Epidemiology of AIDS--statistical analyses

Extending backcalculation to analyse BSE data

We review the origins of backcalculation (or back projection) methods developed for the analysis of AIDS (acquired immunodeficiency syndrome) incidence data. These techniques have been used extensively for >15 years to deconvolute clinical case incidence, given knowledge of the incubation period distribution, to obtain estimates of past HIV (human immunodeficiency virus) infection incidence and short-term predictions of future AIDS incidence. Adaptations required for the analysis of bovine spongiform encephalopathy (BSE) incidence included: stratification of BSE incidence by age as well as birth cohort; allowance for incomplete survival between infection and the onset of clinical signs of disease; and decomposition of the age- and time-related infection incidence into a time-dependent feed risk component and an age-dependent exposure/susceptibility function. The most recent methodological developments focus on the incorporation of data from clinically unaffected cattle screened using recently developed tests for preclinical BSE infection. Backcalculation-based predictions of future BSE incidence obtained since 1996 are examined. Finally, future directions of epidemiological analysis of BSE epidemics are discussed taking into account ongoing developments in the science of BSE and possible changes in BSE-related policies.

Estimation of the incidence of HIV infection

The aim of the method of 'back projection' is to provide estimates of the number of new infections with the human immunodeficiency virus (HIV) as a function of time, by using the numbers of diagnoses of the acquired immune deficiency syndrome (AIDS) together with information on the distribution of the incubation period between infection and diagnosis. Here, the method is investigated with particular reference to cases of HIV infection and AIDS in the United Kingdom.

Psychological defenses and control of AIDS

The well known psychological defenses of avoidance, repression, and denial against the knowledge that one has, or might have, a fatal disease used by a high percentage of individuals, must be considered in control programs for the human immunodeficiency virus (HIV). Current programs of education, voluntary testing, and counseling do not consider this defense mechanism and may therefore provide inadequate control. Compulsory HIV testing programs and development of an "HIV Parole" system may be needed if the current HIV epidemic worsens.

Monte Carlo simulation of the heterosexual selective spread of the human immunodeficiency virus

The spread of human immunodeficiency virus (HIV) by heterosexual intercourse, during the first 2 years following the introduction of the virus among a sexually active and unprotected group of men and women, is modelled by Monte Carlo simulation. A beta distribution of the infectee's risks of infection per infected partner-month is assumed, with the same coefficient of variation as used in previous studies for risk of conception (natural fecundability), but with a mean of 0.04 per infected partner-month, after scaling the distribution. The number of sexual partners that one sex has (here, the women), is assumed to be more variable than for the other, with a mean of 2 partners each. The individual infection risks per infected partner-month are generated initially and do not change, but some random gains and losses of partners occur each month. Infections are updated each month. It is found in this simple model that women who become infected by 2 years had a mean risk of infection (not counting the original infector) only about 13% higher than the others. Some implications of the low selection are noted. Very great variability in the number of infections subsequently due to an index seropositive is found, which prevents easy discrimination in the characteristics of "at-risk" persons.

Incidence of symptoms and AIDS in 146 Swedish haemophiliacs and blood transfusion recipients infected with human immunodeficiency virus

The times from infection with the human immuno-deficiency virus (HIV) to the onset of the first clinical symptom and the development of AIDS were studied prospectively in 98 haemophiliacs and 48 blood transfusion recipients infected with the virus. Patients were followed up for a median of 61 months after infection, the dates of infection being either known exactly or estimated from the interval between the last negative and first positive HIV antibody test result. The rate of progression to AIDS was significantly higher for the transfusion recipients than for the haemophiliacs. The difference in time to the occurrence of the first clinical symptom was less pronounced between the two groups, though pointing in the same direction. The results suggest that on average roughly half of all patients positive for HIV will develop some clinical sign or symptom within five to six years after infection.