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At least 19 recordsLinked to original sources
PubMed2000

Herd immunity and herd effect: new insights and definitions.

The term herd immunity has been used by various authors to conform to different definitions. Earlier this situation had been identified but not corrected. We propose that it should have precise meaning for which purpose a new definition is offered: "the proportion of subjects with immunity in a given population". This definition dissociates herd immunity from the indirect protection observed in the unimmunised segment of a population in which a large proportion is immunised, for which the term 'herd effect' is proposed. It is defined as: "the reduction of infection or disease in the unimmunised segment as a result of immunising a proportion of the population". Herd immunity can be measured by testing a sample of the population for the presence of the chosen immune parameter. Herd effect can be measured by quantifying the decline in incidence in the unimmunised segment of a population in which an immunisation programme is instituted. Herd immunity applies to immunisation or infection, human to human transmitted or otherwise. On the other hand, herd effect applies to immunisation or other health interventions which reduce the probability of transmission, confined to infections transmitted human to human, directly or via vector. The induced herd immunity of a given vaccine exhibits geographic variation as it depends upon coverage and efficacy of the vaccine, both of which can vary geographically. Herd effect is determined by herd immunity as well as the force of transmission of the corresponding infection. Clear understanding of these phenomena and their relationships will help improve the design of effective and efficient immunisation programmes aimed at control, elimination or eradication of vaccine preventable infectious diseases.

Adult↗
PubMed1991

Herd immunity and the HIV epidemic.

Background. Herd immunity describes the collective immunocompetence of a population and its ability to resist disease. The diseases of mycobacteria, salmonella, hepatitis A, cryptosporidia, syphilis, measles, influenza, and numerous others recently have been seen in epidemic proportions in the United States. An association between these superimposed secondary infections and the human immunodeficiency virus (HIV) epidemic can be made since the HIV's imposition on individual immunity has ramifications on a population level through a decline in herd immunity. Conclusion. Exploring these epidemic phenomena as consequential to a reduction in herd immunity can provide a unifying hypothesis to explain existing and predict future infectious disease epidemic dynamics. The benefits of acting upon these implications has advantages for both the HIV infected and the uninfected.

Acquired Immunodeficiency Syndrome↗
PubMedDate not supplied

Economic evaluation of vaccination programs: the impact of herd-immunity.

The unique characteristic of vaccination is that it not only reduces the incidence of disease in those immunized but also indirectly protects nonvaccinated susceptibles against infection (produces herd-immunity). The bulk of economic evaluations of vaccination programs continue to use models that cannot take into account the indirect effects produced by herd-immunity. Here, the authors illustrate the importance of incorporating herd-immunity externalities when assessing the cost-effectiveness of vaccination progams. To do this, they compare 2 methods of estimating the benefits of routine mass vaccination: one that includes herd-immunity (dynamic approach) and one that does not (static approach). Finally, they use the results to clarify a number of misconceptions that are common in the literature concerning herd-immunity and dynamical effects produced by models.

Adolescent↗
PubMed1997

Simulation analysis of the effect of herd immunity and age structure on infection of a cattle herd with bluetongue viruses in Queensland, Australia.

A state-transition model based on Leslie matrix formulation was used to investigate the effects of herd immunity and age structure on the infection of a simulated cattle herd with bluetongue viruses under Australian climatic conditions. Increasing duration of immunity decreased the prevalence of infection. A duration of immunity of 33 months was consistent with prevalence estimates made from previous serological studies of bluetongue virus. Herd prevalence displayed slowly dampening cyclical variation over time (most pronounced when a short duration of immunity was simulated). Increasing calving and mortality risk rates in the simulated herd increased prevalence, whereas increasing age at first calving decreased prevalence. Manipulation of calving rates had the greatest effect on the predicted prevalence of infection in the herd. Simulation of a number of herd-management scenarios suggested that management systems in which cattle are bred early and where high calving rates are achieved are likely to contribute to high levels of infection with bluetongue viruses. Results confirm the importance of management factors in influencing the prevalence of infectious diseases in animal populations.

Aging↗
PubMed2001

Herd immunity after vaccination: how to quantify it and how to use it to halt disease.

In comparison to unvaccinated individuals, vaccinated individuals have fewer clinical symptoms, reduced susceptibility and reduced infectivity. The first two effects of vaccination can mean that each vaccinated individual is protected against clinical symptoms. From experiments and field trials, the extent of individual protection can be determined by a statistical analysis of the resulting data. In addition, there is an effect of the vaccination on the populations in which one or more individuals are vaccinated. This effect on the population is due to the effects of vaccination on susceptibility and infectivity of the vaccinated individuals. The population effect is called herd immunity and is observed as a reduction in chance of becoming infected when being part of a population with some of the individuals vaccinated. Note that the protection by herd immunity applies to vaccinated individuals as well as to unvaccinated individuals. Thus, protection against disease can be achieved not only by vaccinating the individuals that have to be protected but also by vaccinating other individuals in the same population. Such an application of herd immunity is especially important in protecting farm animals. To plan and evaluate vaccination at the population level, the herd immunity needs to be quantified. It will be illustrated that it is possible, not only theoretically but also practically, to quantify herd immunity among farm animals with data from small-scale experiments as well as with data from field trials.

Animal Diseases↗
PubMedDate not supplied

[Status of herd immunity to poliomyelitis in children in the Moscow region].

The paper presents the results of 3-year observations of the status of herd immunity to poliomyelitis in 7 towns of the Moscow region. In individual years antibody for poliomyelitis virus of 3 types was found in 63-74% of the examined children and from 2.7 to 7.5% of children were triple-negative. Insufficient levels of herd immunity to poliovirus types I and III were found. Some factors are analysed which could have some influence on the decline of herd immunity.

Adolescent↗
PubMed2001

Evidence on the infectious etiology of childhood leukemia: the role of low herd immunity (Greece).

OBJECTIVE: Acute lymphoblastic leukemia (ALL) among children may be a rare outcome of a delayed non-specific infection in situations of overall low herd immunity. We evaluated the hypothesis as to whether newly diagnosed ALL cases, compared to their controls, are characterized by lower herd immunity, as reflected in a more seronegative spectrum to several agents, with the exception of a strongly positive response to a single infectious agent, assumed to trigger ALL. METHODS: The study included 94 incident cases of ALL, from all pediatric hematology-oncology units of Greece, and 94, matched for age and gender, controls hospitalized with minor non-infectious conditions. The past exposure to common infections was assessed using 10 serological markers. RESULTS: There was little evidence for an association of ALL with the serology of any of the studied infectious agents among the very young children. In contrast, among children aged 5 years or older, leukemia was inversely associated with seropositivity to Epstein-Barr virus, human herpes virus-6, Mycoplasma pneumoniae and parvovirus B19. CONCLUSIONS: Among children aged 5 years or older the risk of leukemia may be higher when the low herd immunity for several agents is challenged by late infection from an agent that, as a rule, would attack children at a younger age.

Case-Control Studies↗
PubMedDate not supplied

Herd immunity and measles.

The basic concept of herd immunity is directly applicable only under very special conditions. The agents of disease must be restricted to a single host species within which transmission occurs by relatively direct contact, and infection induces solid immunity. Also outbreaks must occur only in randomly mixing populations. In free-living populations, susceptibles are not distributed homogeneously but tend to cluster in subgroups defined by age and by such factors as ethnicity and socioeconomic status. The requisite for occurrence of epidemics, namely a large enough number of susceptibles in frequent contact with each other, exists in virtually all large populations, regardless of the total proportion of the population that is immune. Experience with measles illustrates these conditions. Total prevalence of immunity of greater than or equal to 90% in developing countries does not prevent annual epidemics among the susceptibles, most of whom are children younger than three years of age. Where vaccination is widely practiced, as in the United States since 1962, measles has continued to occur in poorly immunized subgroups that are characterized by low educational level and economic status, very young age, or religious beliefs forbidding acceptance of vaccine. Ultimate success of a systematic immunization program requires knowledge of distribution of susceptibles by age and subgroup and maximal effort to reduce the concentration of susceptibles throughout the community rather than aiming to reach any specific proportion of the overall population.

Child, Preschool↗
PubMed1998

Herd immunity to filarial infection is a function of vector biting rate.

Despite the existence of an impressive body of work on human immune responses against filarial infections, the occurrence of a protective response to infection remains unclear. Here, we use a combined modelling and comparative data analysis framework to address this issue for human infections with the filarial parasite, Wuchereria bancrofti. By analogy with previous work, the analysis involves the comparison of observed field patterns of infection with epidemiological patterns predicted by a mathematical model of parasite immunity. Unlike most other human helminths, which are transmitted by ingestion or dermal penetration, exposure to infection with lymphatic filariasis can be measured explicitly in terms of vector mosquito biting rates, thereby also allowing, probably for the first time, examination of the suggested role of exposure in generating herd immunity to macroparasites. Observed field patterns in this study were derived from 19 different published studies, which gave parallel estimates of community exposure rates and the corresponding age--prevalence patterns of infection, while predictions of the epidemiological impact of herd immunity were obtained using a catalytic model framework. The results provide the first conclusive evidence to date that variations in the observed age--prevalence patterns of infection in filariasis can be effectively explained by the occurrence of an exposure-driven acquisition of herd immunity. We discuss this result in terms of implications for the new World Health Organization-led initiative for the global control of this parasitic disease.

Animals↗
PubMedDate not supplied

Post mass-immunization measles outbreak in Taoyuan County, Taiwan: dynamics of transmission, vaccine effectiveness, and herd immunity.

OBJECTIVES: Analysis of national surveillance data and a seroepidemiologic investigation were conducted to elucidate the causes and epidemiologic characteristics of a measles outbreak in Taoyuan, Taiwan, 1994. METHODS: Measles cases were identified through a national surveillance system. Reported cases and their physician or school nurses were interviewed to trace additional suspect cases and were sampled for serologic diagnosis. Measles-specific IgG and IgM were assayed. A confirmed case was defined as being positive for measles IgM test but not having received measles vaccination within the previous 3 months. RESULTS: The outbreak began in Taoyuan City in December 1993 and continued to spread in primary schools and kindergartens, but caused only sporadic cases in neighboring towns. Among 42 confirmed cases, 15 (38%) were primary school children and 16 (38%) were kindergarten children. Among 24 confirmed cases with a vaccination record, 7 had one dose of vaccination, 4 had two doses of vaccination, and 13 (54%) were unvaccinated. The overall measles susceptible proportion at a kindergarten before the outbreak was 8.1% (17/209) and the overall measles cumulative incidence among susceptibles was 0.65 (11/17). CONCLUSIONS: A measles vaccination coverage of 82% with the first dose at 9 months of age and 63% with the second dose (measles, mumps, and rubella) at 15 months was inadequate to block measles virus circulation in Taoyuan City in 1994. The city center, with a growing population, represents a high risk as an epicenter for measles outbreaks. Measles outbreaks may occur in a school population with 92% herd immunity.

Antibodies, Viral↗
PubMed1999

[The concept of herd immunity applied to the evaluation of vaccination programs].

The article concerns the definition and ways of estimation of indicators of herd immunity as applied to the analysis of the efficiency of vaccination programmes. Some important factors are discussed which determine the proportion of vaccinated individuals in the population necessary for decrease of incidence and eventually for elimination of the disease from a population. The problems related to definition and calculation of herd immunity in populations non uniform in terms of individual risk of infection are discussed.

Communicable Disease Control↗
PubMedDate not supplied

Herd immunity to helminth infection and implications for parasite control.

Despite much research on immunological responses to helminth parasites, knowledge of the dynamic interplay between levels of herd immunity in humans and the rates of exposure, establishment and mortality of parasites remains limited. We describe here a simple mathematical model for the population dynamics of helminth infections which mirrors the development of a degree of acquired immunity within populations which are genetically heterogeneous with respect to immunological responsiveness. We interpret observed patterns in the age-specific intensity of infection and attempt to understand the possible effects of control measures based on chemotherapy and vaccination. Mass chemotherapy can, in some circumstances, reduce the level of herd immunity such that average worm burdens in the adult age classes rise above their precontrol levels. When certain individuals or groups are predisposed to heavy infection, selective or targeted drug treatment can have significantly greater impact than mass or random application. Conversely, model predictions suggest that effective parasite control by vaccination (if and when vaccines become available) is difficult to achieve in communities that are genetically heterogeneous in their ability to mount protective responses to infection.

Age Factors↗
PubMed1997

Measles antibodies and herd immunity in 20- and 40-year-old Norwegians.

The introduction of a measles vaccination programme in Norway in 1969 using one dose of vaccine, and since 1983 two doses, was followed by a substantial decrease in the incidence of the disease. Since 1992, the annual incidence has been less than 20 cases. Small clusters and outbreaks have occasionally been observed among military personnel and unvaccinated children. This paper describes a seroepidemiological investigation of the level of immunity among 1,188 military conscripts, aged 18-28 years (mean 20.7) compared with 695 healthy 40-year-olds. The conscripts had been offered measles vaccine in infancy, in some cases also at 12-13 years of age, but they had also been exposed to wild measles virus, since the virus continued to circulate many years after the vaccination had started. The measles immunity in this group is considered to indicate the immunity level among the first 5 cohorts offered measles vaccine in Norway. The 40-year-olds had grown up in a community with no measles vaccination. Their level of immunity gives an indication of the level finally obtained when there are no vaccinations, and thus of the level that would induce herd immunity against measles in the Norwegian population. The aims of the vaccination programme must be to obtain a corresponding immunity. The results of the investigation show that the percentages with measles antibodies in the respective groups were 92.3 and 98.1. The observation of measles outbreaks among young Norwegian conscripts, as well as reports from several countries on outbreaks in university and college settings with levels of seropositivity of even more than 95%, indicate that the seropositivity in the 20-year-old group may be too low to afford protection, especially when this group is living under close conditions. Consideration should be given to the need for an intensification of the existing vaccination programme to ensure that the protection level needed for herd immunity is reached.

Adult↗