Recombinant Vaccine

1. Live Genetically Modified Vaccines

• In this type, a live pathogenic organism (bacteria or virus) is genetically modified to make it non-pathogenic by deleting or inactivating one or more of its genes. These genetically weakened agents, when introduced into the host, trigger an immune response without causing disease.
• A subtype includes vector-based vaccines, where a gene from a different pathogen is inserted into a harmless carrier virus. For example, Vaccinia virus, a member of the poxvirus family with a large double-stranded DNA genome, has been used to create multivalent vaccines by inserting genes from pathogens like Hepatitis B virus, Herpes simplex virus, and Influenza virus.

The process involves:

• Cloning a portion of vaccinia virus DNA into a plasmid insertion vector.
• Inserting antigenic genes from multiple pathogens into this plasmid.
• Recombining this with the vaccinia virus genome to create a recombinant vaccinia virus capable of expressing foreign antigens.
• This method enables the development of polyvalent vaccines, offering protection against multiple diseases simultaneously.

2. Recombinant Subunit Vaccines

• Subunit vaccines contain only a specific part of the pathogen, usually a protein antigen, rather than the whole organism. These proteins can be synthetically produced or expressed from cloned genes in microbial systems such as yeast (eukaryotic) or E. coli (prokaryotic).
• An excellent example is the Hepatitis B vaccine, where the surface antigen gene HBsAg is cloned into the pMA56 yeast plasmid vector and expressed in Saccharomyces cerevisiae. This gene is fused with a strong promoter and other regulatory sequences to ensure effective expression and selection in yeast cells. The resulting protein is purified and used for immunization.
• This was the first recombinant subunit vaccine licensed for public use in 1987, marketed under names like Recombivax® and Engerix-B®.

3. DNA Vaccines

• DNA vaccines represent a breakthrough in vaccine technology. Here, the gene encoding the antigenic protein is cloned into a plasmid DNA vector, which includes promoter, terminator, origin of replication, and selection markers. This plasmid DNA is then injected directly into the muscle of the patient using methods such as a gene gun or nasal spray.
• Once inside the body, the muscle cells take up the DNA, express the antigen, and initiate a humoral and cellular immune response. DNA vaccines are considered safe, do not involve live pathogens, and can be rapidly developed in response to emerging diseases.

4. RNA Vaccines

• RNA vaccines consist of messenger RNA (mRNA) encoding the desired antigen. Upon injection, antigen-presenting cells (APCs) take up the mRNA and produce the protein antigen, triggering both antibody-mediated and cell-mediated immune responses.

Since RNA is unstable, various modifications like 5’ capping, 3’ poly-A tail, and formulation in lipid nanoparticles (LNPs) are used to improve stability and cellular uptake. RNA vaccines offer several advantages:

• Rapid development and adaptation to viral mutations.
• Cell-free production, reducing the risk of contamination.
• Low-dose requirement, and potential for single-dose protection.

During the COVID-19 pandemic, RNA vaccines such as Pfizer-BioNTech and Moderna emerged as highly effective tools. While India used Covaxin (inactivated virus) and Covishield (adenovirus-based DNA vaccine), RNA vaccine platforms were also rapidly developed globally in response to emerging variants.

• Efforts are ongoing to improve the thermostability of RNA vaccines using lyophilization (freeze-drying) and thermostable nanoparticle formulations, making them easier to store and distribute.