Epidemiology & Public Health

Intermediate

Herd-Immunity Threshold

Minimum fraction of the population that must be immune to prevent sustained spread.

Formula

H = 1 - \frac{1}{R_0}

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Variables

HHerd-immunity threshold

R_0Basic reproduction number

Example

R₀ = 5 → H = 1 − 1/5 = 80 % must be immune

Did You Know?

For measles (R₀ ≈ 15), about 93–95 % of the population must be immunised — one of the hardest thresholds to maintain.

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More in Epidemiology & Public Health

Incidence Rate

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Prevalence

Proportion of a population that has a particular condition at a point in time.

Basic Reproduction Number

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Effective Reproduction Number

Actual average infections per case when part of the population is already immune.

SIR Model — Susceptible

Rate of change of the susceptible population in the SIR compartmental model.

SIR Model — Infected

Rate of change of the infected population — new infections minus recoveries.