Modeling the impact and cost-effectiveness of HIV prevention efforts

OBJECTIVES

This study explores how mathematical models can be used to examine the relationship between the effectiveness and costs of different prevention strategies. The analysis, based on a model designed to simulate the spread of HIV in sub-Saharan Africa, illustrates how the impact and relative cost-effectiveness of a prevention programme are sensitive to a number of factors including the rate of spread of HIV prior to the introduction of the programme, the measure used to evaluate the impact of the programme, and the point when the programme is introduced in the epidemic.

RESULTS

Assessing the impact and cost-effectiveness of different HIV prevention strategies is problematic. Not only are there difficulties in collecting the relevant data, but the impact of a prevention programme may be highly non-linear in character because of the many factors that determine the net rate of viral transmission. The long incubation period of HIV further complicates the analysis as the benefits from preventing a case of infection extend over a number of years.

CONCLUSIONS

The results highlight the need to collect more data on the impact and costs of prevention programmes if we are to ensure that the available resources for HIV prevention are to be used both efficiently and equitably.

Antigenic diversity and the transmission dynamics of Plasmodium falciparum

The average age of humans at their first infection with Plasmodium falciparum is typically less than 1 year in most endemic areas. This has been interpreted as evidence of the high transmissibility of the parasite, with the implication that control of malaria will require high levels of coverage with a potential vaccine. This interpretation is challenged by mathematical models that demonstrate that the long period required to develop immunity to malaria permits a high risk (or low average age) of infection even when parasite transmissibility is low. Patterns of seroconversion to five antigenically distinct isolates of P. falciparum in a highly malarious area of Papua New Guinea indicate that each is only mildly transmissible and that malaria, as a construct of several such independently transmitted strains, has a basic reproductive rate (or transmissibility) that is an order of magnitude lower than other estimates.

Incidental olfactory aplasia: a case report

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Density-dependent processes in the transmission of human onchocerciasis: intensity of microfilariae in the skin and their uptake by the simuliid host

The transmission success of Onchocerca volvulus is thought to be influenced by a variety of regulatory or density-dependent processes that act at various points in the two-host life-cycle. This paper examines one component of the life-cycle, namely, the ingestion of microfilariae by the simuliid vector, to assess the relationship between intake of larvae and the density of parasites in the skin of the human host. Analysis is based on data from three areas in which onchocerciasis is endemic and includes published information as well as new data collected in field studies. The three areas are: Guatemala (Simulium ochraceum s.l.), West and Central Africa (savanna members of the S. damnosum complex), and South Venezuela (S. guianense). The data record experimental studies of parasite uptake by flies captured in the field and fed to repletion on locally infected subjects who harboured varying intensities of dermal microfilarial infection. Regression analyses of log transformed counts of parasite burdens ingested by the flies plotted against log transformed counts of microfilariae per mg of skin revealed little evidence for saturation in parasite uptake by the flies as the intensity in the human host increased. There was a positive and highly significant rank correlation between both variables for the three blackfly species. In an alternative analysis a model was fitted to data on prevalence of flies with ingested microfilariae (mff) versus dermal mean intensities. The model assumed an overdispersed distribution of the number of mff/fly and a given functional relationship between intake and skin load. The results of both approaches were consistent. It is concluded that parasite ingestion by the vector host is not strongly density dependent in the three geographical areas and ranges of dermal loads examined. It therefore appears that this transmission process is of reduced importance as a regulatory mechanism in the dynamics of parasite population growth.

Evaluation of an activated patient diabetes education newsletter

This study evaluated a monthly, activated patient newsletter sent to over 7000 patients in Michigan with diabetes. The newsletter provided concise and action-oriented information about diabetes care. Patients who had signed up to receive the newsletter during the first 4 months of the project (1863) were surveyed to determine how many patients found the newsletter helpful; 80% (1498) of the patients replied. Patients who found the newsletter most helpful were older; had lower incomes, and reported more complications, less understanding of diabetes, and being in poorer overall health. They also were more likely to have non-insulin-dependent diabetes mellitus (NIDDM) than insulin-dependent diabetes mellitus (IDDM). We concluded that the activated patient newsletter is a useful public health/patient education intervention for persons with diabetes. Such a newsletter should be part of a coordinated system of ongoing patient care, education, screening, and social and psychological support.

Primary meningeal glioma

A 59 yr old man presented with headaches and was shown to have a posterior fossa tumor arising from the inferior surface of the tentorium cerebelli. At operation the tumor was discrete from the cerebellum and was thought to be a meningioma. Pathological examination showed the tumor had features similar to those of a meningioma. It consisted of interlacing bundles of spindle cells with a considerable amount of connective tissue. Some mitoses were present. The tumor cells, however, showed abundant staining for glial fibrillary acidic protein indicating their astrocytic nature. The tumor was diagnosed as astrocytoma Grade 2. The tumor 'recurred' 4 mths later and a second surgical removal was attempted. Pathological examination showed features similar to those in the first operative specimen but this time invasion of the cerebellum was present. Deep x-ray treatment (D.X.R.T.) did not alter the tumor growth which proved fatal 7 mths after presentation. The differential diagnosis of an apparently meningeal-based tumor includes the rare entity of primary meningeal glioma. The case is presented as an example of this rare entity which both clinically and pathologically may be mistaken for a meningioma. The prognosis of intracranial solitary primary leptomeningeal gliomas is variable with recurrence and survival being months to years.

Balancing sexual partnerships in an age and activity stratified model of HIV transmission in heterosexual populations

A mathematical model of the transmission dynamics of HIV-1 in a heterosexual population stratified by age, sex, and sexual activity (defined by rates of sexual partner acquisition) is presented. The model represents an extension of previous studies with a special focus on patterns of mixing or contact between sexual activity and different age classes of the two sexes. A range of mixing patterns between these groups is specified for both sexes. Mixing is described on two scales from fully assortative to fully disassortative, with random defined either according to numbers of sexual partnerships or numbers of people. The sexual partnerships in the model are balanced by changes in the rates of sexual partner acquisition between particular groups and a range of changes, from only women changing behaviour to only men changing behaviour, are analysed. The pattern of mixing is most influential in determining the shape and magnitude of the epidemic, but the manner in which people choose partners (i.e. dependent on numbers or proportions in the population) is also important. The relative importance of variation in transmission probabilities and mean rates of partner change on the course of the HIV epidemic is also illustrated. The analysis of the sensitivity of predictions to changing parameters in the force of infection term of the model provides a theoretical basis from which the influence of control strategies and the demographic effects of HIV can be analysed.

Pulse mass measles vaccination across age cohorts

Although vaccines against measles have been routinely applied over a quarter of a century, measles is still persistent in Israel, with major epidemics roughly every 5 years. Recent serological analyses have shown that only 85% of Israelis aged 18 years have anti-measles IgG antibodies. Considering the high transmissibility of the virus and the high level of herd immunity required for disease eradication, the Israeli vaccination policy against measles is now being reevaluated. Motivated by theoretical studies of populations in perturbed environments, we examined the possibility of replacing the conventional cohort vaccination strategy by a pulse strategy--i.e., periodic vaccination of several age cohorts at the same time. Numerical studies of a deterministic age-structured model suggest that vaccination, which renders immunity to no more than 85% of the susceptible children aged 1-7 years, once every 5 years will suffice to prevent epidemics in Israel, where infection rate is highest amongst schoolchildren. The model suggests that by using such a strategy the density of susceptible individuals is always kept below the threshold above which recurrent epidemics will be maintained. Analysis of simpler, non-age-structured, models serves to clarify the basic properties of the proposed strategy. Our theoretical results indicate that the advantages and disadvantages of a pulse strategy should be seriously examined in Israel and in countries with similar patterns of measles virus transmission.

Determinants of the demographic impact of HIV-1 in sub-Saharan Africa: the effect of a shorter mean adult incubation period on trends in orphanhood

Recent evidence suggests that the adult HIV-1 incubation period may be shorter in some sub-Saharan African populations than in Western populations. In this article we use mathematical-model-based simulations to show that, other things being equal, a shorter incubation period can result in smaller but more pronounced HIV-1 epidemics and faster, more acute, changes in demographic features, such as adult mortality, orphanhood and population structure. Empirical studies of orphanhood reveal similar patterns to those found in the simulations, but suggest that migration patterns and structural factors can give rise to greater concentrations of orphans in areas of relatively low HIV-1 prevalence.

Colloid cyst of the 3rd ventricle as a cause of acute neurological deterioration and sudden death

A 13 year old girl presented with a 24 h history of headache and vomiting. There were no focal neurological signs. The diagnoses considered were vascular headache, meningitis and subarachnoid haemorrhage. Lumbar puncture revealed clear cerebrospinal fluid under high pressure and subsequent cerebral computed tomography (CT) scan revealed hydrocephalus. Death occurred some hours after the lumbar puncture. Autopsy revealed a colloid cyst of the 3rd ventricle causing the hydrocephalus. Cerebellar tonsillar herniation was present. This case is reported because although 3rd ventricular colloid cysts are a recognized cause of acute neurological deterioration and sudden death, they rarely present in childhood. In this case, it is considered that lumbar puncture may have hastened death by increasing brain-stem compression due to cerebellar tonsillar herniation. It raises the question of whether cerebral CT scan, where readily available, should be performed prior to lumbar puncture. Pathologically, colloid cysts may be easily missed if the brain is examined fresh as the cysts are fragile and have a tendency to be destroyed or fall out.

Factors controlling the spread of HIV in heterosexual communities in developing countries: patterns of mixing between different age and sexual activity classes

The paper describes the development and analysis of a mathematical model of the spread and demographic impact of HIV in heterosexual communities in developing countries. The model extends previous work in this area by the representation of patterns of mixing between and within different age and sexual activity classes in a two sex structure. Summary parameters are derived to represent different mixing patterns, ranging from assortative via random to disassortative, as are methods to ensure that particular mixing patterns between different age and sexual classes (stratified on the basis of rates of sexual partner change) meet constraints that balance the supply and demand for sexual partners as AIDS induced mortality influences the demographic structure of a population. Analyses of model behaviour rely on numerical methods due to the complexity of the mathematical framework, and sensitivity analyses are conducted to assess the significance of different assumptions and different parameter assignments. Simulated patterns of HIV spread across the two sexes and various age classes are compared with observed patterns in Uganda. The principle conclusion of the study is that the pattern of mixing between age and sexual activity classes, combined with the assumptions made to balance supply and demand between the sexes have a very major influence on the predicted pattern of HIV spread and the demographic impact of AIDS. The paper ends with a discussion of future needs in model development and data acquisition.

Immunological modulation and evasion by helminth parasites in human populations

Helminth parasites are highly prevalent in human communities in developing countries. In an endemic area an infected individual may harbour parasitic worms for most of his or her life, and the ability of these infections to survive immunological attack has long been a puzzle. But new techniques are starting to expose the diverse mechanisms by which these agents modulate or evade their hosts' defences, creating a dynamic interaction between the human immune system and the parasite population.

The relationship between diabetes-related attitudes and patients' self-reported adherence

This study involved 1202 patients who were placed into low adherence or high adherence groups based on their answers to questionnaires. The attitudes of each group were compared for a variety of adherence behaviors. Patients who reported high levels of adherence tended to have attitudes more in accord with diabetes experts. Members of the high adherence group strongly supported the need for special training for health care professionals who treat diabetes, favored team care, accepted the importance of patient compliance, acknowledged the seriousness of non-insulin-dependent diabetes mellitus (NIDDM), and recognized the relationship between glucose control and complications. Differences in attitudes between high- and low adherence groups were more prevalent for difficult adherence areas, eg, diet and exercise, than for easy adherence areas, eg, carrying sweets or diabetic identification. An understanding of patients' attitudes can help diabetes educators and patients develop realistic and relevant self-care plans.

Contact tracing and the estimation of sexual mixing patterns: the epidemiology of gonococcal infections

BACKGROUND AND OBJECTIVES

The concept of a core group of individuals who change sexual partners frequently has played an important role in the study of the epidemiology of sexually transmitted diseases (STDs). It is widely believed that the core group is important in the persistence of infection and that it provides a sensible target for control interventions. GOAL OF THIS STUDY AND STUDY DESIGN: Simple mathematical models are used to assess the significance of sexual mixing between core and noncore groups. Published data from contact tracing studies, which base the definition of the core group on gonorrhea incidence by area of residence, are used to derive estimates of the degree of mixing within and between groups.

RESULTS

Analyses reveal patterns of high mixing within core groups in the urban study sites, which suggests that within-group transmission ensures the persistence of gonococcal infection in the community as a whole.

CONCLUSIONS

An extension of the analysis of contact tracing data to facilitate the development of mathematical models of STD transmission is detailed.

A comparison of the diabetes-related attitudes of health care professionals and patients

The following study compares the diabetes-related attitudes of sampled health care professionals and patients with diabetes. Attitudes were measured with a revised version of the Diabetes Attitude Scale (DAS) which includes seven factors representing attitudes towards: (1) the need for special training in order to provide diabetes care; (2) patient compliance; (3) the seriousness of noninsulin-dependent diabetes (NIDDM); (4) the relationship between blood glucose levels and the complications of diabetes; (5) the impact of diabetes on the patient's life; (6) patient autonomy; and (7) team care. The highest levels of agreement among patients and professionals concerned the seriousness of NIDDM and the relationship between blood glucose control and the development of the complications of diabetes. The most striking finding of the study was that patients tended to express a significantly more judgmental, moralistic attitude toward patient behavior than did health care professionals.

Measles immunization strategies for an epidemiologically heterogeneous population: the Israeli case study

Although the vaccine against measles has been routinely applied over a quarter of a century, measles is still an active disease in Israel. The January 1991 outbreak caused high morbidity in infant and adolescent populations and high mortality, especially among nomad Bedouins in the southern region of the country. The Bedouins form a small fraction of the total Israeli population (ca. 2%), but it is thought that they may experience significantly higher rates of transmission than the majority group. In this work we use deterministic compartmental mathematical models to define the optimal immunization strategy for a population consisting of a majority group characterized by low transmission rates and a minority group characterized by high transmission rates; this study allows both for transmission differences between the two groups, and for possible differences in the average cost (or difficulty) in reaching individuals for vaccination. Our analysis shows that the optimal vaccination policy for such a population involves different strategies for the two groups: a smaller fraction is to be vaccinated in the minority group if transmission in this group is not much larger than in the majority group, whereas, if the difference in transmission is very large, a higher proportion is to be vaccinated in the minority group. The advantage of this non-uniform vaccination policy is that it involves vaccination of a smaller fraction of the total population (and costs less, if there are differential costs between the groups), as compared with the proportion vaccinated under the conventional uniform vaccination policy.(ABSTRACT TRUNCATED AT 250 WORDS)

No reason for complacency about the potential demographic impact of AIDS in Africa

Much uncertainty surrounds the likely demographic impact of AIDS in the worst afflicted regions of the developing world such as sub-Saharan Africa. Various research groups have published projections of future trends but these differ widely with respect to potential impact on net population growth rates. Pessimistic forecasts suggest that AIDS may reverse the sign of a 3% to 4% population growth rate before the establishment of HIV, over time periods of a few to many decades. Optimistic forecasts suggest a decline in population growth rates, but predict that a 3% growth rate before AIDS may be reduced by only about 50% over a period of a few decades. This paper reports new analyses of the demographic impact of AIDS, based on observed age-stratified prevalences of HIV-1 infection amongst women of child bearing age. It is assumed that the observed patterns reflect the final endemic state and the implications of this assumption for adult and infant mortality and female reproductive life expectancy are assessed. It is concluded that a variety of scenarios is possible, depending on the detail of assumptions concerning life expectancy before the arrival of AIDS, the incubation period of the disease, and the rate of vertical transmission. Realistic assumptions do suggest, however, that the observed prevalences of infection in the worst afflicted regions of sub-Saharan Africa are sufficient to reduce current population growth rates to close to, or below, zero in the coming decades.

Dynamic interaction between Leishmania infection in mice and Th1-type CD4+ T-cells: complexity in outcome without a requirement for Th2-type responses

The interaction between Leishmania parasites and Th1 cells is investigated using a simple mathematical model of immunological responses and parasite population growth within the host. The model generates patterns of resistance and susceptibility to infection that mirror observed trends in experimental infections of mice and of humans exposed to infection in areas of endemic transmission. The heterogeneity in outcome predicted by the model can arise either through differences in the values of the parameters that characterize the genetic background of the host or as a consequence of differences in the size of the infecting inoculum of the parasite. Detailed analyses of equilibrium states and of the time course of infection within a host suggest that a limitation in the availability of precursor T-cells, as a consequence of high levels of recruitment into the activated pool, may play a significant role in the progression of infection in susceptible hosts. A brief discussion is presented of the implications of model prediction for therapeutic intervention.

Influencing the attitudes of medical students toward diabetes. Results of a controlled study

OBJECTIVE

To determine the effect of two educational interventions on the diabetes-related attitudes of medical students.

RESEARCH DESIGN AND METHODS

We studied 67 junior and senior medical students who were participating in the University of Michigan Medical School's Family Practice elective clerkship. Students were assigned to one of two interventions. The first was a 1-wk living-with-diabetes behavioral simulation that involved injections, blood glucose monitoring, diet, exercise, and record keeping. The second intervention involved reading an autobiography about living with diabetes and viewing a videotape about the psychosocial impact of diabetes.

RESULTS

No differential impact was found between the two interventions. However, both interventions were followed by a modest positive change in the attitudes of the medical students (which were very positive to begin with) toward the importance of patient autonomy and the value of the team approach to diabetes care. The attitude gains persisted at follow-up for patient autonomy but returned to baseline for team care.

CONCLUSIONS

This study suggests that these two educational interventions resulted in modest increases in the already positive attitudes of medical students toward the importance of patient autonomy and team care in diabetes. However, because the study did not include a group that received no treatment, we cannot be certain on this point. The attitude gain related to team care did not persist at follow-up. These findings are consistent with classical attitude research, which suggests that attitudes are sensitive to influences such as these interventions, but that attitude changes may not persist when those influences are changed or withdrawn. We were not able to find a differential impact between the two interventions and suspect that the general nature of the DAS used as the dependent measure may not have been sensitive enough to capture such a differential impact.