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Immunisation and Herd Immunity

Biology (Year 12) - Pathogen Management Strategies

Ben Whitten

Immunisation is the act of protecting an individual from a particular disease through means of a vaccine, and it also describes the process of someone developing resistance to a specific disease. You may be asking, how do vaccines work? A vaccine stimulates active immunity, which is the production of specific antibodies in a susceptible host during its response to a specific pathogen. It also promotes the formation of memory cells. A serum-type vaccine can be used, however this type of immunity is passive and does not lead to any formation of antibodies or memory cells. An antibody is a special protein which is produced by white blood cells, and they react with/help make pathogens harmless to an organism. Antibodies are also called immunoglobulins, and they are produced by specialised white blood cells called B cells. Vaccination is the administration of a vaccine into the bloodstream for the purpose of triggering immunity, and it is usually done by injection. Immunisation is what happens in your body after you have a vaccination, as the vaccine stimulates your immune system so that it can recognise the disease and protect you from any potential future infection.

Vaccines help develop immunity via triggering a response to antigens, a part of a pathogen which exist on their surface. The pathogen may be dead or attenuated, therefore resulting in minor symptoms. Vaccines consist of a dead, weakened or inactive form of a particular pathogen, e.g., the vaccine may be a pathogen that has had its DNA or RNA removed to prevent pathogen replication but to still supply the necessary antigen to trigger an immune response.

What are the types of immunity? You don't need to know these in particular detail, however there are four main types of immunity:

  1. Active natural immunity (e.g. infection)

  2. Active artificial immunity (e.g. vaccination)

  3. Passive natural immunity (e.g. maternal antibodies)

  4. Passive artificial immunity (e.g. monoclonal antibodies)

What is herd immunity? Herd immunity is the phenomenon that once a particular proportion of a population is immune to a particular disease, susceptible individuals are also better protected from the specific disease. When a high threshold percentage of a population is vaccinated, it is highly difficult for infectious diseases to spread from individual to individual. Picture this; you have measles, and you are surrounded by people who are all vaccinated against measles. The disease cannot be passed on to anyone around you, and it will quickly disappear again as there are too few susceptible hosts for the pathogen to be transmitted to. The lack of susceptible hosts provides protection to vulnerable individuals in a community such as newborns, individuals with compromised immune systems and the elderly. It is important to note, that not all vaccine-preventable diseases are not protected by herd immunity. Tetanus for example, which as you may know has its reservoir in soil, is not preventable by herd immunity. It is infectious, but not contagious. What are the main principles of herd immunity?

  1. A high proportion of the population becomes immune to the disease

  2. Immunity through vaccines or catching disease naturally causes the formation fo specific antibodies and memory cells

  3. Leads to few susceptible people catching the disease, lowering the pathogen population

  4. Infected individuals are more likely to have contact with immune individuals, reducing transmission risk and susceptible people risk

  5. The higher the proportion of immune people, the greater the protection for all individuals

  6. Those who cannot be vaccinated are protected (e.g. those who may go into anaphylaxis following a vaccine)

  7. The proportion of immunity required to achieve herd immunity depends on the virulence of the pathogen

Image: Community Immunity image, Image by National Institute of Health (NIH), Sourced Under a Creative Commons 4.0 License from Wiki Commons

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