(a) What are vaccines? Classify these and discuss DNA vaccines in detail. (b) What are causative organism and vector for dengue, enlist possible ways of prevention from dengue.

(a) What are vaccines?

A vaccine is a biological preparation that trains the immune system to recognise a pathogen without causing the disease itself. It presents an antigen, or the instructions for making one, and the body responds with antibodies, T cells and memory cells. On later exposure, these memory cells produce a faster and stronger response.

Classification

Live attenuated: a weakened living organism, e.g. measles, BCG and oral polio vaccines. Inactivated: a killed organism, e.g. injectable polio and rabies vaccines. Subunit or recombinant: only selected proteins of the pathogen, e.g. hepatitis B and HPV vaccines. Toxoid: an inactivated bacterial toxin, e.g. tetanus and diphtheria vaccines. Polysaccharide and conjugate: bacterial sugar coats, in conjugates joined to a protein, e.g. Hib and pneumococcal vaccines. Viral vector: a harmless virus carries the antigen gene, e.g. Oxford-AstraZeneca COVID-19 vaccine. Nucleic acid: genetic instructions delivered as mRNA (e.g. Pfizer-BioNTech COVID-19 vaccine) or DNA.

DNA vaccines

A DNA vaccine uses a small circular piece of DNA, a plasmid, into which the gene for a pathogen's antigen has been inserted, together with a strong promoter that drives its expression. It contains no living pathogen.

Mechanism: the plasmid is delivered by injection, sometimes helped by electroporation or a needle-free device. It enters the nuclei of muscle, skin and antigen-presenting cells, where the gene is transcribed into mRNA and translated into antigen. The antigen is displayed on MHC class I molecules, activating cytotoxic T cells, and on MHC class II molecules, activating helper T cells and, through them, antibody-producing B cells. The result is both cellular and humoral immunity.

Advantages: they cannot cause the infection they target; they are relatively quick and cheap to design and produce; they are comparatively stable, which eases storage; and they can be redesigned quickly for a new strain.

Limitations: immune responses in humans have often been weaker than in animal studies; delivery into cells is inefficient, so special devices may be required; and the theoretical risk of the DNA integrating into the host genome must be checked. Several DNA vaccines are used in veterinary medicine, and India's ZyCoV-D, authorised for COVID-19 in 2021, was the first for human use.

(b) Dengue

Causative agent: dengue is caused by the dengue virus (DENV), an RNA virus of the family Flaviviridae, with four serotypes (DENV-1 to DENV-4). Strictly speaking, it is a virus and not an organism in the cellular sense.

Vector: the virus spreads through the bite of an infected female Aedes mosquito, chiefly Aedes aegypti and also Aedes albopictus. These mosquitoes bite mostly in the daytime and breed in clean, stagnant water.

Ways of prevention

(i) remove standing water from tyres, pots, coolers, roofs and containers; (ii) cover water tanks, and empty and clean containers every week; (iii) use larvicides in water that cannot be drained; (iv) wear long sleeves and use repellents, especially during the day; (v) use window screens and bed nets, including for daytime sleepers and patients; (vi) carry out targeted indoor and outdoor spraying during outbreaks; (vii) run community cleanliness campaigns and surveillance after the monsoon; (viii) use a licensed vaccine where health authorities recommend one.