How do vector vaccines work to provide immunity?

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2023 Q22 — Science & Technology PYQ with Answer

State PCS 2023 Q22 — Science & Technology PYQ with Answer