Vaccination and immunisation

Vaccination

Receiving a vaccine (e.g., injection/oral dose)

Immunisation

Becoming immune as a result of vaccination

Immunological Memory

Rapid and robust immune response upon re-exposure to a pathogen

Vaccine Mechanism

• Mimics natural infection without disease.

• Induces active immunity + immunological memory.

• Goal: Elicit the same protective response as natural infection

Ideal vaccine characteristics

• Safe - no serious side effects

• stable

• Affordable

• Easy to administer

• long-term protection

• stops transmission - Induces mucosal & systemic immunity

• antigen presence

Vaccine Types: Live attenuated

Weakened pathogen e.g. MMR

Vaccine Types: Inactivated

Chemically or heat-inactivated pathogen e.g. Salk polio vaccine

Vaccine Types: Subunit

Purified parts of pathogen e.g. HBV surface antigen

Vaccine Types: Toxoid

Detoxified toxins e.g. tetanus

Vaccine Types: Viral vector

Modified viruses expressing pathogen genes e.g. AstraZeneca (ChAdOx1)

Vaccine Types: DNA

Plasmid encoding antigen e.g. experimental

Vaccine Types: mRNA

Encapsulated mRNA encoding antigen e.g. Pfizer, Moderna

Adjuvants

• Boost immune response

• Example: Alum (Aluminium hydroxide)

• Modes:

  • Slow antigen release

  • Create local inflammation

Poliomyelitis (Polio) serotypes

• Type I - Brunhilde

• Type II - Lansing - eradicated

• Type III - Leon

• Immunity to one serotype ≠ protection against others → vaccines must include all 3.

• affects mostly children

Polio vaccines

• OPV (Oral)

• IPV (Salk)

OPV

• Live attenuated

• Oral

• Cheap, mucosal + systemic immunity, herd immunity

• May revert, no use in immunocompromised

IPV

• Inactivated

• injection

• safe, cannot revert

• No mucosal immunity, costlier

Tetanus

• Caused by Clostridium tetani toxin

• Entry via dirty wounds

• Causes lockjaw, seizures, death

• Toxoid vaccine neutralizes toxin

HBV Vaccine

• Originally from infected blood → now made via recombinant DNA

• Expressed in yeast or CHO cells

• Safer, effective, cheape

Covid-19 vaccine

• ChAdOx1 (AstraZeneca): Chimpanzee adenovirus vector

• Encodes SARS-CoV-2 spike protein

• Triggers cellular and humoral response

DNA Vaccine

• Uses plasmid vector

• Goes to nucleus → mRNA → antigen

• Risk: genomic integration

RNA Vaccine

• mRNA in lipid nanoparticle

• Direct translation in cytoplasm

Primary vaccine failure

No initial immune response

Secondary vaccine failure

Immunity wanes over time

Herd immunity

• Enough immune individuals → disease cannot spread

• Protects vulnerable: infants, immunocompromised

Why No Vaccines for Some Diseases?

• HIV - Rapid mutation, antigenic diversity

• Parasites - Immune evasion, chronic infection

• Financial disincentive → impacts low-income populations

Challenges: Vaccine Hesitancy

• WHO: One of the Top 10 Global Health Threats

• Caused by misinformation, mistrust

• Reported in >90% countries