Herd immunity (or Community Immunity) Itis the level of resistance of a community or group of people to a particular disease. OR Group protection beyond that afforded by the protection of immunised individuals.
1854 – Measles epidemic in Faroe islands –(High attack rate & CFR) later epidemic wave declined with the build up of herd immunity Provides an immunological barrier to the spread of disease in the human herd It is considered best left for those who cannot safely receive vaccines because of a medical condition such as an immune disorder or for organ transplant recipients.
Elements Contributing to HerdImmunity1. Occurrence of clinical and subclinical infection in the herd2. Immunisation of the herd3. Herd structure
Herd structure Never constant Notonly includes the host belonging to the herd species but also animal host , insect vectors, social environmental factors that favour or inhibit the spread of infection from host to host Plays decisive role in the immune status of herd
Herdimmunity high occurrence of epidemic highly unlikely Thiscan be achieved by ongoing immunisation programme to reduce the susceptible number of person to a small proportion of the population elimination of disease in due course Achieved in case of Diphtheria ,Polio, Measles.
In case of small pox it was not herd immunityBut ,1. Elimination of source of infection2. Surveillance3. Containment measures With the abolition of vaccine against small pox ,its herd immunity will naturally decrease in course of time
A disease in which herd immunity does not protect the individual ???? Reason??
Theproportion of immune individuals in a population above which a disease may no longer persist is the herd immunity threshold
Estimated Herd Immunity thresholds for vaccine preventable diseases Diseases DiseaseTransmission R0 Herd immunity threshold Diphtheria Saliva 6-7 85% Measles Airborne 12-18 83 - 94% Mumps Airborne droplet 4-7 75 - 86% Pertussis Airborne droplet 12-17 92 - 94% Polio Fecal-oral route 5-7 80 - 86% Rubella Airborne droplet 5-7 80 - 85% Smallpox Social contact 6-7 83 - 85% R0 is the basic reproduction number, or the average number of secondary infectious cases that are produced by a single index case in completely susceptible population.
Herdimmunity may be determined by Serological epidemiology. Studies have shown that it is neither possible nor it is necessary to achieve 100% herd immunity in a population to hault an epidemic or control disease.