Vaccine

 

Vaccine

Definition

A vaccine is a biological preparation containing weakened, killed, or purified components of a pathogen (or its genetic material) that stimulates the immune system to develop protective immunity against a specific disease without causing the disease itself. Vaccines train the immune system to recognize and respond rapidly to future infections.

Introduction

Vaccination is one of the greatest achievements in medical science. It has significantly reduced morbidity and mortality due to infectious diseases such as smallpox, polio, diphtheria, tetanus, measles, and hepatitis. According to the World Health Organization (WHO), immunization prevents approximately 3.5–5 million deaths annually worldwide and protects against more than 30 life-threatening diseases.

Important Terms

Term	Definition
Vaccine	Biological preparation that induces immunity
Vaccination	Administration of a vaccine
Immunization	Process of becoming protected against a disease
Antigen	Substance that triggers an immune response
Antibody	Protein produced by B lymphocytes against antigens
Booster Dose	Additional dose given to maintain immunity
Herd Immunity	Community protection when a large proportion is immunized

 

History of Vaccination

1. Variolation

• Ancient method used in China and India.
• Involved exposure to small amounts of smallpox material.

2. Edward Jenner (1796)

Edward Jenner

• Observed that milkmaids infected with cowpox were protected from smallpox.
• Inoculated a boy with cowpox material.
• Demonstrated protection against smallpox.
• Known as the Father of Immunology.

3. Louis Pasteur

Louis Pasteur

• Developed vaccines for:
• Rabies
• Anthrax
• Chicken cholera

Principles of Vaccination

Vaccines mimic natural infection and stimulate adaptive immunity. When a vaccine enters the body:

Step 1: Antigen Recognition

• Antigen-presenting cells (APCs) recognize vaccine antigens.

Step 2: Activation of Lymphocytes

• Helper T cells become activated.
• B cells produce antibodies.

Step 3: Memory Cell Formation

• Memory B cells and T cells are generated.

Step 4: Long-Term Protection

• Upon future exposure, memory cells rapidly destroy the pathogen.

Vaccines provide active artificial immunity.

Types of Immunity

A. Innate Immunity

Characteristics:

• Present from birth
• Non-specific
• Immediate response

Examples:

• Skin
• Mucous membranes
• Phagocytes

B. Acquired (Adaptive) Immunity

1. Active Immunity

Produced by the body's own immune system.

Natural Active Immunity

• Develops after natural infection.

Example: Recovery from chickenpox.

Artificial Active Immunity

• Produced by vaccination.

Example: Polio vaccine.

2. Passive Immunity

Ready-made antibodies are transferred.

Natural Passive Immunity

• Maternal antibodies through placenta and breast milk.

Artificial Passive Immunity

• Administration of antisera or immunoglobulins.

Examples:

• Anti-rabies serum
• Anti-snake venom

Classification of Vaccines

1. Live Attenuated Vaccines

Contain weakened but living microorganisms.

Features

• Strong immune response
• Long-lasting immunity
• Usually require fewer doses

Examples

• BCG
• MMR
• Oral Polio Vaccine (OPV)
• Varicella vaccine
• Yellow fever vaccine

Advantages

• Strong humoral and cellular immunity
• Long-term protection

Disadvantages

• Cannot be used in severely immunocompromised individuals
• Require cold chain maintenance

2. Inactivated (Killed) Vaccines

Contain killed microorganisms.

Examples

• Inactivated Polio Vaccine (IPV)
• Rabies vaccine
• Hepatitis A vaccine

Advantages

• Safer
• No risk of causing disease

Disadvantages

• Multiple doses needed
• Booster doses required

3. Toxoid Vaccines

Contain inactivated bacterial toxins.

Examples

• Tetanus toxoid
• Diphtheria toxoid

Features

• Produce immunity against toxins rather than bacteria

4. Subunit Vaccines

Contain purified antigenic components.

Examples

• Acellular pertussis vaccine
• Hepatitis B vaccine

Advantages

• Highly safe
• Few adverse effects

Disadvantages

• Require boosters

5. Recombinant Vaccines

Produced by genetic engineering.

Example

• Hepatitis B vaccine

Produced using recombinant DNA technology.

6. Conjugate Vaccines

Polysaccharide antigens linked with proteins.

Examples

• Hib vaccine
• Pneumococcal conjugate vaccine

Useful in infants where pure polysaccharides are poorly immunogenic.

7. mRNA Vaccines

Contain messenger RNA coding for pathogen proteins.

Examples

• COVID-19 mRNA vaccines

Advantages

• Rapid development
• No live pathogen involved

Disadvantages

• Storage challenges

8. Viral Vector Vaccines

Use harmless viruses to deliver genetic material.

Examples

• Ebola vaccine
• Some COVID-19 vaccines

Components of a Vaccine

1. Antigen

Disease-causing component that stimulates immunity.

2. Adjuvant

Enhances immune response.

Examples: Aluminum salts

3. Stabilizers

Maintain vaccine potency.

Examples: Sugars, Gelatin

4. Preservatives

Prevent contamination.

Example: Thimerosal

5. Residual Antibiotics

Prevent bacterial contamination during manufacture.

Examples:

• Neomycin
• Streptomycin

Herd Immunity

Definition

Protection of an entire population when a sufficiently large proportion of individuals are immune.

Importance

• Protects unvaccinated individuals
• Reduces disease transmission
• Helps eradicate diseases

Example

• Smallpox eradication

Cold Chain

Definition

A system for maintaining vaccines at recommended temperatures from manufacture to administration.

Purpose

• Preserve vaccine potency
• Prevent vaccine failure

Equipment

• Walk-in coolers
• Deep freezers
• Ice-lined refrigerators
• Vaccine carriers

Temperature

Most vaccines:

• +2°C to +8°C

Adverse Events Following Immunization (AEFI)

Common Reactions

Local

• Pain
• Swelling
• Redness

Systemic

• Fever
• Malaise
• Headache

Rare Reactions

• Anaphylaxis
• Severe allergic reactions
• Neurological complications

Benefits of vaccination greatly outweigh risks.

Vaccine Failure

Primary Vaccine Failure

No adequate immune response develops.

Causes:

• Improper storage
• Host immune defects

Secondary Vaccine Failure

Protection wanes over time.

Solution: Booster doses

Vaccine-Preventable Diseases

Disease	Vaccine
Tuberculosis	BCG
Polio	OPV/IPV
Diphtheria	DPT
Tetanus	DPT/Td
Pertussis	DPT
Measles	MMR/MR
Mumps	MMR
Rubella	MMR
Hepatitis B	Hepatitis B vaccine
Rabies	Rabies vaccine
HPV infection	HPV vaccine

Smallpox Eradication

Milestones

Year	Event
1796	Jenner develops vaccine
1967	WHO eradication program begins
1977	Last natural case
1980	WHO declares eradication

Smallpox remains the first and only human disease completely eradicated through vaccination.

Advantages of Vaccines

1. Prevent infectious diseases.
2. Reduce mortality and morbidity.
3. Provide long-term immunity.
4. Protect vulnerable populations.
5. Prevent epidemics and pandemics.
6. Reduce healthcare costs.
7. Contribute to disease eradication.

Limitations of Vaccines

1. Some require boosters.
2. Cold-chain dependent.
3. Rare adverse effects.
4. Not 100% effective.
5. Some pathogens mutate rapidly.

NEET Quick Revision Box

• Vaccine = biological preparation inducing immunity.
• Vaccination produces artificial active immunity.
• Edward Jenner = Father of Immunology.
• Smallpox vaccine = first successful vaccine.
• BCG protects against tuberculosis.
• OPV = live attenuated vaccine.
• IPV = killed vaccine.
• DPT contains diphtheria toxoid, tetanus toxoid, and pertussis antigen.
• Booster doses maintain immunity.
• Herd immunity protects the entire community.
• WHO estimates immunization prevents 3.5–5 million deaths annually.
• Smallpox is the only human disease eradicated through vaccination.
• Vaccines induce immunological memory through the formation of memory B cells and memory T cells, enabling a faster and stronger secondary immune response upon subsequent exposure to the pathogen.

National Immunization Schedule (India)

The following table summarizes the Universal Immunization Programme (UIP) schedule from birth to adolescence, which is the schedule most commonly tested in medical entrance examinations.

Age	Vaccines Given
At Birth	BCG, OPV-0 (Oral Polio Vaccine), Hepatitis B Birth Dose
6 Weeks	OPV-1, Pentavalent-1 (DPT + Hepatitis B + Hib), Rotavirus Vaccine (RVV)-1, fIPV-1 (Fractional Inactivated Polio Vaccine), PCV-1 (Pneumococcal Conjugate Vaccine)
10 Weeks	OPV-2, Pentavalent-2, RVV-2
14 Weeks	OPV-3, Pentavalent-3, fIPV-2, RVV-3, PCV-2
9–12 Months	MR-1 (Measles-Rubella), JE-1 (Japanese Encephalitis – endemic areas only), PCV Booster, Vitamin A (1st dose)
16–24 Months	MR-2, JE-2 (endemic areas only), DPT Booster-1, OPV Booster, Vitamin A (2nd dose)
2–5 Years	Vitamin A doses 3–9 (every 6 months until 5 years of age)
5–6 Years	DPT Booster-2
10 Years	Td (Tetanus + Adult Diphtheria)
16 Years	Td (Tetanus + Adult Diphtheria)
Pregnant Woman	Td-1, Td-2 (or Td Booster if adequately immunized within previous 3 years)

High-Yield Facts

Vaccine	Age
BCG	Birth
Hepatitis B Birth Dose	Within 24 hours of birth
First MR Dose	9–12 months
Second MR Dose	16–24 months
OPV Booster	16–24 months
DPT Booster-1	16–24 months
DPT Booster-2	5–6 years
Td	10 years & 16 years
First Vitamin A Dose	9 months
Rotavirus Vaccine	6, 10, 14 weeks
Pentavalent Vaccine	6, 10, 14 weeks
PCV	6 weeks, 14 weeks, Booster at 9–12 months

One-Line Revision

BCG–OPV–HepB at birth → Pentavalent series at 6,10,14 weeks → MR at 9 months → DPT/OPV booster at 16–24 months → DPT booster at 5 years → Td at 10 and 16 years.