Vaccination

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Key Knowledge:
  • Vaccination programs and their role in maintaining herd immunity for a specific disease in a human population


Vaccinations are an example of artificial active immunity (they contain antigens to trigger immunity, but do not cause disease)

  • Vaccines can limit pathogen spread and reduce the mortality rate or crippling effects associated with a particular disease
  • However, protection may not be lifelong (booster shots may be required) and some individuals may have adverse reactions


Vaccinations induce long-term immunity to specific pathogenic infections by stimulating the production of memory cells

  • A vaccine is a weakened or attenuated form of the pathogen that contains antigens but is incapable of triggering disease
  • The antigenic determinants in a vaccine may be conjugated to an adjuvant, which functions to boost the immune response
  • The body responds to an injected vaccine by initiating a primary immune response, which results in memory cells being made
  • When exposed to the actual pathogen, the memory cells trigger a more potent secondary immune response
  • As a consequence of this more potent immune response, disease symptoms do not develop (individual is immune to pathogen)


vaccination


The length of time a person is immune to infection following a vaccination depends on how long the memory cells survive for

  • Memory cells may not survive a lifetime and individuals may subsequently require a booster shot to maintain immunity


Herd Immunity

Vaccination confers immunity to vaccinated individuals but also indirectly protects non-vaccinated individuals via herd immunity

  • Herd immunity is when individuals who are not immune to a pathogen are protected from exposure by the large amounts of immune individuals within the community
  • Herd immunity is important for vulnerable populations that may not be able to be vaccinated – including the elderly, extremely young and immune compromised individuals


The proportion of the population required to be vaccinated to confer herd immunity will be dependent on a number of factors:

  • Disease spread will be dependent on the mode of transmission and period of infectivity (e.g. length of incubation period)
  • Population density and social behaviour patterns will also determine the relative risk of exposure within a population
  • For a moderately contagious, airborne pathogen (such as coronavirus), experts may aim for 80-90% immunity levels


herd immunity


Extension:  Small Pox

Vaccination programmes can be used to eliminate a disease within a region, or even eradicate it worldwide

  • Smallpox was targeted by the World Health Organisation in 1967 and declared officially eradicated in 1980


The eradication of smallpox by vaccination was successful for a number of reasons:

  • Smallpox was easily identifiable due to overt clinical symptoms, which helped to limit potential transmission
  • Transmission only occurred via direct contact and there were no animal vectors or reservoirs to sustain the infectious agent
  • The infection period was short lived (3 – 4 weeks) and the virus was stable and didn’t mutate into alternate strains
  • There was global cooperation and immunity was long-term so repeated booster shots were unnecessary


 
Ted Talks: How we Conquered the Deadly Smallpox Virus