How Do Vaccines Work?

Vaccines are biological substances used to stimulate antibody production and thus achieve immunity against disease, which are administered to the body in various ways.

Vaccines are administered to healthy people and people at-risk in order to protect both individuals and the society from diseases and their related complications by establishing herd immunity. With their biological content rendered harmless, vaccines help the body identify and develop an immune response to previously un-encountered microbes and/or toxins. As a result, when the body encounters the infectious agent, it activates the immune response previously developed by the vaccine to fight the agent. This mechanism protects the person from infection and makes him/her immune to that disease without contracting the disease. The immunization attained by vaccination usually stays in the body for life and when the body encounters the same disease agent again, it fights to render it ineffective. However, some vaccines may require booster vaccinations to ensure life long immunity. 

Immunization is one of the most important public health interventions used against vaccine-preventable diseases and deaths. Infectious diseases and epidemics spread with greater difficulty in societies with efficient vaccination campaigns.

Microbes are everywhere, both in our environment and in our body. When a susceptible person encounters a harmful organism, this can lead to illness and death.

There are many ways the body can defend itself against microbes. Skin, mucus, and cilia (moving microscopic hairs that sweep microbes away from the lungs) form the first layer of our body’s defense as physical barriers preventing germs from entering the body.

Normally, when a microbe infects the body, our body's defense, the immune system, is triggered. The immune system attacks and destroys the microbe.

Microbes can be bacteria, viruses, parasites or fungi that can cause disease in the body. “Antigen” is the subcomponent of a microbe that causes antibody formation and allows recognition of the microbe by our immune system. One can think of the antibodies produced in response to the antigen of the microbe as soldiers in our body's defense system. Every antibody or soldier in our immune system is trained to recognize a particular antigen. When the human body is exposed to a microbe for the first time it takes time for the immune system to respond and produce the specific antibodies. During this period, the person tends to get sick. Once antigen-specific antibodies are produced, they work with the rest of the immune system to destroy the microbe and stop the disease.

Antibodies against a kind of microbe usually do not protect against another microbe; they are microbe-specific. The body follows up the production of antibodies in its initial response to an antigen, with the generation of antibody-producing memory cells. If the body is exposed to the same microbe multiple times, the antibody response gets quicker and more effective than the first time, because more and more memory cells are ready to pump out antibodies against the microbe.

This means that if the person is exposed to the microbe in the future, the immune system can protect against the disease by reacting instantly.

Vaccines usually contain weakened or inactivated parts of a specific organism (antigen) that stimulate the immune system in the body. The weakened version of the microbe or the inactivated antigen do not cause disease in the vaccinated person. It instead will trigger the immune system to recognize the antigen of the microbe, imitating the first response to the microbe in an unvaccinated person and ensuring immunization to further exposure of the microbe.

Some vaccines require multiple doses administered over a period of weeks or months. This may be necessary to allow for the development of memory cells and the production of longer-lasting antibodies. In this way, the body is trained to fight a specific disease-causing organism by building up its memory to fight quickly when exposed again in the future.