Fitting the HIV Epidemic in Zambia: A Two-Sex Micro-Simulation Model

BACKGROUND: In describing and understanding how the HIV epidemic spreads in African countries, previous studies have not taken into account the detailed periods at risk. This study is based on a micro-simulation model (individual-based) of the spread of the HIV epidemic in the population of Zambia, where women tend to marry early and where divorces are not frequent. The main target of the model was to fit the HIV seroprevalence profiles by age and sex observed at the Demographic and Health Survey conducted in 2001. METHODS AND FINDINGS: A two-sex micro-simulation model of HIV transmission was developed. Particular attention was paid to precise age-specific estimates of exposure to risk through the modelling of the formation and dissolution of relationships: marriage (stable union), casual partnership, and commercial sex. HIV transmission was exclusively heterosexual for adults or vertical (mother-to-child) for children. Three stages of HIV infection were taken into account. All parameters were derived from empirical population-based data. Results show that basic parameters could not explain the dynamics of the HIV epidemic in Zambia. In order to fit the age and sex patterns, several assumptions were made: differential susceptibility of young women to HIV infection, differential susceptibility or larger number of encounters for male clients of commercial sex workers, and higher transmission rate. The model allowed to quantify the role of each type of relationship in HIV transmission, the proportion of infections occurring at each stage of disease progression, and the net reproduction rate of the epidemic (R(0) = 1.95). CONCLUSIONS: The simulation model reproduced the dynamics of the HIV epidemic in Zambia, and fitted the age and sex pattern of HIV seroprevalence in 2001. The same model could be used to measure the effect of changing behaviour in the future

Profile: Agincourt health and socio-demographic surveillance system.

The Agincourt health and socio-demographic surveillance system (HDSS), located in rural northeast South Africa close to the Mozambique border, was established in 1992 to support district health systems development led by the post-apartheid ministry of health. The HDSS (90 000 people), based on an annual update of resident status and vital events, now supports multiple investigations into the causes and consequences of complex health, population and social transitions. Observational work includes cohorts focusing on different stages along the life course, evaluation of national policy at population, household and individual levels and examination of household responses to shocks and stresses and the resulting pathways influencing health and well-being. Trials target children and adolescents, including promoting psycho-social well-being, preventing HIV transmission and reducing metabolic disease risk. Efforts to enhance the research platform include using automated measurement techniques to estimate cause of death by verbal autopsy, full 'reconciliation' of in- and out-migrations, follow-up of migrants departing the study area, recording of extra-household social connections and linkage of individual HDSS records with those from sub-district clinics. Fostering effective collaborations (including INDEPTH multi-centre work in adult health and ageing and migration and urbanization), ensuring cross-site compatibility of common variables and optimizing public access to HDSS data are priorities

VARIATIONS IN THE AGE PATTERN OF INFANT AND CHILD MORTALITY WITH SPECIAL REFERENCE TO A CASE STUDY IN NGAYOKHEME (RURAL SENEGAL)

Risks of death below age 5 years vary widely across human populations. This thesis investigates some environmental factors of--and sex differences in--the age pattern of mortality in infancy and childhood and analyzes the impact of this age pattern on Brass multipliers. The first chapter, based on a study of 456 national life tables, reviews variations in the pattern of mortality as it is affected by the level of development, climate and the sex composition of the population. An index of the pattern of infant and child mortality is, defined: RCIM, the ratio of child to infant mortality = (4 + e(,5) - e(,1))/(1 + e(,1) - e(,0)). Life expectancy is shown to be a major predictor of RCIM, with high mortality populations showing higher child mortality relative to infant mortality. Climate also has a strong impact on RCIM, with tropical areas having the highest ratio when controlling for life expectancy. Finally, females have a higher ratio than males as a result of their lower infant mortality. Chapter 2 investigates the relationship between the distribution of death by cause and regional pattern of mortality in 21 high-mortality populations. However this analysis is largely inconclusive because of the inaccuracy of data on causes of death. A second part of the dissertation (Chapters 3 and 4) is an indepth investigation of a rural area of Senegal (Ngayokheme). The pattern of mortality exhibits an outstanding tropical pattern, with extremely high child mortality. The major discrepancy between this and other patterns is the extremely high death rate at 18-35 months. Although this is the weaning period, there is no evidence that mortality is higher just after weaning. Mortality is also very high between 6 and 18 months which is related to a high prevalence from malaria. The study compares two periods (1963-1971 and 1972-1981) during which mortality below age 5 declines dramatically; however pattern of mortality is shown to remain the same. Comparison with other tropical areas is done. Sex differences in this area show specific features when compared to national life tables. Females have the same probability of dying below age 5 as males, but with a different structure: females have higher neonatal mortality, which is hypothesized to be related to low sex ratios at birth. They also have higher mortality from measles but lower mortality from respiratory diseases. . . . (Author\u27s abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UM

VARIATIONS IN THE AGE PATTERN OF INFANT AND CHILD MORTALITY WITH SPECIAL REFERENCE TO A CASE STUDY IN NGAYOKHEME (RURAL SENEGAL)

Risks of death below age 5 years vary widely across human populations. This thesis investigates some environmental factors of--and sex differences in--the age pattern of mortality in infancy and childhood and analyzes the impact of this age pattern on Brass multipliers. The first chapter, based on a study of 456 national life tables, reviews variations in the pattern of mortality as it is affected by the level of development, climate and the sex composition of the population. An index of the pattern of infant and child mortality is, defined: RCIM, the ratio of child to infant mortality = (4 + e(,5) - e(,1))/(1 + e(,1) - e(,0)). Life expectancy is shown to be a major predictor of RCIM, with high mortality populations showing higher child mortality relative to infant mortality. Climate also has a strong impact on RCIM, with tropical areas having the highest ratio when controlling for life expectancy. Finally, females have a higher ratio than males as a result of their lower infant mortality. Chapter 2 investigates the relationship between the distribution of death by cause and regional pattern of mortality in 21 high-mortality populations. However this analysis is largely inconclusive because of the inaccuracy of data on causes of death. A second part of the dissertation (Chapters 3 and 4) is an indepth investigation of a rural area of Senegal (Ngayokheme). The pattern of mortality exhibits an outstanding tropical pattern, with extremely high child mortality. The major discrepancy between this and other patterns is the extremely high death rate at 18-35 months. Although this is the weaning period, there is no evidence that mortality is higher just after weaning. Mortality is also very high between 6 and 18 months which is related to a high prevalence from malaria. The study compares two periods (1963-1971 and 1972-1981) during which mortality below age 5 declines dramatically; however pattern of mortality is shown to remain the same. Comparison with other tropical areas is done. Sex differences in this area show specific features when compared to national life tables. Females have the same probability of dying below age 5 as males, but with a different structure: females have higher neonatal mortality, which is hypothesized to be related to low sex ratios at birth. They also have higher mortality from measles but lower mortality from respiratory diseases. . . . (Author\u27s abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UM