This type of vaccine contains only a small amount
of the antigenic portion of the of the pathogen needed to create an immune
response in order to provide protection. They are becoming the most popular
design for modern vaccines because they do not contain any live components of
the pathogens which means anyone is able to receive the vaccine (2). Therefore,
they are considered safer than live attenuated vaccines (3). This also makes
them easier to produce (1).
However, they do have some downsides, since they
do not contain any live components this the body sometimes will not to create a
proper immune response because the vaccine fails to stimulate the immune system
(2, 3). So the proper combination of antigens with the correct properties to
stimulate the immune system is needed. There is no guarantee of lasting
immunity so boosters are needed (3). Therefore, adjuvants are added to these
vaccines in order to help the vaccine to elicit a strong protective immune
response to induce long term immunity (1). Subunit Vaccines can also be further
categorized into Protein-based, polysaccharide, and conjugate, toxoid, virus-like
particles (VLPs), and nanoparticles (1).
In 1980’s with the help of genetic engineering
protein-based vaccines were developed using recombinant DNA technology allowing
DNA from two or more sources to be combined. The first protein-based vaccine or
protein based vaccine to be develop is the hepatitis B vaccine that uses
antigens from a hepatitis B virus protein that was produced by yeast cells to
code for that protein (1). They function
by also presenting the antigen to the immune system without any live components
by using a specific isolated protein of the pathogen. However, a downside is
that if they are denatured they could bind to different antibodies the protein
pathogen (3). Another vaccine commonly made with this method is acellular
pertussis (aP). It has inactivated pertussis toxin (protein) and could have one
or more other bacterial components, of course the toxin is detoxified so it is
does not cause harm (3).
Polysaccharides,
or sugars, actually form the outer coating of many bacteria (1). Polysaccharide
vaccines are used to prevent bacterial infections by creating a response
against the molecules in the pathogens capsule, and while small they are not
very immunogenic (1, 3). Consequence they tend to not be as effect in young
children and infants under 18 to 24 months old. They also induce only short
term immunity since the immune response is slow and there is a slow rise in
antibodies. Therefore, not creating an immune memory (3). The first ever
licensed vaccine using this method was Haemophilus
influenza type B (Hib), however failed to create proper immune responses in
infants (1).
Conjugate vaccines also create a response
against the pathogens capsule. However, compared to polysaccharide vaccines
they are created by using technology to bind the polysaccharide to a carrier
protein that is actually able to induce a long term response. This type of
vaccine is even able to work in infants. Today Hib, pneumococcal, and
meningococcal infections have conjugate vaccines (1,3).
Toxoid vaccines use the inactivated form of
toxin, called toxoid, that the bacteria secrete to create an immune response
against disease causing proteins/toxins. Vaccines such diphtheria and tetanus
are examples this type of vaccine (1).
Virus-like particles (VLPs) are also based on
recombinant protein antigens. They induce an immune response similar to natural
viruses but VLPs are non-infectious since they do not have the genetic material
that the virus need in order to replicate inside cells (1).
Scientist are currently developing new ways to
present subunit antigens to the immune system. NIAID scientist actually develop
an experimental vaccine that uses the protein ferritin, that is able to self-assemble
into microscopic pieces called nanoparticles that display a protein antigen. Currently
there is an experimental nanoparticle-based influenza vaccine being evaluated
in early stages of human trials. It is also being tested for the development of
vaccines against MERS coronavirus respiratory virus (RSV) and Epstein Barr
virus (1).
Resources:
- National
Institute of Allergy and Infectious Diseases. (2019, July 1). Vaccine Types.
Retrieved April 29, 2020, from https://www.niaid.nih.gov/research/vaccine-types
- Skwarczynski,
M., & Toth, I. (2017). Introduction. Micro
and Nanotechnology in Vaccine Development, xvii-xviii. doi: 10.1016/b978-0-323-39981-4.00027-0
- World Health
Organization. (n.d.). Subunit Vaccines. Retrieved April 29, 2020, from https://vaccine-safety-training.org/subunit-vaccines.html
