What is herd immunity?
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When enough of a population is immune that transmission chains break, indirectly protecting those who can't be vaccinated. The threshold varies by disease contagiousness — around 95% for measles, lower for less transmissible infections.
It protects those who cannot be vaccinated — newborns, people on chemotherapy, those with immune deficiency — which is the main ethical argument for vaccination.
It only holds while coverage stays high. Measles outbreaks reappear quickly when coverage drops even a few percentage points below threshold.
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