Herd Immunity

Definitions

• Herd immunity refers to proportion of subjects possessing immunity within a given population.
• Herd effect signifies reduction of infection or disease in unimmunized segment resulting from immunizing a population proportion.
• Herd immunity threshold denotes minimum population proportion requiring immunization to eliminate disease.
• Herd protection describes unimmunized individuals remaining protected within a herd due to protection rendered by immunized members.
• Herd immunity results from immunization or infection transmitted from human to human.
• Herd effect results from immunization or health interventions reducing transmission probability.

Measurement Modalities

• Herd immunity measured by testing population samples for specific immune parameters.
• Herd effect measured by quantifying disease incidence decline in unimmunized segments following immunization program institution.

Pathophysiology And Mechanisms

• Contagious disease transmission chains experience disruption when large numbers become immune or less susceptible.
• Simultaneous vaccination protects large proportion of susceptible individuals.
• Transmission chain breaks due to reduced carriage of microorganisms by vaccinated individuals.
• Risk of disease decreases significantly even among unimmunized individuals.
• Strategy proves essential for eradicating poliovirus and controlling measles epidemics.
• Finland successfully eliminated poliomyelitis when three-dose inactivated poliovirus vaccine coverage reached 51 percent.

Factors Influencing Herd Effect

• Phenomenon requires effective vaccine preventing infection rather than merely preventing clinical disease.
• Demands diseases where humans serve as exclusive source or chief reservoir.
• Efficacy of vaccine directly impacts magnitude of herd effect.
• Concept remains strictly inapplicable to infectious but non-contagious diseases.
• Vaccines possessing historically low protective efficacy demonstrate blunted herd effect.

Vaccine Profile Comparison