Some vaccination strategies are specifically designed to achieve herd immunity. Vaccination against measles, for example, aims to immunize enough people, so that the virus is no longer able to circulate in the population.  

In theory, herd immunity can also be reached naturally if an epidemic is allowed to spread in the population until most people catch the disease and develop immunity after infection or die. However, this only works for diseases that produce long-lasting immunity and whose pathogens do not change rapidly. For infections where immunity fades quickly – like some common cold viruses – or where the germ frequently mutates, as with seasonal flu, herd immunity through natural infection is not achievable. 

During the COVID-19 pandemic, some suggested that society should simply allow the virus to circulate freely and that herd immunity would develop. However, such a strategy would have come at immense human and social cost, including the loss of many lives.  

Herd immunity is never permanent. New susceptible individuals constantly enter the population through birth, while immunity may weaken over time. As a result, vaccination and other preventive measures remain necessary to prevent new epidemics reappear. Epidemiologists use mathematical models to study how births, deaths, vaccination and other prevention measures affect epidemics and how to reach herd immunity.  

Measles offers a classic example of these processes. Before widespread vaccination, the interplay between births and disease-acquired immunity after infection produced cyclic epidemics. Vaccination made it possible to stop virus circulation in a population and protect the most fragile individuals of society.