Scientific Basis of Vaccines

Question 1

Describe with examples of each, the main types of vaccines - live attenuated, killed whole-organism, sub-unit, and conjugated vaccines

Answer 1

Live Attenuated Vaccines:

• Uses a pathogen that is still ‘alive’ but its virulence is diminished
• These vaccines can trigger strong cellular and antibody responses, lifelong immunity with only one or two doses
• E.g. the MMR vaccine, yellow fever vaccine or oral polio vaccine (OPV)

Killed or Inactivated Vaccines:

• Prepared from microorganisms that have been killed or inactivated by heat or chemicals
• Safer than live vaccines but require multiple doses (boosters)
• E.g. injectable polio vaccine (IPV), the hepatitis A vaccine and rabies vaccine

Subunit Vaccines:

• Include only the essential pieces of the pathogen (like a specific protein or sugars on the surface) rather than the entire microorganism
• E.g. Hepatitis B vaccine
• A special type of subunit vaccine is a conjugate vaccine. In these vaccines, the subunits are chemically attached (conjugated) to a carrier protein, which enhances the immune response. E.g Hib vaccine
• This technique is often used when the subunit itself is not good at inducing an immune response, such as surface polysaccharides

Recombinant Vaccines:

• A type of subunit vaccine that are produced through genetic engineering
• A piece of the pathogen’s DNA is inserted into another cell or organism, produced proteins are harvested and purified to serve as vaccines
• E.g. HPV and mRNA vaccines for COVID don’t contain live viruses and instead use a small piece of the pathogen’s genetic material to induce an immune response

Question 2

Explain the need for boosting with sub-unit vaccines

Answer 2

A limitation of subunit vaccines is their reduced immunogenicity, or their ability to provoke an immune response therefore boosting is required:

Intensifying Immune Memory:

• Immunological memory refers to the immune system’s ability to remember and quickly eliminate pathogens that it has encountered previously
• Subunit vaccines, given their low immunogenicity, may not stimulate a sufficiently strong memory response with a single dose, necessitating booster shots
• After the booster shot, the number of memory cells (B and T lymphocytes) specific for the antigens in the vaccine increases
• Ensuring a swift and efficient immune response if the person encounters the disease-causing pathogen in the future

Affinity Maturation and Quality of Immune Response:

• After an immune response, some B cells remain to become memory B cells which have BCRs that can recognise the same antigen
• BCRs can have varying affinities to the antigen
• Affinity maturation allows B cells that produce higher-affinity antibodies to be selected for survival
• This process continues over time, meaning subsequent exposures (through booster shots) can lead to the production of antibodies with increasingly higher affinity for the antigen

Broadening the Immune Response:

• Boosters can also expand the scope of the immune response
• For instance, many viruses have several different strains. If the vaccine includes antigens from different strains, a single vaccination may not efficiently prompt immune responses against all these strains

Overcoming Immune Tolerance:

• The immune system becomes less responsive to an antigen after repeated exposure
• Booster shots present a high quantity of antigen to the immune system, thereby provoking a potent immune response despite tolerance
• Also provoke the maturation of naive T cells into effector T cells

Question 3

Explain the term herd immunity

Answer 3

Herd immunity is a form of indirect protection from infectious diseases that occurs when a large percentage of a population becomes immune to an infection

1) Basis of Herd Immunity:

• Spread of contagious diseases is hindered in a community where a large proportion of individuals are immune, either through previous encounters or vaccines
• These immune individuals act as a barrier, as the pathogen cannot infect them and thus less likely to be spread to others

2) Benefits of Herd Immunity:

• Protecting those who cannot be vaccinated, such as newborns, the elderly, and immunodeficient or when vaccine has not induced immunity

3) Threshold for Heard Immunity:

• Varies with each disease, i.e. against measles - 95%, as it is highly contagious. For polio, 80%

4) Role of Vaccination:

• Vaccines create immunity without causing illness thereby safely increasing the proportion of the population that’s immune

5) Challenges to Achieving Heard Immunity:

• Achieving herd immunity is not always possible. For some diseases, such as HIV and malaria, there are no effective vaccines
• Pathogens may mutate rapidly, meaning that the immunity acquired from last year’s strain may not protect against this year’s strain, as seen with the influenza virus

Question 4

Describe the main aims of vaccination programmes and explain how vaccines contribute to public health

Answer 4

Main Aims of the Vaccination Programs:

1) Prevention of Disease:

• Protect individuals from contracting diseases by stimulating an immune response that remembers the pathogen
• Vaccines prepare the body to fight off infections effectively if the individual is later exposed to the disease

2) Herd Immunity:

• When a large enough proportion of the population is immunised (through vaccines), it indirectly protect those who aren’t by reducing the chances of an outbreak
• Protecting those who have specific allergies, immune-compromised, elderly and newborns

3) Eradication of Diseases:

• E.g. Polio where widespread vaccination eradicated the disease entirely as they could not replicate

Contribution to Public Health:

1) Reducing Morbidity and Mortality:

• Vaccines have dramatically decreased the number of people getting sick or dying from diseases, increasing life expectancy

2) Cost-Effective Healthcare:

• By preventing diseases, vaccines reduce the cost of healthcare spending on treatments and hospitalisation

3) Prevention of Antibiotic Resistance:

• By reducing the number of bacterial infections, vaccines slow down the development of antibiotic resistance

4) Preserving Economic Stability:

• By preventing illness, vaccines help to maintain a productive workforce and reduce the economic burden of diseases

5) Promoting Health Equi