herd immunity

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herd im·mu·ni·ty

the resistance to invasion and spread of an infectious agent in a group or community, based on the resistance to infection of a high proportion of individual members of the group; resistance is a product of the number susceptible and the probability that susceptibles will come into contact with an infected person.
Synonym(s): group immunity

herd immunity

Etymology: ME, heord, group; L, immunis, free from
the level of disease resistance of a community or population.

herd immunity

A measure of the transmission potential of a disease, which corresponds to the number of secondary infections produced by a typical case of an infection in a population that is totally susceptible, and can be quantified by counting the number of secondary cases following the introduction of an infection into a totally susceptible population.

Herd immunity occurs when a significant proportion of the population (or the herd) have been vaccinated, and this provides protection for unprotected individuals. The larger the number of people who are vaccinated in a population, the lower the likelihood that a susceptible (unvaccinated) person will come into contact with the infection. It is more difficult for diseases to spread between individuals if large numbers are already immune, and the chain of infection is broken.

The herd immunity threshold is the proportion of a population that need to be immune in order for an infectious disease to become stable in that community. If this is reached—for example due to immunisation—then each case leads to a single new case and the infection will become stable within the population, that is, R=1. If the threshold is surpassed, then R<1 and the disease will die out.

For an epidemic to occur in a susceptible population, R0 must be >1 for the number of cases to increase. If the R0 is < 1 the number of cases decreases, as occurs when a new vaccine is introduced.

This is an important measure used in infectious disease control and immunisation and eradication programmes.

Factors affecting R0
• Rate of contacts in the host population;
• Probability of infection being transmitted during contact;
• Duration of infectiousness.

herd im·mu·nity

(hĕrd i-myū'ni-tē)
The resistance to invasion and spread of an infectious agent in a group or community, based on the resistance to infection of a high proportion of individual members of the group.

herd immunity

resistance of a population to a particular PATHOGEN due to IMMUNITY of a large percentage of that population to the pathogen.


a group of animals, usually cattle, or pigs, or related wild animal species, which live a collective life together. This may be a natural pattern of behavior or be imposed by a human operated management system.

herd abnormality
an abnormality detectable only by examination of epidemiological data, e.g. milk yield per hectare, conception rate to first service.
herd composition
includes bulls (where applicable), cows in milk, dry cows, heifers not yet calved, bred heifers, virgin heifers, yearlings, calves weaned and suckers or at foot. Called also herd structure.
herd diagnosis
a diagnosis made to fit a herd problem which may be, for example, a low reproduction rate, or wool yield, or win rate at the races.
dairy herd
herd used exclusively for milk production.
herd epidemic
an epidemic confined to one herd.
herd fertility control scheme
programs based on surveillance of all reproduction data and comparison of indexes with preset targets. Correction of inefficiencies may be implemented by the farmer but diagnosis of the cause and treatments and prophylaxes are largely the province of the veterinarian.
herd health program
a health management system based on periodic visits to the herd by a veterinarian to check the status of a series of identifiable health parameters including production, reproductive efficiency, mastitis prevalence, calf survival, cow culling and mortality rate, fecal egg counts. Superior programs also include production management so that genetics, nutrition, housing, disease control and financial management are coordinated in a wholefarm approach.
herd immunity
a level of resistance in a herd or flock which is sufficient to prevent the entry of a particular disease into, or its spread within, the herd. The resistance may be innate, a genetically based resistance, or acquired as a result of previous exposure to the particular agent or of vaccination. The general usage of the term relates to the prevention of spread of infection at an epidemic level. So that in a herd in which there are 70 to 80% of immune animals there may be sporadic cases but the prevalence is unlikely to be significant. The same comments apply to larger populations, e.g. a wild animal or companion animal population which is really not managed as a herd.
herd level test
test performed on the entire herd or an adequate sample of it.
rolling herd average (RHA)
the average milk production per herd per year based on the 12 months just finished. Upon completion of a new test record, usually at monthly intervals, the record for the same period of the previous year is deducted and the new record is added, then a new rolling 365-day average is calculated. RHSs are updated with each new test. See also moving average.
herd sampling
examination, either physical or clinical pathological, of a herd to determine the herd status in a particular epidemiological parameter.
herd size
a critical factor in planning for productivity efficiency. May be quoted as the number of animals of a particular age or stage of production, e.g. milking cows, assuming that other, usually young, stock are also carried on the farm.
herd structure
see herd composition (above).
herd udder health
status of the herd with respect to the prevalence of quarter infection, clinical mastitis, teat lesions.
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