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

Farlex Partner Medical Dictionary © Farlex 2012

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.

Medical Dictionary for the Health Professions and Nursing © Farlex 2012

herd immunity

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

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

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References in periodicals archive

Herd immunity is particularly important for shielding those groups in the population who are more vulnerable to communicable diseases, including individuals who:

Protecting the Herd

Killing mosquitoes--a strategy some countries have used to try to curb Zika's reach--could actually hinder herd immunity, letting the next epidemic strike sooner, the team's simulations suggest.

Zika peaking in Latin America: widespread infections to end soon--but virus will return

The researchers, whose work is published in the journal Science, estimated that infections from the mosquito-borne virus will become so widespread in affected countries that populations will develop what is called "herd immunity." This occurs when a high percentage of a population has become immune to an infection either through developing natural immunity or through vaccination, making a wider outbreak less likely.

Zika outbreak to end in 2 to 3 years, scientists predict

WASHINGTON -- The prevalence of human papillomavirus (HPV) types 16 and 18 has significantly decreased, but only among vaccinated women; national data show no evidence that the vaccine targeting them has conferred herd immunity or encouraged a shift toward lower-risk types.

HPV vaccine doesn't provide herd immunity or crossprotection

The original concept of herd immunity was coined by Dr.

Who calls the shots?

Disease flare-ups such as this are noteworthy, and to some people frightening, because it is generally assumed that there exists what is known as "herd immunity" (see discussion below) that should prevent such outbreaks.

The law and immunization in the United States

Likewise, virtually no one, including those who work in healthcare settings, properly understands the complicated multifactor effects of how accurate the antigen selection was for the current flu season, how many people it takes to elicit herd immunity in a region, the likelihood of infectivity from someone who is ill, and how the whole matrix of probabilities interplay to make certain areas resistant or susceptible.

Vaccine production is only half of our due diligence: addressing the conversation about risks and side effects of vaccination

"We also find weak evidence of herd immunity effects, whereby hospitalization rates are reduced in adults.

Population-level data support ACIP flu vaccine advice

It has thus been decided to inject anti-polio vaccines to the susceptible children besides continuing with oral polio vaccination that provides herd immunity to the kids, said Jam Mehtab Hussain Dahar.

Marked decline in polio incidence leads to IPV introduction in Sindh

Herd immunity occurs when the vaccination rate is high enough in a community to protect unvaccinated individuals from infection.

A moral obligation: encouraging a community of immunity is a practical way for Catholic schools to teach kids to love their neighbors

carriage of the bacteria is important for understanding their ability to promote herd immunity."

University participates in study of two vaccines