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Similar to several kinds of modern vaccines, the protein subunit vaccine improves on simplicity of the classic vaccine by removing the need to include viral particles at all.
In a classic vaccine, the reason we introduce the virus to the host at all is for “antigenicity,” the ability to express key parts of the virus (called antigens) from which the immune system’s antibodies can be created. Once the body produces adequate neutralizing antibodies, the actual introduction of the virus in its true state.
A protein subunit is just one protein which forms a protein complex by folding along with other protein molecules. This is a process called “coassembly” and the end result is a protein complex.
As it happens, a protein subunit can be an antigen. This is the key insight that allows the protein subunit vaccine to skip the step of introducing the virus or viral particles; it skips right to the antigen.
There are two methods to creating a subunit vaccine. In one approach, the protein subunit is generated in the host genetically. The proper gene of the virus’s antigen is added into a viral or yeast vector. A vector is a harmless virus that allows for genetic transport into cells. The hepatitis B vaccine works this way but by using an attenuated (weakened) bacteria.
The second approach is a bit simpler. The vaccine could administer just a specific protein found in the virus, similar to injecting the antigens directly. The drawback of this approach is the risk proteins could unfold or “denature” in a way that a different antigens are introduced. If this is the case, the vaccine could be ineffective or potentially worse, create immunity for something we don’t want the body to immunize from.