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Biomedical subjects

R M Anderson

Publications and source records attributed to R M Anderson.

At least 199 records · Page 11Linked to original sources
PubMed1992

Preliminary studies of sexual networks in a male homosexual community in Iceland.

In this article, we present the preliminary results of a study on sexual networks in a homosexual community in Iceland. The network data are transformed into a mixing matrix, which suggests that the pattern of sexual contact within the community is disassortative (i.e., sexual contacts occur largely between individuals of disparate sexual activity levels). The implications for the spread of human immunodeficiency virus (HIV) in the community are assessed by comparison with epidemics generated by proportionate and assortative (like with like) mixing, using a mathematical model of HIV transmission. The potential magnitude of the epidemic associated with the data is shown to be larger than both proportionate and assortative alternatives, although the early rate of growth is lower for the mixing matrix implied by the data. The assumptions made concerning behavioural changes triggered by alterations in population structure as a consequence of AIDS-induced mortality do not significantly alter the shape of the predicted epidemic. The problems in data acquisition and interpretation are discussed.

Adult↗
PubMed1991

Vaccine safety versus vaccine efficacy in mass immunisation programmes.

In community-wide immunisation programmes against childhood infections there is a conflict between the interests of the individual (vaccine safety and efficacy) and the interests of the community (vaccine uptake and level of herd immunity). Studies suggesting that the complication rate is greater with the high efficacy Urabe Am 9 mumps vaccine than with the lower efficacy Jeryl Lynn vaccine, have led to concern about whether the higher efficacy mumps vaccine should be introduced or retained in nationwide mass immunisation programmes. We describe the use of a mathematical model to assess benefits and risks to both individual and community, and illustrate this method by reference to immunisation programmes based on these vaccines. On the basis of current epidemiological data on viral transmission and vaccine coverage in England and Wales, data on vaccine-associated and infection-associated complication rates, and vaccine efficacies estimated from clinical trials, our analyses suggest there is little to choose between the two vaccines, but that overall performance depends on the level of vaccine coverage in a defined population. In community-based programmes, the greater apparent safety of the Jeryl Lynn vaccine (fewer vaccine-induced complications) is offset by the greater apparent efficacy of the Urabe Am 9 vaccine (fewer complications due to natural infection). The findings suggest that it may not always be in the interests of the community to use the vaccine with the lowest complication rate.

Age Factors↗
PubMed1991

Gonococcal infection and human fertility in sub-Saharan Africa.

An analysis is presented of the influence of Neisseria gonorrhoeae on human population growth in regions of sub-Saharan Africa where gonococcal infections are prevalent in sexually active adults. Combining epidemiological and demographic data within the framework of a mathematical model, we show that gonorrhoea has a major impact on fertility and, concomitantly, on net population growth in areas with a high prevalence of untreated infections. Specifically, a 20% prevalence in sexually active adults is predicted to induce a 50% reduction in net population growth. Model predictions are in good agreement with observed data from Uganda, and the sensitivity of the prediction to various complications, such as heterogeneity in sexual behaviour, is assessed. The analysis suggests that the predicted increase in fertility arising from expanded sexually transmitted disease (STD) control programmes in Africa to help combat the spread of human immunodeficiency viruses (HIV-1 and HIV-2) will help to offset the predicted demographic impact of AIDS in the worst afflicted areas. In other areas the rise in fertility associated with effective STD control will need to be countered by the linkage of STD control programmes with family planning initiatives.

Adolescent↗
PubMed1991

Antigenic diversity thresholds and the development of AIDS.

Longitudinal studies of patients infected with HIV-1 reveal a long and variable incubation period between infection and the development of AIDS. Data from a small number of infected patients show temporal changes in the number of genetically distinct strains of the virus throughout the incubation period, with a slow but steady rise in diversity during the progression to disease. A mathematical model of the dynamic interaction between viral diversity and the human immune system suggests the existence of an antigen diversity threshold, below which the immune system is able to regulate viral population growth but above which the virus population induces the collapse of the CD4+ lymphocyte population. The model suggests that antigenic diversity is the cause, not a consequence, of immunodeficiency disease. The model is compared with available data, and is used to assess how the timing of the application of chemotherapy or immunotherapy influences the rate of progress to disease.

Acquired Immunodeficiency Syndrome↗
PubMed1991

The spread of HIV-1 in Africa: sexual contact patterns and the predicted demographic impact of AIDS.

The spread of HIV-1 in Africa is examined here in the light of recent information on the main epidemiological and behavioural determinants of transmission. Mathematical models incorporating demographic, epidemiological and behavioural processes are used to assess the potential demographic impact of the disease AIDS. These analyses highlight the significance of patterns of sexual behaviour, and in particular networks of sexual contact, on the predicted spread of infection. Current data reveal substantial variations in the degree of spread between and in countries, but new analyses support earlier predictions that in the worst-afflicted areas AIDS is likely to change population growth rates from positive to negative values in a few decades.

Acquired Immunodeficiency Syndrome↗
PubMed1991

Potential of community-wide chemotherapy or immunotherapy to control the spread of HIV-1.

Whether zidovudine (3'-azido-3'-deoxythymidine, AZT) should be offered to symptomless individuals infected with human immunodeficiency virus type-1 (HIV-1), in the hope of delaying or even preventing progression to AIDS, has been much debated. The discussion has focused on the efficacy of the drug in delaying progression to disease, the severity of its side-effects, and the likelihood of its prolonged and widespread use resulting in zidovudine-resistant strains of the virus. Little attention has been given to the degree to which treatment reduces the infectiousness of symptomless patients, and to the concomitant implications for the overall transmission rate of HIV-1 in the community. Here we use simple mathematical models to show that community treatment with antiviral drugs or immunotherapies that lengthen the incubation period of AIDS without significantly reducing the infectiousness of treated individuals, can increase the rate at which HIV-1 infection spreads (which is fairly obvious) and can even, under certain circumstances, increase the AIDS-related death rate in the community (which is less obvious).

Acquired Immunodeficiency Syndrome↗
PubMed1991

Drugs, sex and HIV: a mathematical model for New York City.

A data-based mathematical model was formulated to assess the epidemiological consequences of heterosexual, intravenous drug use (IVDU) and perinatal transmission in New York City (NYC). The model was analysed to clarify the relationship between heterosexual and IVDU transmission and to provide qualitative and quantitative insights into the HIV epidemic in NYC. The results demonstrated the significance of the dynamic interaction of heterosexual and IVDU transmission. Scenario analysis of the model was used to suggest a new explanation for the stabilization of the seroprevalence level that has been observed in the NYC IVDU community; the proposed explanation does not rely upon any IVDU or sexual behavioural changes. Gender-specific risks of heterosexual transmission in IVDUs were also explored by scenario analysis. The results showed that the effect of the heterosexual transmission risk factor on increasing the risk of HIV infection depends upon the level of IVDU. The model was used to predict future numbers of adult and pediatric AIDS cases; a sensitivity analysis of the model showed that the confidence intervals on these prediction estimates were extremely wide. This prediction variability was due to the uncertainty in estimating the values of the models' thirty variables (twenty biological-behavioural transmission parameters and the initial sizes of ten subgroups). However, the sensitivity analysis revealed that only a few key variables were significant in contributing to the AIDS case prediction variability; partial rank correlation coefficients were calculated and used to identify and to rank the importance of these key variables. The results suggest that long-term precise estimates of the future number of AIDS cases will only be possible once the values of these key variables have been evaluated accurately.

Female↗
PubMed1991

Discussion: the Kermack-McKendrick epidemic threshold theorem.

The paper reviews the work of Kermack and McKendrick on the development of simple mathematical models of the transmission dynamics of viral and bacterial infectious agents within population of hosts. The focus of attention is centred on the notion of a threshold density of susceptible hosts to trigger an epidemic and recent extensions of this idea as expressed in the definition of a basic or case reproductive rate of infection. The main body of the paper examines recent developments of the basic Kermack-McKendrick model with an emphasis on deterministic models that describe various types of heterogeneity in the processes that determine transmission between infected and susceptible persons. Particular attention is given to the role of behavioural heterogeneity within the framework of a contact or mixing matrix which defines "who acquires infection from whom".

Acquired Immunodeficiency Syndrome↗
PubMed1991

Model-based comparisons of measles immunization strategies using high dose Edmonston-Zagreb type vaccines.

The World Health Organization has recently made a new recommendation for measles immunization in areas with very high rates of transmission. The new policy will involve a switch to the high potency Edmonston-Zagreb (E-Z) vaccine (a strain with improved efficacy in the presence of maternally-derived, specific antibody), targeting immunization at six-month-old infants. We present a mathematical model of measles transmission that can be used to assess the likely impact of cohort immunization on the incidence of infection using the new strain of vaccine. Data requirements of the model are discussed, with special reference to the critical issue of age-specific vaccine efficacy. Currently available information on seroconversion rates achieved with the new strain are summarized. Results are presented which endorse the new WHO recommendations, predicting considerable benefits, in particular, to those most at risk of measles morbidity and mortality. Furthermore, integration of E-Z with the third diphtheria, pertussis, tetanus (DPT) dose at three-four months should be considered as an alternative option, particularly if combined with some additional coverage at nine months of age.

Humans↗
PubMed1991

Sexual contact patterns between men and women and the spread of HIV-1 in urban centres in Africa.

A simple model of the transmission of HIV-1 by heterosexual contact and from mother to unborn infant is developed to assess the influence of patterns of mixing between low and high sexual activity classes of the two sexes on the pattern of spread of the virus and the demographic impact of AIDS. Numerical studies of model behaviour are based, where possible, on parameter estimates derived from epidemiological studies of HIV-1 spread in Africa. Analyses reveal that the assumed pattern of mixing, ranging from assortative (like with like) through random (proportional) to disassortative (like with unlike), has a very major impact on the predicted spread of the virus and the concomitant demographic impact of AIDS. Patterns of strong assortative mixing are predicted to generate the least spread and demographic impact, by comparison with proportional or disassortative mixing. Analyses also reveal that the rules governing behaviour changes, once AIDS-induced mortality changes the structure of the population (i.e. the numbers in the low and high sexual activity classes of the two sexes), have a very significant influence on the course of the epidemic. Where possible, predicted patterns are compared with observed trends in Africa.

Acquired Immunodeficiency Syndrome↗
PubMed1991

The challenge of translating scientific knowledge into improved diabetes care in the 1990s.

The problems associated with translating scientific knowledge into improved diabetes care are formidable but solvable. The American public has already made important changes in its health-related behavior. Millions of people have stopped smoking cigarettes, and many people have changed other behaviors to reduce their risk of cardiovascular disease and cancer. Many of the changes required to improve the quality of diabetes care in the 1990s will involve significant changes in our society's health-care policies and its health-related values and behaviors. Such changes, although often difficult to bring about, have the potential to have a profound and lasting impact on the health of our society. Many patients with diabetes have already implemented new care practices such as self-monitoring of blood glucose and multiple insulin injection regimens. As better technology and more evidence for the efficacy of rigorous blood glucose control become available, they will also be translated into practice. Diabetes care can be improved but to do so significant scientific and societal resources will have to be brought to bear on the problem of understanding and influencing the health-related behavior of people with diabetes and the health-care professionals and institutions providing care for them.

Demography↗
PubMed1991

Learning to empower patients. Results of professional education program for diabetes educators.

The patient empowerment approach to diabetes education is intended to enable patients to make informed decisions about their own diabetes care and to be fully responsible members of the health-care team. Facilitating patient empowerment requires a specific set of skills and attitudes on the part of diabetes educators. A professional education program designed to facilitate the acquisition and enhancement of the requisite skills and attitudes was designed, implemented, and evaluated. The program involved adhering to a simulated diabetes care regimen for 3 days followed by a 3-day intensive skills-based workshop. The 23 educators who participated in the first two offerings of this program made significant gains in their counseling skills and demonstrated a positive change in attitude.

Attitude↗