How do vaccines protect individuals and populations from disease?
Active and passive immunisation, the use of antigens and adjuvants in vaccines, the establishment of herd immunity and the herd immunity threshold, and the difficulties posed by antigenic variation and by vaccine uptake.
An SQA Higher Human Biology answer on immunisation, covering active and passive immunity, the antigens and adjuvants used in vaccines, the establishment of herd immunity and the herd immunity threshold, and the challenges of antigenic variation and of achieving high vaccine uptake.
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What this dot point is asking
The SQA wants you to explain active and passive immunisation, describe how vaccines use antigens and adjuvants to produce immunity, explain herd immunity and the herd immunity threshold, and describe the difficulties caused by antigenic variation and by low vaccine uptake.
Active and passive immunisation
Active immunisation (vaccination) uses a vaccine that contains antigens from the pathogen in a form that does not cause the disease (for example, a weakened or dead pathogen, or part of it). The antigens trigger a specific immune response, so the body makes antibodies and, crucially, memory cells. Because memory cells survive, the protection is long-lasting.
Passive immunisation instead gives the person ready-made antibodies. This provides immediate protection, useful after a dangerous exposure, but because the person does not make their own memory cells, the protection is short-lived.
How vaccines work
This is exactly the immunological memory of the specific immune response, used deliberately to protect against a disease without the person having to catch it first.
Herd immunity
The herd immunity threshold is the percentage of the population that must be immune for herd immunity to take effect. The more infectious the disease, the higher this threshold. Herd immunity is important because it protects people who cannot be vaccinated, such as very young babies or those with weakened immune systems.
Difficulties with immunisation
Two main problems make immunisation harder:
- Antigenic variation. Some pathogens (such as the influenza virus) regularly change their antigens. Memory cells made against the old antigens no longer recognise the new ones, so immunity is lost and the vaccine must be updated, which is why a new flu vaccine is needed each year.
- Low vaccine uptake. If too few people are vaccinated, the population stays below the herd immunity threshold, and outbreaks can occur. Uptake can fall because of public concern about safety, complacency, or difficulty accessing the vaccine.
Examples in context
Example 1. The yearly flu vaccine. Because the influenza virus shows antigenic variation, last year's memory cells and vaccine no longer match this year's strains. A new vaccine with updated antigens is produced each year, illustrating the challenge antigenic variation poses.
Example 2. Protecting newborn babies. Very young babies cannot be fully vaccinated, but if enough of the surrounding population is immune, herd immunity stops diseases spreading to them. A fall in vaccine uptake removes this protection and outbreaks can reach the vulnerable.
Try this
Q1. State what an adjuvant does in a vaccine. [1 mark]
- Cue. It enhances the immune response to the antigen, making the vaccine more effective.
Q2. Explain why a more infectious disease has a higher herd immunity threshold. [1 mark]
- Cue. Because it spreads more easily, a greater proportion of the population must be immune to stop it spreading.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20194 marksExplain how a vaccine produces long-term immunity, and state the role of an adjuvant in a vaccine.Show worked answer →
A 4-mark answer needs the immune mechanism of vaccination and the role of the adjuvant.
A vaccine contains antigens from a pathogen, in a form that does not cause the disease. When the vaccine is given, the antigens trigger a specific immune response: the matching lymphocytes are selected and multiply, producing antibodies and, importantly, memory cells.
The memory cells survive long term. If the person later meets the real pathogen, the memory cells produce a fast, strong secondary response that destroys it before illness develops, giving long-term immunity.
An adjuvant is a substance added to a vaccine to enhance the immune response to the antigen, making the vaccine more effective. Award (1) vaccine contains antigens, (2) these trigger a specific response producing memory cells, (3) memory cells give a fast secondary response on later exposure, and (4) an adjuvant enhances the immune response to the antigen.
SQA Higher 20214 marksExplain what is meant by herd immunity and the herd immunity threshold, and describe one reason a vaccination programme might fail to achieve it.Show worked answer →
This is a 4-mark question on herd immunity.
Herd immunity is the protection of a population that occurs when a large enough proportion of people are immune (usually through vaccination). Because so few people can catch and spread the pathogen, even the non-immune individuals are protected, as the pathogen cannot spread easily.
The herd immunity threshold is the percentage of the population that must be immune for herd immunity to take effect. The more infectious the disease, the higher this threshold.
A programme can fail to reach the threshold if vaccine uptake is too low, for example because of public concern about safety or difficulty accessing the vaccine. Antigenic variation can also undermine it. Award (1) herd immunity protects the population when many are immune, (2) non-immune individuals are protected because spread is reduced, (3) the threshold is the percentage immune needed, and (4) a valid reason such as low uptake or antigenic variation.
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