Subunit vaccines are defined as those containing one or more pure or semi-pure antigens. Since the technology for growing viruses to high titers in cell cultures advanced, it became practicable to purify virus and viral antigens.
It is now possible to identify the peptide sites encompassing the major antigenic sites of viral antigens, from which highly purified subunit vaccines can be produced.
But some times increasing purification may lead to loss of immunogenicity and this may necessitate coupling to an immunogenic carrier protein or adjuvant, such as aluminum salt.
In addition to using a whole protein as a vaccine, it is possible to identify individual epitomes within these protective proteins and develop peptide vaccines.
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The potential advantages of using subunits as vaccines are the increased safety and less antigenic competition, ability to target the vaccines and differentiation of vaccinated animals from infected animals is easy.
Since only a few components are included in the vaccine ability to target the vaccines at the site where immunity is required is possible. Examples of purified subunit vaccines include the HA vaccines for influenza A and B, and hepatitis B surface antigens (HBs Ags) derived from the plasma of carriers.
Potential disadvantages of subunit vaccines
(1) The disadvantage of subunit vaccines is that they generally require strong adjuvants and these adjuvants often induce tissue reactions.
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(2) Duration of immunity is generally shorter than the live vaccines.
(3) The major disadvantage of peptide vaccines is that they often need to be linked to carriers to enhance their immunogenicity.
(4) A pathogen can escape immune responses to a single epitope versus multiple epitope vaccines.