Use of vaccines to provide protection for individuals and populations; concept of herd immunity
A vaccine trains your immune system to fight a specific pathogen without you getting ill. When enough people in a population are vaccinated, even unvaccinated individuals gain protection — this is herd immunity.
Formula
Herd immunity threshold = 1 − (1 ÷ R₀)
Real World
The UK's measles herd immunity threshold is approximately 95% vaccination coverage — when coverage dropped to 80% following the discredited Wakefield MMR scare in the late 1990s, measles cases surged.
Exam Focus
When explaining herd immunity, state explicitly that unvaccinated individuals are protected because transmission chains are broken — do not just say 'fewer people get infected'.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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