During the years that the herd was being replenished by purchase abortions were frequent, but that practice was discontinued, and the heifer calves born in the herd have been raised. … Thus a herd immunity seems to have developed as a result both of keeping the aborting cows and raising the calves.

The Spread of Bacterial Infection. The Problem of Herd-Immunity [title]. … Consideration of the results obtained over the last five years, … has led us to believe that the question of immunity as an attribute of a herd should be studied as a separate problem, closely related to, but in many ways distinct from, the problem of the immunity of an individual host.

It is likely that many animals in affected herds have developed an inapparent or nonclinical form of the disease because there have been no records of recurrence of the disease on the same farm—an indication that a herd immunity developed.

The larger the number of immunised people in the community the less easy is the spread of disease from one person to another. … For the purpose of creating ‘herd immunity’, as this process is sometimes called, it is necessary to achieve 80 per cent immunisation of the community.

Jews, long exposed to some diseases in the confined quarters of European ghettos, had developed herd immunities through a long process of natural selection.

The disease incidence appeared to have a cyclic course. After an outbreak, the disease and virus disappeared and the herd immunity gradually waned in the next 2 to 3 years. A new outbreak was the consequence of the reintroduction of transmissible gastroenteritis virus (TGEV).

For herd immunity to occur, the vaccine must prevent transmission of the virus as well as prevent disease.

Some proponents of natural rearing condemn vaccinations; they prefer using homeopathic nosodes (medicine prepared from the diseased part or discharge of something, which supposedly works as well as vaccination). These people point to their dogs' good health as proof that nosodes work. However, their good fortune is probably the result of herd immunity, that is, as long as most dogs are vaccinated, the unvaccinated dogs rarely come in contact with the infectious agents.

Herd immunity is based on the notion that if a herd (a population or a group) is mostly protected from a disease by immunization then the chance that a major epidemic will occur is limited. Jonas Salk, one of the developers of the polio vaccine, suggested that if a herd immunity level of 85% exists in a population, a polio epidemic will not occur. Herd immunity is also viewed as the resistance a population has to the invasion and spread of an infectious disease.

Many examples of herd immunity have been described, illustrating the importance of indirect protection for predicting the short- and long-term impact of vaccination programs, for justifying them economically, and for understanding the nature of the immunity induced by various vaccines.

At above 80% immunity, the simulation indicates that no transmission can occur, even when mosquito densities are high (5 per person). Similar herd immunities (82–87%) halt transmisson of poliomyelitis and diphtheria. At lower herd immunities, transmission occurs at increasingly lower vector densities.

A few weeks ago, the U.K. had almost no social-isolation measures in place, and according to some reports, the government planned to let the virus run its course through the population, with the exception of the elderly, who were to be kept indoors. The idea was to let enough people get sick and recover from the mild version of the disease, to create "herd immunity." Things changed swiftly after an epidemiological model from Imperial College London projected that without drastic interventions, more than half a million Britons would die from COVID-19.

Herd immunity

Meaning

Noun

Synonyms