Statistical models for prevalent cohort data.
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Acquired Immunodeficiency Syndrome↗
Biomedical subjects
P J Solomon
Publications and source records attributed to P J Solomon.
Estimates for different stages of HIV/AIDS disease.
Software for estimating current and future numbers in different stages of the natural history of HIV infection has been developed. This is based on a general multistage model for the natural history of HIV/AIDS disease, allowing for treatment effects. The multistage model has been incorporated into the general back-projection procedure. For illustration, Australian AIDS data are used here.
Acquired Immunodeficiency Syndrome↗
Predicting AIDS deaths and prevalence in Australia.
OBJECTIVE: To describe a method for predicting deaths from, and the prevalence of, the acquired immunodeficiency syndrome (AIDS) in Australia and to obtain ranges of projections for the next five years based on data observed to 30 September 1989 and reported by January 1990. The projections are important components of planning for future health-care needs. PATIENTS: All patients with AIDS in Australia who were known to the National Centre in HIV Epidemiology and Clinical Research. The data used in the analysis were date of diagnosis of AIDS, date of death if the patient had died, and State or Territory of diagnosis. RESULTS: For Australia as a whole, the doubling time of AIDS incidence changed from 0.83 years before mid 1987 to 4.34 years after that time. Five hundred and seventy patients were diagnosed with AIDS to 30 June 1987 of whom 487 had died by 30 September 1989 and the estimated mean survival time was 1.36 years. Of 1037 patients diagnosed with AIDS after June 1987, 352 had died and the estimated mean survival time was 2.42 years. Combining the estimated survival distribution with the projected new cases of AIDS, we forecast that there will be between 530 and 680 deaths from AIDS in Australia in the year mid 1991 to mid 1992. Estimated cumulative deaths to mid 1994 range from 3390 to 4250. We predict that there will be between 1370 and 1850 people living with AIDS during the year ending mid 1992, and that this number will increase to between 1760 and 2830 for the year ending mid 1994. We also predict that the prevalence of AIDS in New South Wales will lie between 820 and 1620 for the year ending mid 1994, and that the prevalence in Victoria will lie between 270 and 740 in the same year. CONCLUSIONS: Any value in the range of predictions for AIDS incidence we give is equally likely and there is good agreement with the data now observed from mid 1989 to mid 1990. The dramatic increase in the doubling time is the result of a number of factors predicted by epidemic theory and the availability of treatment, zidovudine in particular. Our estimates of deaths and prevalence have been influenced by the quality of the available death data. The observed number of deaths in the year mid 1989 to mid 1990 has exceeded the number forecast for that year. Substantial improvements in survival were associated with the introduction of zidovudine into clinical practice in mid 1987 but the death rate has risen rapidly again in the very recent past, possibly due to patients becoming increasingly refractory to treatment with zidovudine (before or after a diagnosis of AIDS). Our approach to forecasting can be adjusted to accommodate a transient effect of treatment as further data accumulate.
Acquired Immunodeficiency Syndrome↗
Sun exposure in Australian adolescents.
BACKGROUND: Little information is available on the sun-related behavior of teenagers despite the considerable resources spent to decrease sun exposure in this age group. OBJECTIVE: Our purpose was to describe the sun exposure behavior of Australian adolescents and define characteristics that predict use of sun protection. METHODS: Cross-sectional study of a random sample of 972 school students 13 to 15 years of age from three different locations in Australia (two urban and one rural) using a diary to document sun exposure and sun protection on two consecutive weekends. RESULTS: More than 80% of adolescent boys in each place and more than 60% of adolescent girls in both of the large cities spent more than 2 hours outdoors during the peak ultraviolet (UV) periods on each weekend. Neither sunscreen nor hats were used for more than half the time spent in the sun; however, shirts were worn most of the time. Students who wore hats were more likely to be boys (odds ratios [OR] = 2.2, 95% confidence intervals [CI] 1.40 to 3.44) and live in the rural region (OR = 4.6, 95% CI 2.36 to 9.04). Students who used sunscreen tended to have skin that sunburned easily (OR = 3.2, 95% CI 1.27 to 7.88) and score highly on the knowledge questions (OR = 2.9, 95% CI 1.46-5.69). This model was not a good predictor of behavior on a subsequent weekend, possibly because behavior was highly variable, with 35% to 50% of students changing their pattern of protection use from one weekend to the next. CONCLUSION: Adolescents spend long periods on summer weekends in the sun and do not follow recommended sun protection guidelines. The high variability of sun-related behavior makes modeling and consequent development of education programs a difficult task.
Adolescent↗
Sensitivity analyses for the backcalculation method of AIDS projections.
In Australia, AIDS is a notifiable disease and the data have been of a relatively high quality. In this article, the cases among Australian residents diagnosed by mid-1989 and reported by January 1990 are used to evaluate the sensitivity of estimates of numbers infected with the human immunodeficiency virus (HIV) and of projections of numbers of new AIDS diagnoses to major uncertainties in the method of backcalculation. We find that the estimates and long-term projections are sensitive to all uncertainties, including the incubation period distribution, the new infection intensity distribution, and the level of data aggregation chosen for analysis. Further, we find the projection that is least sensitive to all uncertainties, namely AIDS diagnoses for the next year (mid-1989 to mid-1990), falls in a relatively narrow range that encompasses the number that had been reported by April 1991.
Acquired Immunodeficiency Syndrome↗
Predicting the course of AIDS in Australia.
There have been urgent demands for knowledge about the epidemic of the acquired immunodeficiency syndrome (AIDS) in Australia. Accurate predictions are important for efficient allocation and planning of limited health-care resources. Ideal data for this purpose would be reliable knowledge of the past and present incidence of human immunodeficiency virus (HIV) infection. However, since the incidence of the infection is unknown predictions can only be based on historical data of the incidence of AIDS. In this article, we show the limitations of such predictions by examining a broad range of mathematical models that successfully track the observed data (1187 cases diagnosed to December 31, 1988). In addition, we describe a simple method for prediction in subgroups where the numbers of cases observed so far are small. Four models representing different forms of departure from the simple exponential model provide the best fits to the Australian AIDS data. Regional variability and a possible effect resulting from the introduction of zidovudine were incorporated into the models. Significant regional variability in the course of the epidemic was observed between New South Wales, Victoria and the rest of the country. For Australia as a whole, the doubling time changed from less than one year before mid 1987 to more than two years after this time. Model fits were improved by fitting the models to just the four years of data from 1985. The models give comparable predictions for the first year (1989) of around 600 new cases. However, by 1993 the predictions vary considerably, ranging from 500 to 2300 new cases. It is predicted that between 3100 and 6700 cases are likely to be diagnosed in Australia between 1989 and 1993. The results from the subgroup prediction demonstrate that when the observed number of cases is small, then the range of predictions for a future time interval is very wide. For reliable long-term predictions that are necessary for public health planning, basic information on the past and present incidence of HIV infection is urgently needed.
Acquired Immunodeficiency Syndrome↗
Effect of zidovudine on survival of patients with AIDS in Australia.
Since the first case of AIDS in Australia was diagnosed in December 1982, there have been substantial improvements in the treatment of AIDS-related conditions. In particular, zidovudine was widely introduced into clinical practice in Australia in June 1987. In order to evaluate its effect, we compared the survival of patients diagnosed before and after July 31, 1987 using data available in early 1989. Survival distributions were compared by means of Kaplan-Meier curves and by fitting exponential survival models incorporating a special feature of the data. Before August 1, 1987 the overall distribution of survival times for patients with AIDS in Australia is well described by an exponential distribution with a mean of 1.04 years. The corresponding median survival time for this period was 8.8 months. For patients diagnosed with AIDS after July 31, 1987 the median survival time had not been attained by December 31, 1988. However, the estimated mean survival time increased to 2.7 years. Survival times were found to be remarkably stable over the different regions of Australia. We have shown that substantial improvements in survival of patients diagnosed with AIDS in Australia are associated with the widespread availability of zidovudine from mid 1987. To the best of our knowledge this study is the first of its kind to show a major shift in the distribution of survival associated with the introduction of antiviral therapy.
Acquired Immunodeficiency Syndrome↗
Accommodating change due to treatment in the method of back projection for estimating HIV infection incidence.
This note shows how the method of back projection, which is being widely applied to predict the incidence of HIV infection, can be extended to incorporate distributional changes due to a treatment effect, such as zidovudine (commonly known as AZT). By way of example we consider one of the approaches to back projection and apply the method to some Australian data.
Acquired Immunodeficiency Syndrome↗
The inheritance of height in a Finnish population.
Although height is widely cited as the classic polygenic trait, there have been few large-scale studies of its inheritance since that of Pearson and Lee 80 years ago (Pearson and Lee 1903). Values of heritability derived in many standard genetics texts are based on those data. The data of Rissanen and Nikkila (1977) on 2869 individuals in 392 three-generation families provide an opportunity to make comparisons in a contemporary European population. We examine here height differences between regions, sexes and generations, and consider the form of the population distribution. Analysing the data within the classic polygenic framework we obtain a much higher spouse correlation in height than shown by earlier studies, a smaller heritability, and only a small effect of common-sib environment or genetic dominance deviations.
Body Height↗