Vaccines train and prepare the body’s natural defenses, the immune system, to recognize and fight viruses and bacteria. If the body were exposed to these pathogens after vaccination, it would be prepared to destroy them quickly, thereby preventing disease.

When a person is vaccinated against a disease, their risk of infection is also reduced, making them much less likely to pass the virus or bacteria to other people. The more people in a community that are vaccinated, the fewer vulnerable people there will be, thereby reducing the chances that an infected person will transmit the pathogen to others. This reduces the chances of a pathogen circulating within the community and protecting those who cannot get the corresponding vaccine from a disease, (due to clinical conditions such as allergies or age).

The term “herd immunity” refers to the indirect protection against an infectious disease that is achieved when a population becomes immune, either as a result of vaccination or of having previously presented the infection. Herd immunity does not imply that people who have not been vaccinated or who have not developed the infection are immune. In contrast, herd immunity occurs when people who are not immune but live in a community where the immunity ratio is high have a lower risk of contracting a disease compared to people who are not immune and live in a community where the immunity ratio is low.

In communities where the immunity ratio is high, people who are not immune have a lower risk of contracting the disease compared to the risk they would normally have, but that reduction in risk derives from the immunity of people of the community in which they live (i.e. herd immunity) not from the fact that they are immune. Even after herd immunity is first achieved and unvaccinated people are found to have a lower risk of developing the disease, the risk can be further lowered by increasing vaccination coverage. In cases where vaccination coverage is very extensive, it is possible that people who are not immune may have a risk of contracting the disease similar to that of those who are truly immune.

Herd immunity

When a person is vaccinated against a disease, it is very likely that they are protected against that disease. However, not all people can be vaccinated. Some, with pre-existing diseases that weaken their immune systems, (for example, cancer or HIV), or those with severe allergies to some components of vaccines, may not be able to receive certain vaccines. These people may be protected if they live among other people who are vaccinated. When many people in a community are vaccinated, the circulation of the pathogen is difficult because most people are immunized. Therefore, the more people who are vaccinated, the less likely it is that a person who cannot protect himself with vaccines is at risk of being exposed to pathogens. This is called herd immunity.

This is very important not only for people who cannot get vaccinated, but also for those who may be more susceptible to the diseases we vaccinate against. No single vaccine provides 100% protection, and herd immunity does not offer full protection to those who cannot safely be vaccinated. However, herd immunity offers these people a substantial degree of protection, thanks to the fact that the people around them are vaccinated

Vaccination not only protects you, but also people in the community who cannot be vaccinated. If you can get vaccinated, please do so.

Source: WHO