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
| Feature | Vaccines With Herd Effect | Vaccines Without Herd Effect |
|---|
| Key Examples | Oral poliovirus vaccine, Measles vaccine, Conjugated pneumococcal vaccine, Haemophilus influenzae type b conjugate vaccine. | Tetanus toxoid, Bacillus Calmette-Guérin vaccine, Diphtheria vaccine. |
| Primary Mechanism | Prevents infection and mucosal carriage. | Protects primarily against clinical disease without clearing carriage. |
| Epidemiological Reservoir | Humans act as exclusive or chief reservoir. | Environmental or non-human reservoirs frequently exist. |
| Public Health Impact | Protects unimmunized contacts via reduced transmission. | Protects exclusively the vaccinated individual. |
| Contagiousness | Highly contagious transmission pathways disrupted. | Non-contagious or transmission unaffected by vaccine. |
