This memory aid covers Memory Aids within Vaccination and Herd Immunity for GCSE Biology. How vaccines work, types of vaccines, population immunity, vaccination programs It is section 12 of 15 in this topic. Use it for quick recall, then test yourself straight afterwards so the memory aid becomes usable in an answer.
Topic position
Section 12 of 15
Practice
18 questions
Recall
21 flashcards
Memory Aids
Vaccination timeline — five steps:
- 1. Inject — vaccine antigens introduced into body
- 2. Recognised — B cells with matching receptors are activated
- 3. Response — primary immune response occurs; antibodies produced
- 4. Remember — memory B cells formed and stored
- 5. Ready — on real exposure, rapid secondary response prevents disease
Herd immunity threshold formula: Threshold = 1 - (1/R₀). Example: measles R₀ = 15. Threshold = 1 - 1/15 = 1 - 0.067 = 0.933 = 93.3%. More contagious disease → higher R₀ → need higher coverage. Less contagious → lower R₀ → lower coverage sufficient.
Live vs killed vaccines — SILK:
- S — Stronger immunity (live)
- I — Immunocompromised cannot have live vaccines
- L — Lasts longer (live vaccines usually need fewer boosters)
- K — Killed (inactivated) vaccines are safer but weaker
Jenner connection: Jenner noticed milkmaids with cowpox did not get smallpox. Cowpox antigens are similar enough to smallpox antigens that antibodies/memory cells against cowpox also recognise smallpox. This is cross-immunity — the founding principle of vaccination.
Quick Check: The measles virus has a basic reproduction number (R₀) of approximately 15. Calculate the herd immunity threshold for measles and explain why achieving this level of vaccination is so important for protecting babies under 12 months who cannot yet be vaccinated.
Threshold = 1 - (1/R₀) = 1 - (1/15) = 1 - 0.0667 = 0.9333 = approximately 93.3%. This means about 93-95% of the population must be immune to prevent measles outbreaks. Babies under 12 months cannot receive the MMR vaccine because maternal antibodies (transferred before birth) would neutralise the vaccine antigens, preventing an immune response. Until they are vaccinated at 12 months, they are completely vulnerable to measles. If vaccination coverage in the surrounding population exceeds 93%, the virus cannot spread efficiently — even if it enters the population, it encounters immune individuals before reaching the unvaccinated baby. If coverage falls below this threshold, outbreaks occur and these babies are at serious risk, since measles in infants can cause severe complications including pneumonia and brain damage.
Quick Check: Explain why a live attenuated vaccine for influenza would be expected to produce stronger, longer-lasting immunity than a killed (inactivated) influenza vaccine.
A live attenuated vaccine contains weakened but living virus that can replicate briefly in the host. Because the virus replicates, it presents antigens to the immune system over an extended period and in a variety of contexts (surface proteins, internal proteins revealed during replication), generating a broad and sustained immune response. This closely mimics natural infection and produces large numbers of long-lived memory B and T cells. A killed vaccine contains non-replicating virus particles that present antigens only once and at a lower concentration. The immune response is typically weaker and shorter-lasting, which is why annual flu vaccination is needed and why some killed vaccines require adjuvants (immune-stimulating additives) or multiple doses to generate protective immunity.
Quick Check: In a population of 10,000 people, measles vaccination rates drop from 95% to 80%. Evaluate the likely public health consequences of this drop, using the concept of herd immunity threshold.
The herd immunity threshold for measles is approximately 95% (R₀ ≈ 15-18). When coverage was 95%, herd immunity was maintained — the virus could not spread efficiently because nearly every contact an infected person had was with an immune individual. At 80% coverage, 2,000 people are susceptible. The virus can now jump between susceptible individuals and sustain outbreaks. Vulnerable groups most at risk include: babies under 12 months (cannot be vaccinated yet), immunocompromised individuals (cannot be vaccinated), and the small percentage for whom the vaccine did not produce full immunity. An outbreak in a population of 10,000 with 80% coverage could infect hundreds of susceptible individuals. This analysis shows why even a relatively small decrease in vaccination rates below the threshold can have disproportionately large public health consequences.