UPSC Prelims 2021 Q65: Recombinant Vector Vaccines — Explained
With reference to recent developments regarding 'Recombinant Vector Vaccines', consider the following statements:
1. Genetic engineering is applied in the development of these vaccines.
2. Bacteria and viruses are used as vectors.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Answer: (c) Both 1 and 2
Statement-wise explanation
- 1: correct — A recombinant vector vaccine is made with recombinant DNA technology, which is genetic engineering. The gene for an antigen of the target pathogen is cut and inserted into the genome of a harmless or weakened carrier, the vector. Covishield (ChAdOx1 nCoV-19) is an example: it is a replication-deficient chimpanzee adenovirus that carries the gene for the SARS-CoV-2 spike protein, and the genes the virus needs to replicate have been deleted.
- 2: correct — Both viruses and bacteria are used as vectors. Viral vectors include adenoviruses (Covishield; Sputnik V, which uses Ad26 and Ad5) and vesicular stomatitis virus (Ervebo, the Ebola vaccine WHO prequalified on 12 November 2019). Bacterial vectors include attenuated Salmonella Typhi (Ty21a), Listeria monocytogenes and BCG, which have been engineered to carry foreign antigens and tested in many animal and human trials.
How to eliminate
Statement 1 can be worked out from the name. 'Recombinant' means DNA from different sources is joined together, and that is genetic engineering by definition. Once Statement 1 is true, the options narrow to 1 only or Both 1 and 2. Statement 2 contains no absolute word such as 'only' or 'always'. It just says bacteria and viruses are used, so one real example of each is enough to make it true. Most candidates in 2021 knew Covishield and Sputnik V as viral-vector vaccines. The harder part is remembering that bacteria are also used as vectors, mainly attenuated Salmonella, Listeria and BCG. Remember: a vector is any harmless carrier organism, a virus or a bacterium, that delivers an antigen gene.
Know more
How a recombinant vector vaccine works
A vector is a carrier. Scientists take the gene for one antigen of a dangerous pathogen and insert it into a harmless or weakened microbe. When that microbe enters the body, it makes the antigen inside our cells. The immune system learns to recognise the antigen without ever meeting the real pathogen. Because the antigen is made inside cells, these vaccines trigger both antibodies and T-cell (cell-mediated) immunity.
Types of vector
| Vector type | Examples | Vaccine / use |
|---|---|---|
| Non-replicating virus | Chimpanzee adenovirus (ChAdOx1) | Covishield (COVID-19) |
| Non-replicating virus | Human adenovirus Ad26 + Ad5 | Sputnik V (COVID-19) |
| Nasal adenovirus vector | Chimpanzee adenovirus | iNCOVACC/BBV154, nasal COVID-19 vaccine (EUA 2022) |
| Replicating virus | Vesicular stomatitis virus (VSV) | Ervebo (Ebola) |
| Bacterium | Attenuated Salmonella Typhi Ty21a, Listeria, BCG | Experimental vector vaccines, cancer immunotherapy trials |
Strengths: strong immune response, often from a single dose; no need to handle the live pathogen; bacterial vectors can be taken by mouth and are cheap to produce.
Limitations: pre-existing immunity to the vector (for example, to common human adenoviruses) can make the vaccine less effective. This is why Covishield uses a chimpanzee adenovirus and Sputnik V uses two different vectors for its two doses.
Don't confuse with other platforms
- mRNA vaccines (Pfizer, Moderna): mRNA in a lipid nanoparticle; no living carrier.
- DNA vaccines (ZyCoV-D, India, EUA 20 August 2021): a naked plasmid carries the gene; no vector organism.
- Inactivated vaccines (Covaxin): the whole pathogen is killed.
- Subunit / recombinant protein vaccines (hepatitis B, Corbevax): the protein is made in yeast or cell culture and purified. Genetic engineering is used, but the microbe is not injected as a carrier.
In the news
In 2021 India's COVID-19 vaccination drive relied on Covishield, a recombinant chimpanzee-adenovirus vector vaccine given restricted emergency approval in January 2021. Sputnik V, a combined adenovirus vector vaccine, got import permission in April 2021. 'Viral vector' became a common term in the news, and UPSC tested the general platform rather than a single brand.