BPSC 69th CCE Prelims General Studies · Question 22 of 144

Correct — C, By placing the virus in a modified version of a different virus. A viral-vector vaccine works by borrowing a harmless carrier virus and using it as a delivery van. The gene coding for one distinctive protein of the target pathogen is spliced into the genome of a second, unrelated virus that has first been crippled so it cannot replicate in humans; that carrier — the vector — infects a few cells, hands over the gene, and the recipient's own cells then manufacture the target antigen and display it. The immune system meets that antigen, raises neutralising antibodies against it and primes killer T cells, all without ever encountering the disease-causing organism. India's most widely used COVID-19 vaccine is exactly this: Covishield is the Oxford–AstraZeneca design ChAdOx1 nCoV-19, a replication-deficient chimpanzee adenovirus carrying the full-length coding sequence of the SARS-CoV-2 spike protein, manufactured at scale by the Serum Institute of India in Pune. Russia's Sputnik V uses two human adenoviruses, Ad26 for the first dose and Ad5 for the second, and Ervebo against Ebola — a live recombinant vesicular stomatitis virus engineered to express the Zaire ebolavirus glycoprotein — had already put the platform into licensed use before COVID-19 made it famous. One honest caveat about the wording: strictly, what is placed inside the modified carrier virus is the GENE for one antigen of the target virus, not the target virus itself. Option (c) is loose on that point, but it is the only option that names a second, modified carrier virus at all, and that carrier is the defining feature of the platform.

• (a)By introducing a weakened or inactivated virus into the body — This describes the two oldest vaccine platforms, not the vector platform. A live-attenuated vaccine (BCG, oral polio, measles-rubella) uses a weakened whole organism; an inactivated vaccine (Covaxin/BBV152, the Salk injectable polio vaccine, Sinovac's CoronaVac) uses a whole organism killed with a chemical such as beta-propiolactone. In both, the pathogen itself is what enters the body — there is no carrier virus and no gene transfer, which is precisely what the question is asking about.
• (b)By directly attacking and destroying pathogens in the body — No vaccine of any kind does this. Vaccination is active immunisation — it trains the immune system in advance and leaves behind memory B and T cells, so the fighting is done later and by the body itself. Direct destruction of a pathogen already present is the job of an antibiotic or antiviral drug, or of passive immunisation with ready-made monoclonal antibodies or antisera. The option is testing whether the candidate can tell prophylaxis from therapy.
• (d)By entering directly into the cells and enabling them to create spike proteins — The best distractor on the item, because it is true of a different platform and shares the vector platform's final step. This is the mRNA vaccine — Pfizer-BioNTech's Comirnaty, Moderna's Spikevax, India's GEMCOVAC-19 — where naked genetic instructions wrapped in a lipid nanoparticle enter the cell with no carrier virus at all. Both routes end with the cell making spike protein; the discriminating word is 'directly'. A vector vaccine reaches the cell inside another virus, and that is the whole distinction being tested.

Every vaccine has the same goal — show the immune system a recognisable piece of a pathogen, safely, so that memory cells are in place before the real infection arrives — and the platforms differ only in how that piece is presented. The classical routes deliver the antigen ready-made: live-attenuated (a weakened whole organism), whole inactivated (a killed organism), subunit or protein (a purified antigen, as in Corbevax and hepatitis B vaccines) and toxoid (a chemically neutralised bacterial toxin, as for tetanus and diphtheria). The genetic routes deliver instructions instead and let the recipient's own cells build the antigen — viral-vector vaccines, DNA-plasmid vaccines such as India's needle-free ZyCoV-D, and mRNA vaccines. Because the antigen made inside a cell is also presented on MHC class I molecules, the genetic platforms tend to raise a cytotoxic T-cell response as well as antibodies, which whole-inactivated vaccines do less well. Vectors are chosen for harmlessness and for novelty to the human immune system: adenoviruses are the workhorses, made replication-incompetent by deleting the E1 gene, and vectors need not be viral at all — attenuated Salmonella and vaccinia have both been used.

The four options are not random: each names a real vaccine platform, so the item is a platform-identification test disguised as a mechanism question. Strike (b) first, because it describes no vaccine at all — a vaccine never attacks a pathogen, it prepares the body to. Strike (a) next: 'weakened or inactivated' is the textbook definition of the live-attenuated and killed platforms, which is what Covaxin is and what Covishield is not. That leaves (c) and (d), and here the trap is real, because a vector vaccine also ends with the cell producing spike protein — the two platforms differ only in how the instructions get in. The single discriminating word is 'directly' in option (d): mRNA arrives naked inside a lipid bubble, with no carrier organism, while a vector vaccine arrives packaged inside a second, modified virus. Option (c) is the only one that mentions a carrier virus, and that carrier is the definition of the platform. A useful memory anchor for a candidate in Bihar or anywhere else in India: the two vaccines rolled out on the same day in January 2021 sat on opposite platforms — Covishield a chimpanzee-adenovirus vector, Covaxin a whole inactivated virus.

• Covishield is ChAdOx1 nCoV-19 (AZD1222) — a replication-deficient chimpanzee adenovirus carrying the full-length coding sequence of the SARS-CoV-2 spike protein. Designed at Oxford with AstraZeneca and manufactured by the Serum Institute of India, Pune, it accounted for the overwhelming majority of doses given in India's COVID-19 drive.
• Sputnik V (Gam-COVID-Vac, Gamaleya Institute) deliberately uses two different human adenovirus vectors — Ad26 for dose one and Ad5 for dose two — because antibodies raised against the first vector would blunt a repeat dose. This 'anti-vector immunity' problem is also why Oxford chose a chimpanzee adenovirus, to which almost no human has prior immunity.
• Vectors are not necessarily viral: bacteria such as attenuated Salmonella, and viruses such as adenovirus and vaccinia, are all used as recombinant vectors, and building any of them requires genetic engineering — the exact pair of statements UPSC asked candidates to validate in 2021.
• Covaxin (BBV152, Bharat Biotech with ICMR-NIV) is a whole-virion inactivated vaccine adjuvanted with an imidazoquinoline compound on alum — the platform described by option (a) — so India's two January 2021 rollouts sat on different platforms entirely.
• Ervebo (rVSV-ZEBOV) against Zaire ebolavirus is a live recombinant vesicular stomatitis virus engineered to express the Ebola surface glycoprotein; approved in the United States and the European Union in 2019, it had the viral-vector platform in licensed human use before the COVID-19 vaccines appeared.

• Calling Covishield an mRNA vaccine — a mistake UPSC planted deliberately in 2022; Covishield is a chimpanzee-adenovirus vector vaccine and no mRNA vaccine was ever part of India's mass rollout
• Treating options (c) and (d) as the same thing because both end with the cell producing spike protein; the platform is defined by how the instructions arrive, not by what the cell then makes
• Assuming a 'vector' must be a virus — bacteria are used as recombinant vectors too, and the word 'vector' in disease transmission (a mosquito) means something quite different again

BPSC asks the mechanism in plain English with one platform per option and no product names, so the candidate must know what a vector vaccine physically does rather than which brand uses it. UPSC comes at the same syllabus from the other side — matching real products to platforms (2022: is Covishield mRNA? is Sputnik V vector-based? is Covaxin inactivated?) or testing a definition through a statement pair, as in its 2021 item on recombinant vector vaccines using bacteria and viruses as vectors.

• practice — not a real PYQ

  Which one of the following COVID-19 vaccines used in India is based on a viral vector platform?

  • (a)Covaxin
  • (b)Covishield
  • (c)Corbevax
  • (d)GEMCOVAC-19

  Answer(b) Covishield — a replication-deficient chimpanzee adenovirus carrying the SARS-CoV-2 spike gene. Covaxin is whole-virion inactivated, Corbevax is a protein subunit vaccine and GEMCOVAC-19 is an mRNA vaccine.

• practice — not a real PYQ

  In a viral vector vaccine, what is it that the vector actually carries into the recipient's cells?

  • (a)A live but weakened form of the disease-causing virus
  • (b)Ready-made antibodies against the disease-causing virus
  • (c)The gene coding for one antigen of the disease-causing virus
  • (d)A chemically inactivated toxin produced by the pathogen

  Answer(c) The gene coding for one antigen of the disease-causing virus — the vector is only a delivery vehicle, and the recipient's own cells then build that antigen. (a) describes a live-attenuated vaccine, (b) passive immunisation and (d) a toxoid vaccine.