Vaccines and immunity

The first principle

The immune system learns: the first time it meets a germ it responds slowly, but it keeps memory cells, and the second time it responds within days and far more strongly. A vaccine uses this by giving the first lesson safely, showing the body a harmless form or a piece of a pathogen so that memory is in place before the real infection arrives. Every vaccine type is a different answer to one question, how to show enough of the germ to teach the immune system without causing the disease.

The core

How immunity works

• Innate immunity is the first, general line. Skin and mucus, stomach acid, white cells such as neutrophils and macrophages that swallow invaders, inflammation and fever act within minutes to hours against anything foreign, but they do not remember a particular germ.
• Adaptive immunity is specific and has memory. It recognises an antigen, a particular molecule on a pathogen, through two kinds of lymphocytes: B cells, which mature into plasma cells that release antibodies to neutralise and tag germs in body fluids, and T cells, of which helper T cells coordinate the response and cytotoxic T cells kill the body's own cells once a virus has infected them.
• Memory makes the second response fast. After an infection or a vaccine, long lived memory B and T cells remain, so the secondary response produces more antibody, of higher affinity, within a few days; this is why one exposure can protect for years.
• Active and passive immunity differ in who makes the antibodies. Active immunity comes from one's own response to infection or a vaccine and lasts, while passive immunity is borrowed, as antibodies crossing the placenta, in breast milk or in an injection of antiserum for snake bite or rabies, and fades in weeks or months.
• Herd immunity protects the unvaccinated. If each case would infect R₀ others in a fully susceptible population, transmission falls once more than a fraction 1 − 1/R₀ of people is immune; for measles, with an R₀ of about 12 to 18, that is roughly 92 to 95 per cent, which is why measles needs very high coverage.

What a vaccine shows the body

• Live attenuated vaccines use a weakened germ that multiplies a little without causing disease, such as BCG against tuberculosis, the measles and rubella vaccine, oral polio and rotavirus vaccines; they give strong, long immunity, but are avoided in people with weak immunity and in pregnancy.
• Inactivated vaccines use a killed germ, such as injectable polio vaccine, rabies vaccine and Covaxin, and are safer but usually need several doses and boosters.
• Subunit, toxoid and conjugate vaccines use only a part: a protein such as the hepatitis B surface antigen made in yeast, a disabled toxin (toxoid) for tetanus and diphtheria, or a bacterial sugar coat joined to a carrier protein so that infants, whose immune systems respond poorly to sugars alone, respond to it, as in the Haemophilus influenzae type b and pneumococcal vaccines.
• Viral vector vaccines put the gene for a pathogen protein inside a harmless virus that enters cells and makes the protein there, as in Covishield, which used a chimpanzee adenovirus.
• Nucleic acid vaccines deliver the gene itself: an mRNA vaccine carries mRNA in a lipid nanoparticle and the cell's ribosomes make the antigen before the mRNA breaks down, while a DNA vaccine carries a gene in a plasmid. Katalin Karikó and Drew Weissman received the 2023 Nobel Prize in Physiology or Medicine for modifying the bases of mRNA so that it does not set off an inflammatory reaction, which made such vaccines workable.
• Adjuvants are substances added to a vaccine, such as aluminium salts, that stimulate the innate response so that a small amount of antigen produces stronger immunity.

Dimensions

• Health and equity. Vaccines prevent disease rather than treat it, so their benefit is largest for poor households that cannot afford treatment; the public health system, health policy and coverage gaps between States belong to GS2 Social Justice.
• Economic. India accounts for nearly 60 per cent of global vaccine production, supplies vaccines to more than 100 countries and meets more than 65 per cent of the routine immunisation requirements of the World Health Organization (WHO), led by the Serum Institute of India, the world's largest maker by doses, and by Bharat Biotech and Biological E.
• Scientific and strategic. The ability to design, test and make a vaccine quickly is a matter of national security in a pandemic, as Covid showed when exporting countries kept their own supplies first.
• Diplomatic. Under Vaccine Maitri in 2021 India supplied Covid vaccines to many countries as gifts, commercial sales and through the global COVAX facility, and India supplies a large share of the vaccines that Gavi, the Vaccine Alliance, and the United Nations Children's Fund (UNICEF) buy for poorer countries.
• Social and ethical. Vaccine hesitancy, driven by misinformation and distrust, can undo coverage, and compulsory vaccination raises the question of individual choice against protection of others.

In practice

• The Universal Immunisation Programme (UIP), launched in 1985, gives vaccines free against twelve diseases: tuberculosis, polio, diphtheria, pertussis, tetanus, hepatitis B, Haemophilus influenzae type b disease, measles, rubella, rotavirus diarrhoea, pneumococcal pneumonia, and Japanese encephalitis in endemic districts. Mission Indradhanush, from December 2014, runs drives to reach children and pregnant women who were missed.
• Results. India's last case of wild polio was in January 2011 and the WHO South East Asia Region, including India, was certified polio free in March 2014, while maternal and neonatal tetanus was declared eliminated in 2015; measles and rubella elimination is the next target.
• Digital records. The Electronic Vaccine Intelligence Network (eVIN) tracks vaccine stock and storage temperature across the cold chain, and U-WIN, launched nationally in October 2024, records every dose given to pregnant women and children up to 16 years under the UIP, building on the CoWIN platform used for Covid vaccination.
• Covid brought new platforms into Indian production. Covishield (viral vector, made by the Serum Institute under licence from Oxford and AstraZeneca), Covaxin (inactivated, by Bharat Biotech with the Indian Council of Medical Research's National Institute of Virology), ZyCoV-D (by Zydus, the first DNA vaccine authorised for humans anywhere, in August 2021), Corbevax (protein subunit, by Biological E), Gemcovac (mRNA, by Gennova Biopharmaceuticals) and iNCOVACC (nasal, by Bharat Biotech).
• HPV vaccine. The Serum Institute developed Cervavac, India's own quadrivalent vaccine against the human papillomavirus that causes most cervical cancer, approved in 2022, and at the end of February 2026 the government launched a national campaign giving a single dose of HPV vaccine to girls aged 14.
• Approval and quality. A new vaccine passes laboratory and animal tests, then phase 1 trials for safety in tens of volunteers, phase 2 for dose and response in hundreds, and phase 3 for efficacy in thousands, before the Central Drugs Standard Control Organisation (CDSCO) licenses it under the New Drugs and Clinical Trials Rules 2019; each batch is tested at the Central Drugs Laboratory, Kasauli, and adverse events following immunisation are monitored after rollout. In October 2024 WHO found that India's vaccine regulatory system met its benchmark of a functional system at maturity level 3, which supports WHO prequalification of Indian vaccines for United Nations purchase.

Issues and way forward

• The last mile and zero dose children. Coverage is lowest in urban slums, migrant families and remote tribal areas, because of poor access and missed records rather than missing vaccines; tracking each child on U-WIN and running Indradhanush style drives where gaps are found is the answer.
• The cold chain. Most vaccines must stay between 2 and 8 °C and some mRNA vaccines far colder, so a power cut can spoil a stock; eVIN monitoring, solar refrigerators and more heat stable vaccines reduce losses.
• Hesitancy and misinformation. Rumours on social media spread faster than corrections, so trusted local workers such as Accredited Social Health Activists (ASHAs), clear communication of side effects and quick, open investigation of serious adverse events matter as much as supply.
• Research and innovation. Indian firms excel at making licensed vaccines cheaply at scale but have developed fewer new ones; funding for early stage research, the mRNA and other platforms built during Covid, and the Mission COVID Suraksha model of financing candidates through trials are the way forward.
• Quality and trust abroad. A large exporter must keep every batch reliable, so sustaining WHO maturity level 3, moving towards level 4, and strengthening State drug regulators protect both patients and India's reputation.

Common confusions

• Vaccination and immunisation. Vaccination is giving the vaccine, while immunisation is the process by which the person becomes immune, which may need several doses.
• Antigen and antibody. An antigen is the foreign molecule that is recognised, while an antibody is the protein the body makes against it.
• Active and passive immunity. Active immunity is made by one's own body and lasts, while passive immunity is received ready made and fades.
• mRNA vaccine and gene therapy. An mRNA vaccine's message is read for a few days and then destroyed, and it never enters the nucleus or changes the person's DNA, while gene therapy is meant to change how a person's genes work for a long time.
• Efficacy and effectiveness. Efficacy is how well a vaccine works in a controlled trial, while effectiveness is how well it works in real use across a population.

For the exam

• Keyword bank: innate and adaptive immunity, B and T cells, memory cells, primary and secondary response, herd immunity threshold, live attenuated, inactivated, conjugate, viral vector, mRNA vaccine, adjuvant, cold chain, Universal Immunisation Programme, Mission Indradhanush, U-WIN.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: India's Universal Immunisation Programme gives free vaccines against twelve diseases, and India accounts for nearly 60 per cent of global vaccine production, supplying more than 100 countries.
  • Introduction with a source: In October 2024 the World Health Organization found that India's vaccine regulatory system met its benchmark for a functional system at maturity level 3, the basis on which Indian vaccines reach United Nations buyers.
  • Conclusion: India's success with vaccines rests as much on finding the last child as on making the dose, so U-WIN, the cold chain and trusted health workers deserve the same priority as new platforms.
  • Conclusion: Covid showed that a country that can design and make its own vaccines can protect itself and others, and India should turn its manufacturing strength into strength in discovery.