Before talking about the vaccine, we first need to understand the process of how harmful organisms behave in the body. Harmful organisms that can cause diseases are called pathogens, which can be bacteria, viruses, parasites, or even fungi. Each pathogen has a specific part that is unique to the organism or the disease it causes. Certain molecules from the pathogen need to be in the body to trigger an immune response. These molecules are called antigens.

When an immune response is triggered, the body starts producing antibodies. They are like soldiers in our body’s defense system. Each antibody can recognize one antigen and neutralize it. It can take time for our body to produce the required number of antibodies to destroy the pathogen and stop whatever disease it causes. Once it is stopped, our immune system creates antibody memory cells to remember the pathogen in the future. If the same pathogen happens to be in our body again, the antibodies will react faster and more effectively than the first time because the memory cells are ready to release antibodies at any moment.

Now, we are ready to talk about vaccines. A vaccine contains a weakened or harmless antigen that triggers an immune response. This version of an antigen is non-dangerous and will not cause disease within the body. Having this weakened fragment within the body is enough to learn how to build the specific antibody for this antigen. So, if a harmful organism appears with this antigen, our immune system will know how to defeat it and immediately attack it to destroy the sickness.

Once a person is vaccinated, they are safe from a particular disease. However, not all people can have a vaccine. People with underlying health conditions, such as cancer or HIV, or allergies to specific components in a vaccine cannot be vaccinated. Despite that, they are still protected from getting a disease if they live among vaccinated people. They are immune to the pathogen and therefore cannot spread it. This is called herd immunity.

Of course, a vaccine does not guarantee absolute protection; however, it significantly decreases the risk of the disease outbreak.