A prophylactic vaccine is a preparation administered to a healthy person to prevent a future infection by training the body’s defenses to combat a specific pathogen. By introducing a harmless version or component of the pathogen, the vaccine prepares the immune system for a potential encounter. This process allows the body to build a defense without experiencing the illness itself. The result is an immune system ready to mount a rapid response if the actual pathogen invades.
The Mechanism of Immune Prevention
When a vaccine is administered, it introduces molecules known as antigens to the body. These antigens are specific parts of a pathogen, like a surface protein, that the immune system recognizes as foreign. Specialized antigen-presenting cells (APCs) engulf these antigens and display them to other immune cells. This action initiates a primary immune response.
This initial activation engages two main types of lymphocytes: B-cells and T-cells. Helper T-cells become activated and, in turn, help activate B-cells. The activated B-cells then multiply and differentiate into plasma cells, which are responsible for producing antibodies. These antibodies are proteins tailored to bind specifically to the introduced antigen, marking it for destruction. Concurrently, cytotoxic T-cells are activated to identify and eliminate any body cells that might become infected with the pathogen.
A primary outcome of this response is the creation of immunological memory. A subset of the activated B-cells and T-cells develop into long-lasting memory cells. These cells persist in the body for months, years, or even a lifetime, retaining the “blueprint” for recognizing that specific pathogen.
Should the vaccinated individual later be exposed to the actual pathogen, these memory cells trigger a secondary immune response. This response is substantially faster and more powerful than the primary one. Memory B-cells quickly generate a large volume of antibodies, while memory T-cells rapidly multiply to destroy infected cells. This swift action often neutralizes the pathogen before it can cause noticeable illness, preventing or significantly reducing the severity of the disease.
Key Prophylactic Vaccine Technologies
Vaccine technology presents antigens to the immune system in various ways. Live-attenuated vaccines use a weakened form of the living virus or bacterium. This version of the pathogen can still replicate but does so poorly, so it does not cause disease in people with healthy immune systems. Because it closely mimics a natural infection, it generates a robust and long-lasting immune response. The measles, mumps, and rubella (MMR) vaccine is an example.
Inactivated vaccines use a version of the pathogen that has been killed with chemicals, heat, or radiation. Since the pathogen is dead, it cannot replicate, making these vaccines suitable for a wide range of individuals. The immune response they generate is sometimes less intense than that from live vaccines, occasionally requiring multiple doses or boosters to maintain immunity. The inactivated polio vaccine is an example of this technology.
Other platforms use only specific pieces of a pathogen, which avoids introducing the entire organism. Subunit, recombinant, and polysaccharide vaccines use a specific protein (subunit) or a sugar chain (polysaccharide) from the pathogen. Recombinant technology allows scientists to produce large quantities of these specific antigens for use in a vaccine. The human papillomavirus (HPV) and hepatitis B vaccines fall into this category.
Recent advancements include mRNA and viral vector vaccines, which use the body’s cellular machinery to produce the antigen. An mRNA vaccine delivers genetic material (messenger RNA) that instructs cells to build a specific pathogen protein. A viral vector vaccine uses a modified, harmless virus to carry genetic instructions into the cells. In both cases, the body produces the antigen internally, which is then presented to the immune system. The Pfizer-BioNTech and Moderna COVID-19 vaccines are examples of mRNA technology, while the Johnson & Johnson COVID-19 vaccine uses a viral vector.
The Therapeutic Vaccine Contrast
While prophylactic vaccines are for prevention, therapeutic vaccines are used for treatment. They are administered to individuals already sick with a disease, such as cancer or a chronic infection. The goal is to stimulate or redirect the patient’s immune system to attack existing diseased cells, retraining it to fight cells it previously ignored.
The mechanism of a therapeutic vaccine involves teaching the immune system to identify markers on diseased cells. For example, therapeutic cancer vaccines help immune cells recognize and destroy tumor cells. These vaccines introduce antigens associated with the cancer, prompting a targeted attack against the malignant cells.
The fundamental difference is their purpose. Prophylactic vaccines are given to healthy people to build immunological memory against a future pathogen. Therapeutic vaccines are a form of immunotherapy used to amplify the immune response against an already established disease.