A subunit vaccine trains the immune system by presenting only specific, purified components of a pathogen, such as proteins or sugars. These components are selected because they are recognized by the body’s defenses and can trigger a protective response. Instead of exposing the immune system to the entire disease-causing organism, this approach provides a targeted feature of the pathogen. This allows the immune system to learn how to identify and fight off a threat without encountering the whole pathogen.
Creating Subunit Vaccines
Producing subunit vaccines often involves recombinant DNA technology. Scientists first identify a particular antigen, a specific protein or sugar molecule on the pathogen’s surface, known to provoke a strong immune reaction. The gene that codes for this chosen antigen is then isolated from the pathogen’s DNA.
This gene is then inserted into a harmless organism, such as yeast or bacteria. These organisms act as biological “factories,” producing large quantities of the target antigen. The process typically involves growing these modified organisms in large fermentation tanks. The antigen is then extracted, purified, and formulated into the final vaccine.
Mechanism of Immune Stimulation
Subunit vaccines are inherently safe because they contain only pathogen fragments, not the entire organism, so they cannot cause the disease. Once injected, these purified subunits are recognized by antigen-presenting cells like macrophages and dendritic cells. These cells engulf and process the subunits, breaking them down into smaller pieces.
The processed pieces of the antigen are then displayed on the surface of these antigen-presenting cells. This presentation alerts other immune cells, specifically T-cells and B-cells, activating a targeted immune response. Because the purified subunit alone might not be sufficient to fully alert the immune system, adjuvants are often added to the vaccine formulation. Adjuvants amplify the immune response, encouraging the body to produce more antibodies and memory cells for robust, lasting protection.
Types and Examples of Subunit Vaccines
Subunit vaccines encompass several categories, each utilizing different components of pathogens to stimulate immunity.
Protein Subunit Vaccines
Protein subunit vaccines use purified proteins from the pathogen, triggering an immune response against these specific viral or bacterial structures. Examples include the Hepatitis B vaccine, which uses a surface antigen produced in yeast, the acellular pertussis vaccine, which targets specific proteins from the bacterium causing whooping cough, and the Novavax COVID-19 vaccine, presenting the SARS-CoV-2 spike protein.
Polysaccharide Vaccines
Polysaccharide vaccines employ sugar chains, or polysaccharides, found on the outer coating of certain bacteria. These vaccines prime the immune system to recognize and respond to these bacterial capsular polysaccharides. An example is the pneumococcal polysaccharide vaccine, which helps protect against Streptococcus pneumoniae.
Conjugate Vaccines
Conjugate vaccines chemically link a polysaccharide to a carrier protein. This conjugation enhances the immune response, particularly in infants and young children whose immune systems may not respond strongly to polysaccharides alone. Examples include the Haemophilus influenzae type b (Hib) vaccine, protecting against severe infections like meningitis, and the pneumococcal conjugate vaccine, both of which have reduced disease incidence in children.
Subunit Vaccines in the Broader Vaccine Landscape
Subunit vaccines offer a high safety profile due to their precise composition. Unlike whole-pathogen vaccines, which use the entire (weakened or inactivated) germ, subunit vaccines contain only isolated pieces. This means subunit vaccines cannot replicate or cause disease, making them suitable for a broader range of individuals, including those with compromised immune systems.
Newer vaccine platforms, such as mRNA and viral vector vaccines, differ in their antigen presentation. While subunit vaccines directly deliver the pre-made antigen, mRNA and viral vector vaccines provide the body with genetic instructions. These instructions enable the body’s own cells to produce the antigen, which then triggers an immune response. Subunit vaccines can be understood as “delivering the protein,” whereas mRNA and viral vector vaccines are “delivering the recipe” for the body to create the protein itself.