What is the mechanism by which vaccines protect against diseases?

Vaccines protect against diseases by stimulating the immune system to recognize specific pathogens. When a vaccine is administered, it introduces a harmless component of the pathogen, such as inactivated or attenuated viruses, or pieces of the pathogen like proteins or sugars. This presence prompts the immune system to create a response, which includes the production of antibodies and the activation of immune cells that can recognize and remember the pathogen.

If the vaccinated individual later encounters the actual pathogen, their immune system can quickly respond because it has already learned to recognize and attack it, often preventing the development of the disease or reducing its severity. The process does not involve directly killing pathogens in the bloodstream, providing immediate immunity, or modifying pathogens. Instead, it builds a memory within the immune system, enabling a faster and stronger response upon subsequent exposures to the disease-causing agents. This is the fundamental principle underlying vaccination and its effectiveness in controlling infectious diseases.