, only 40% is helpful in reducing the incidence, although that does not constitute herd immunity.
Vaccine effectiveness also depends on the type of pathogen. Some vaccines prevent a person from getting infected. However, the COVID vaccine has thus far only prevented severe disease and is not very effective at preventing infection.
There is a downside to herd immunity—if the herd immunity is very high, the incidence of the pathogen in the population is so low that a person does not get naturally "boosted" by encountering it from time to time. This is one reason why boosters are needed for many vaccines.
