Herd immunity and herd effect: new insights and definitions

Area of Science:

• Epidemiology
• Public Health
• Immunology

Background:

• The concept of herd immunity is frequently used with varying definitions.
• Previous literature has identified but not corrected these definitional inconsistencies.

Purpose of the Study:

• To propose a precise definition for herd immunity.
• To introduce and define the term 'herd effect' for indirect protection.
• To differentiate between herd immunity and herd effect for improved public health strategies.

Main Methods:

• Defining herd immunity as the proportion of immune subjects in a population.
• Defining herd effect as the reduction in infection in the unimmunised due to immunisation.
• Proposing measurement methods for both herd immunity (immune parameter testing) and herd effect (incidence decline in unvaccinated).

Main Results:

• Herd immunity is defined as the proportion of immune subjects within a population.
• Herd effect is defined as the reduction of infection in the unimmunised segment resulting from immunisation.
• Herd immunity is applicable broadly, while herd effect is specific to human-transmitted infections and interventions reducing transmission.

Conclusions:

• Clarifying the definitions of herd immunity and herd effect is crucial.
• Understanding these concepts aids in designing effective immunisation programmes.
• Accurate definitions support disease control, elimination, and eradication efforts.