What is Herd Immunity?

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Infectious diseases caused by microorganisms such as bacteria and viruses can spread widely within a community. In particular, densely populated areas like big cities allow rapid disease transmission, because people are frequently in close proximity. When a certain proportion of the community acquires immunity for a disease, then further spread is limited, because immune individuals usually cannot get re-infected or infect others. This is when we reach herd immunity for this particular disease.

There are two main ways to achieve herd immunity. The first way is for the pathogen to infect a large number of people, who then recover and become immune. The second way is through vaccination, which provides future immunity against the disease by stimulating the natural pathways that lead to it. Nowadays, many infectious diseases are either eliminated or controlled through vaccination, such as measles, chickenpox, and the seasonal flu. In fact, a vaccine is the safest way towards herd immunity, leading to less sick people and therefore a lower overall number of disease-related deaths and complications.

Depending on the size of the community, as well as the infectiousness of the disease, the percentage of people that must become immune before herd immunity is achieved can vary. For example, measles requires 93-95% vaccination rates, whereas polio requires around 80-86%. This is why timely vaccination of children at the recommended schedule is really important; skipping or delaying a dose does not only increase chances of disease susceptibility for the child, but also contributes to a decrease in herd immunity and therefore allows previously controlled diseases to reappear like the recent resurgence of the measles.

Herd immunity is highly important for our society, because it is an important tool to curtail or even eliminate infectious diseases. Furthermore, it protects some of the most vulnerable members in our communities from getting sick, such as the elderly, those who can’t have a vaccine for medical reasons, or the immunocompromised, as well as pregnant women and infants. For this reason, there is extensive research and funding dedicated towards the development of safe and effective vaccines.

More resources on herd immunity and vaccination: