Vaccination and herd immunity

Vaccination is a medical procedure that aims to provide immunity against infectious diseases. It involves introducing a vaccine into the body, which contains antigens from a specific pathogen (e.g., bacteria, virus).

• Antigens: These are molecules (often proteins or carbohydrates) found on the surface of pathogens that can trigger an immune response.
• Immune Response: When a vaccine is administered, the body's immune system recognises the antigens as foreign and begins to produce:
  • Antibodies: Specific proteins that bind to and neutralise the pathogen.
  • Memory Cells: Long-lived lymphocytes (B and T cells) that 'remember' the pathogen. If the body encounters the actual pathogen later, these memory cells enable a much faster and stronger secondary immune response, preventing the disease from developing.

Vaccination provides active artificial immunity, meaning the body actively produces its own antibodies and memory cells as a result of deliberate exposure to antigens (not through natural infection).

Vaccines work by mimicking a natural infection without causing the disease itself. This 'training' of the immune system prepares it to fight off future real infections.

Here's a step-by-step breakdown:

1. Antigen Presentation: The vaccine introduces antigens (e.g., weakened virus, bacterial toxin, or parts of a pathogen) into the body.
2. Immune Cell Recognition: Immune cells, such as macrophages and dendritic cells, recognise these antigens as foreign.
3. Activation of Lymphocytes: These antigen-presenting cells then activate specific B-lymphocytes and T-lymphocytes.
4. Antibody Production: Activated B-lymphocytes differentiate into plasma cells, which produce large quantities of specific antibodies that target the vaccine's antigens.
5. Memory Cell Formation: Both B and T lymphocytes also develop into memory cells. These cells persist in the body for years, sometimes decades.
6. Future Protection: If the vaccinated individual is later exposed to the actual pathogen, the memory cells rapidly recognise it and trigger a swift and robust secondary immune response, producing antibodies much faster and in greater amounts than the primary response. This neutralises the pathogen before it can cause illness.

This process is why vaccinated individuals are protected from diseases like measles, polio, and tetanus.

Different types of vaccines are designed to elicit an immune response in various ways:

• Live-attenuated vaccines: Contain a weakened (attenuated) form of the living pathogen. They provoke a strong, long-lasting immune response. Examples: Measles, Mumps, Rubella (MMR), Varicella (chickenpox).
  • Advantage: Strong, long-lasting immunity, often with a single dose.
  • Disadvantage: Cannot be given to immunocompromised individuals or pregnant women due to the risk of the weakened pathogen causing disease.
• Inactivated vaccines: Contain pathogens that have been killed, usually by heat or chemicals. They are less effective than live-attenuated vaccines and often require multiple doses (boosters). Examples: Polio (injectable), Influenza (flu shot), Hepatitis A.
  • Advantage: Safer for immunocompromised individuals as the pathogen is dead.
  • Disadvantage: Weaker immune response, requiring booster shots.
• Subunit vaccines: Contain only specific parts (subunits) of the pathogen, such as proteins or polysaccharides, that are highly antigenic. Examples: Hepatitis B, HPV, Pertussis (whooping cough, often part of DTaP).
  • Advantage: Very safe, as they contain no live components.
  • Disadvantage: May require adjuvants (substances that enhance the immune response) and multiple doses.
• Toxoid vaccines: Contain inactivated bacterial toxins (toxoids). These toxins are responsible for the disease symptoms. The immune system learns to neutralise the toxin. Examples: Tetanus, Diphtheria.
  • Advantage: Protects against the toxin's effects, not necessarily the bacteria itself.
  • Disadvantage: Immunity may wane over time, requiring boosters.

Herd immunity (also known as community immunity) occurs when a significant portion of a population becomes immune to an infectious disease, making its spread from person to person unlikely. This provides indirect protection to individuals who are not immune.

How it works:

• When a high percentage of the population is vaccinated (or has recovered from the disease), there are fewer susceptible individuals for the pathogen to infect.
• This reduces the chances of an infected person coming into contact with a susceptible person, thereby breaking the chain of transmission.
• The pathogen struggles to find new hosts, and its circulation within the community decreases significantly or even stops.

Importance of Herd Immunity:

• Protects Vulnerable Individuals: It protects those who cannot be vaccinated, such as:
  • Infants too young to receive certain vaccines.
  • Individuals with compromised immune systems (e.g., cancer patients undergoing chemotherapy, organ transplant recipients).
  • People with severe allergies to vaccine components.
  • Those for whom a vaccine is medically contraindicated.
• Disease Eradication: High levels of herd immunity are crucial for the potential eradication of diseases (e.g., smallpox has been eradicated, polio is close).
• Reduces Outbreaks: It prevents large-scale outbreaks, reducing hospitalisations and deaths.

The threshold for achieving herd immunity varies depending on the contagiousness of the disease (measured by its basic reproduction number, R0). Highly contagious diseases like measles require a very high vaccination rate (e.g., 95%) to achieve herd immunity, while less contagious diseases may require a lower percentage.