Principles of Vaccination (OCR A Level Biology)

• The principles underpinning vaccinations were discovered by Edward Jenner in the 1700s when he developed the first smallpox vaccine
• 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
• Vaccination programmes are offered to citizens by the government as a major component of health protection
  • The young in the population are given vaccines to protect them from diseases that were once common and caused serious harm
  • For example, in the UK babies are vaccinated against polio and measles

• A country may not have had any cases of a particular disease for several years however international travel means there is the possibility that a disease could be reintroduced at any time by travellers coming from other countries
• By having their citizens vaccinated against diseases governments can prevent serious epidemics from occurring

Effectiveness of vaccines

• Very few drugs are effective against viruses which is why vaccines are critical in controlling the spread of viruses
• 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 due to:
• People having 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
• Viruses having the capacity to change their surface antigens (the targets of vaccines) by:
  • Antigenic drift – over time there are small changes in the structure and shape of antigens (within the same strain of virus)

  • Antigenic shift – there are major changes in antigens (within the same strain of virus)
  • Antigenic concealment – the pathogen ‘hides’ from the immune system by:
    • Living inside cells
    • Coating their bodies in host proteins
    • Parasitising immune cells such as macrophages and T cells (eg. HIV)
    • Remaining in parts of the body that are difficult for vaccines to reach (eg. Vibrio cholerae – cholera, remains in the small intestine)

  • Cross-breeding – different strains of the virus invade the same cell, producing new viruses with antigens from different strains (essentially the strains swap antigens with each other)
    • The strains of influenza viruses that cause human influenza have been known to crossbreed with viruses that cause similar diseases in other animals
    • This crossbreeding can produce new strains of the human influenza virus that cause pandemics (as no individuals have immunity against them)
    • Every year the World Health Organisation (WHO) tries to provide information about strains that are likely to spread in order to aid government decisions and the development of flu vaccines

Herd immunity and ring 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
  • Governments will often vaccinate as many people as possible

• 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

• Ring immunity is another way by which mass vaccination programmes can work
• People living or working near a vulnerable (or infected) person are vaccinated in order to prevent them from catching and transmitting the disease
• The vaccinated individuals do not spread the pathogen onto others so those vulnerable individuals "within the ring" are protected as the people they interact with will not have the disease

The challenges of 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)

Types of 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 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

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