AQA A Level Biology复习笔记2.6.1 Vaccines

• A vaccine is a suspension of antigens that are intentionally put into the body to induce artificial active immunity. A specific immune response where antibodies are released by plasma cells
• There are two main types of vaccines:
  • Live attenuated
  • Inactivated

   

• Vaccines are administered either by injection or orally (by mouth)
• The vaccinations given by injection can be into a vein or muscle
• Vaccinations produce long-term immunity as they cause memory cells to be created. The immune system remembers the antigen when reencountered and produces antibodies to it, in what is a faster, stronger secondary response

Effectiveness of vaccines

• Vaccines can be:
  • Highly effective with one vaccination giving a lifetime’s protection (although less effective ones will require booster / subsequent injections)
  • Generally harmless as they do not cause the disease they protect against because the pathogen is killed by the primary immune response

   

• Unfortunately, there can be problems with vaccines:
  • People can have a poor response (eg. they are malnourished and cannot produce the antibodies – proteins or their immune system may be defective)
  • Antigenic variation – the variation (due to major changes) in the antigens of pathogens causes the vaccines to not trigger an immune response or diseases caused by eukaryotes (eg. malaria) have too many antigens on their cell surface membranes making it difficult to produce vaccines that would prompt the immune system quickly enough
  • Antigenic concealment – this occurs when the pathogen ‘hides’ from the immune system by living inside cells or when the pathogen coats their bodies in host proteins or by parasitising immune cells such as macrophages and T cells (eg. HIV) or by remaining in parts of the body that are difficult for vaccines to reach (eg. Vibrio cholerae – cholera, remains in the small intestine)

   

• The principles underpinning vaccinations were discovered by Edward Jenner in the 1700s when he developed the first smallpox vaccine

Live attenuated vaccines

• Live attenuated vaccines contain whole pathogens (e.g. bacteria and viruses) that have been ‘weakened’
• These weakened pathogens multiply slowly allowing for the body to recognise the antigens and trigger the primary immune response (plasma cells to produce antibodies)
• These vaccines tend to produce a stronger and longer-lasting immune response
• They can be unsuitable for people with weak immune systems as the pathogen may divide before sufficient antibodies can be produced
• An example of this type of vaccine is the MMR (Measles, Mumps and Rubella)

Inactivated vaccines

• Inactivated vaccines contain whole pathogens that have been killed (‘whole killed’) or small parts (‘subunit’) of the pathogens (eg. proteins or sugars or harmless forms of the toxins – toxoids)
• As inactivated vaccines do not contain living pathogens they cannot cause disease, even for those with weak immune systems
• However, these vaccines do not trigger a strong or long-lasting immune response like live attenuated vaccines. Repeated doses and/or booster doses are often required
• Some people may have allergic reactions or local reactions (eg. sore arm) to inactivated vaccines as adjuvants (eg. aluminium salts) may be conjugated (joined) to the subunit of the pathogen to strengthen and lengthen the immune response
• An example of a whole killed vaccine is the polio vaccine
• An example of a toxoid subunit vaccine (where inactivated versions of the toxins produced by pathogens are used) is Diphtheria

Herd Immunity

• Herd immunity arises when a sufficiently large proportion of the population has been vaccinated (and are therefore immune) which makes it difficult for a pathogen to spread within that population
• Those who are not immunised are protected and unlikely to contract it as the levels of the disease are so low
• It is very important as it allows for the individuals who are unable to be vaccinated (e.g. children and those with weak immune systems) to be protected from the disease
• The proportion of the population that needs to be vaccinated in order to achieve herd immunity is different for each disease
• If vaccination rates fall below the required level then herd immunity can break down
  • There was an outbreak of Measles in Swansea in 2012 due to a lack of vaccine uptake

   

Eradicating disease

• Eradicating disease presents a challenge
  • On one hand some pathogens are simply complicated and present with disease processes that are not straightforward and so a successful vaccine has not been developed
  • On the other hand, diseases that could be eradicated where a vaccine does exist, have not been eliminated because too few in the community have been vaccinated

   

• It has also been difficult to eradicate other infectious diseases due to:
  • Unstable political situations in areas such as Africa, Latin America and parts of Asia, perhaps resulting in civil unrest or wars
  • Lack of public health facilities (poor infrastructure, few trained personnel, limited financial resources)

   

Smallpox

• The eradication of Smallpox is a success story, but its success had specific reasons that cannot be universally replicated in the struggle to eliminate disease
• Smallpox is a highly contagious disease caused by a virus that exists in two forms: Variola minor and Variola major, the latter being the worst of the two, with a death rate of 12 to 30%
• Smallpox was transmitted by direct contact and caused red spots (which filled with pus) to cover the body. People who recovered were disfigured as a result of scabs that formed from these spots. It also affected the eyes resulting in permanent blindness for many who recovered
• The WHO began an eradication programme against Smallpox in 1967, stating their intention to eradicate the virus within ten years. The WHO did not declare smallpox eradicated until 1980
• The programme focused on:
  • Vaccination – the aim was to vaccinate more than 80% of populations at risk and if a case of smallpox was reported ring vaccination would occur (where everyone in the household with the reported case, the surrounding 30 households, relatives and anyone else who had contact would get vaccinated)
  • Surveillance

   

• Its success was attributed to:
  • The virus was stable – it did not mutate therefore its surface antigens did not change, therefore the same vaccine could be used worldwide which made it cheap to produce the vaccine
  • The vaccine was a ‘live attenuated’ one, being produced from a harmless strain of a similar virus
  • The vaccine could be transported without becoming unviable, as it could be freeze-dried and kept at high temperatures for up to 6 months, thus it was suitable for the tropics
  • The symptoms made it easy to identify infected people (surveillance was possible)
  • Humans being the only reservoirs of infection and there were no carriers making it easier to break the transmission pathway
  • The consistency of the effort, vaccination, surveillance and containment of all outbreaks on a global scale

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