Herd immunity

Written by

in

 

NEET PG High-Yield: Herd Immunity

Herd immunity (community immunity) refers to the resistance of a population to the introduction and spread of an infectious agent, based on the high proportion of individuals who are immune.

Key Concept: Critical Threshold

The threshold level of immunity required to stop the spread of a disease is calculated using the Basic Reproduction Number (R_0):

Herd\ Immunity\ Threshold = 1 - \frac{1}{R_0}

Important Characteristics

Feature Description
Mechanism Reduces the probability that an infected person will encounter a susceptible person.
Vulnerable Groups Protects those who cannot be vaccinated (e.g., immunocompromised, infants).
Dynamic Nature Depends on the $R_0$ of the pathogen; a higher R_0 requires a higher immunity threshold.

High-Yield NEET PG Pearls:

  • Not Absolute: Herd immunity does not protect against diseases that are not transmitted person-to-person (e.g., Tetanus).
  • Examples: Measles (R_0 \approx 12-18) requires very high coverage (~95%); Polio requires lower coverage (~80%).
  • Limitation: It is a population-level protection measure; it does not replace the need for individual-level vaccination.