Vaccines

Vaccines are biological preparations designed to provide active acquired immunity against specific infectious diseases. They work by stimulating the body’s immune system to recognise and combat pathogens such as viruses or bacteria without causing the disease itself. Vaccines are among the most important public health innovations in human history, credited with eradicating or controlling many life-threatening diseases such as smallpox, polio, diphtheria, measles, and COVID-19.

Historical Background

The concept of vaccination dates back to the late 18th century. In 1796, Edward Jenner, an English physician, discovered that exposure to cowpox protected humans against smallpox, a deadly viral disease. This early practice, termed vaccination (from vacca, the Latin word for cow), laid the foundation for modern immunology.
In the 19th century, Louis Pasteur expanded the principles of vaccination by developing vaccines against rabies and anthrax, demonstrating that weakened or inactivated forms of pathogens could induce immunity. Since then, vaccines have evolved through scientific advances in microbiology, molecular biology, and biotechnology, forming a cornerstone of preventive medicine.

Principle of Immunity

The human immune system protects the body through two types of responses:

• Innate immunity: The first line of defence, non-specific and immediate.
• Adaptive immunity: A specific immune response that develops upon exposure to a pathogen and provides long-lasting protection through memory cells.

Vaccines mimic infection by introducing antigens (disease-causing components such as proteins or inactivated microbes) into the body. These antigens trigger the production of antibodies and the activation of T-cells without causing illness. When the individual encounters the actual pathogen later, the immune system recognises it and mounts a rapid, effective defence.
This process results in immunological memory, the basis for long-term protection.

Types of Vaccines

Modern vaccines use a variety of scientific approaches to stimulate immunity. The main types include:

1. Live Attenuated Vaccines
   • Contain weakened forms of the pathogen that cannot cause severe disease in healthy individuals.
   • Induce strong, long-lasting immunity with one or two doses.
   • Examples: Measles, Mumps, and Rubella (MMR) vaccine, Oral Polio Vaccine (OPV), BCG (for tuberculosis), and Yellow Fever vaccine.
   • Limitations: Not suitable for immunocompromised individuals.
2. Inactivated (Killed) Vaccines
   • Contain pathogens that have been inactivated by heat or chemicals.
   • Safer but generally require multiple doses or boosters.
   • Examples: Inactivated Polio Vaccine (IPV), Hepatitis A, and Rabies vaccines.
3. Subunit and Conjugate Vaccines
   • Contain purified parts of the pathogen (proteins, polysaccharides, or toxins).
   • Conjugate vaccines combine weak antigens with stronger carrier proteins to enhance immune response.
   • Examples: Hepatitis B, Human Papillomavirus (HPV), Haemophilus influenzae type b (Hib), and Pneumococcal vaccines.
4. Toxoid Vaccines
   • Contain inactivated toxins produced by bacteria, used when the toxin is the main cause of disease.
   • Examples: Diphtheria and Tetanus vaccines.
5. mRNA Vaccines
   • Introduce a small piece of genetic material (messenger RNA) that instructs cells to produce a harmless version of a pathogen’s protein, triggering immunity.
   • Examples: Pfizer-BioNTech and Moderna COVID-19 vaccines.
   • Advantages: Rapid development and high efficacy.
6. Vector-Based Vaccines
   • Use a harmless virus (vector) to deliver genetic material encoding the pathogen’s antigen.
   • Examples: Oxford-AstraZeneca COVID-19 vaccine, Johnson & Johnson (Janssen) vaccine, and Sputnik V.
7. DNA and Protein Subunit Vaccines (Emerging Technologies)
   • Use either plasmid DNA or recombinant proteins to generate immune responses, offering stability and safety advantages.

Vaccine Development and Testing

The creation of a vaccine is a rigorous, multi-phase process to ensure safety, efficacy, and quality.

1. Preclinical Stage: Laboratory and animal testing to assess safety and immune response.
2. Clinical Trials:
   • Phase I: Small group testing for safety and dosage.
   • Phase II: Expanded trials to evaluate immune response and side effects.
   • Phase III: Large-scale testing for efficacy and rare side effects.
3. Regulatory Approval: Health authorities (e.g., World Health Organization (WHO), U.S. Food and Drug Administration (FDA), European Medicines Agency (EMA), Central Drugs Standard Control Organisation (CDSCO) in India) review data before authorisation.
4. Post-Market Surveillance (Phase IV): Ongoing monitoring for adverse effects and long-term performance.

Immunisation Schedules

Vaccines are administered according to specific immunisation schedules tailored by national health authorities. For instance, the Universal Immunisation Programme (UIP) in India and the Centers for Disease Control and Prevention (CDC) in the United States outline age-specific vaccination charts for infants, children, adolescents, and adults.
Common childhood vaccines include:

• BCG – against tuberculosis
• DPT – diphtheria, pertussis, tetanus
• Polio (OPV/IPV)
• Hepatitis B
• MMR – measles, mumps, rubella
• Hib and rotavirus vaccines

Adult immunisations target influenza, tetanus boosters, and conditions like shingles, pneumococcal disease, and hepatitis.

Importance of Vaccination

Vaccines play a critical role in public health and disease prevention by:

• Reducing morbidity and mortality: Preventing millions of deaths annually.
• Eradicating diseases: Smallpox was eradicated globally in 1980 through vaccination.
• Preventing epidemics: Control of polio, measles, and other infectious diseases.
• Herd Immunity: When a large proportion of a population is immunised, disease transmission is minimised, protecting unvaccinated individuals such as infants and immunocompromised people.
• Economic benefits: Lowering healthcare costs by preventing illness and loss of productivity.

Challenges and Controversies

1. Vaccine Hesitancy: Misinformation, religious beliefs, and fear of side effects can lead to refusal or delay in vaccination, undermining herd immunity.
2. Cold Chain and Distribution: Vaccines require strict temperature control during storage and transportation, posing logistical challenges in low-resource settings.
3. Emerging Pathogens: New diseases such as COVID-19, Ebola, and Zika demand rapid vaccine development using advanced technologies.
4. Intellectual Property and Access Inequality: Patent protection and commercial pricing often limit vaccine access in developing countries. Global initiatives like COVAX aim to promote equitable distribution.
5. Mutation and Waning Immunity: Pathogen mutations (e.g., influenza, SARS-CoV-2 variants) can reduce vaccine efficacy, necessitating booster doses or reformulated vaccines.

Global Vaccination Initiatives

International organisations play a vital role in global immunisation efforts:

• World Health Organization (WHO): Coordinates global immunisation policies and disease eradication programmes.
• UNICEF: Supports vaccine access for children in developing nations.
• GAVI, the Vaccine Alliance: Promotes equitable access to vaccines through funding and partnerships.
• COVAX Facility: A global initiative ensuring fair COVID-19 vaccine distribution.

These collaborations have dramatically reduced vaccine-preventable diseases worldwide.

Ethical and Social Dimensions

Vaccination embodies collective responsibility. While it protects individuals, its broader goal is community health. Ethical debates centre on mandatory vaccination policies, balancing individual freedom with public safety. Informed consent, transparency, and trust between governments, scientists, and the public are essential for maintaining vaccination programmes.

Future of Vaccines

Advances in biotechnology continue to reshape vaccine science. Future developments may include:

• Universal vaccines targeting all strains of influenza or coronaviruses.
• Personalised vaccines for cancer and autoimmune diseases.
• Microneedle patches and oral vaccines for easier delivery.
• mRNA and nanoparticle-based platforms for rapid response to emerging infections.