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S M Blower

Publications and source records attributed to S M Blower.

32 records · Page 2Linked to original sources
PubMed1995

An analysis of the process of human immunodeficiency virus sexual risk behavior change.

Few studies have tried to identify the process of risk behavior change. In this paper, we present a method for the analysis of the process of risk behavior change; the methodology involves using time series data to form transition probability matrices. We apply the method to human immunodeficiency virus (HIV) sexual risk behavioral data from a cohort of gay men in Amsterdam. We determine the process of risk behavior change in this cohort, and we demonstrate that it is not possible to deduce the process simply from an examination of the static pattern. Our results imply that the risk behavior change that is observed in this cohort may be viewed as a homogeneous one-step Markovian process. This process of risk behavior change has implications for the long-term prediction of HIV seroconversion rates and for the design and evaluation of HIV behavioral intervention studies and vaccine trials. Our results also have implications for the interpretation of relapse behavior.

Cognition↗
PubMed1994

Prophylactic vaccines, risk behavior change, and the probability of eradicating HIV in San Francisco.

Theory is linked with data to assess the probability of eradicating human immunodeficiency virus (HIV) in San Francisco through the use of prophylactic vaccines. The necessary vaccine efficacy levels and population coverage levels for eradication are quantified. The likely impact of risk behavior changes on vaccination campaigns is assessed. The results show it is unlikely that vaccines will be able to eradicate HIV in San Francisco unless they are combined with considerable reductions in risk behaviors. Furthermore, if risk behavior increases as the result of a vaccination campaign, then vaccination could result in a perverse outcome by increasing the severity of the epidemic.

AIDS Vaccines↗
PubMed1993

Imperfect vaccines and herd immunity to HIV.

A number of prophylactic vaccines against human immunodeficiency virus (HIV) have passed through phase I clinical trials, and phase II clinical trials are now being planned. These vaccines are not expected to be perfect and might fail in a number of different ways. This paper shows how to equate different aspects of imperfection in a prophylactic vaccine in terms of impact upon levels of herd immunity, and hence upon the vaccine coverage required for eradication. Such comparisons reveal that an otherwise perfect vaccine that gives protection which wanes with a half-life of 10 years is only as good as a vaccine that works in 30% of people giving them complete, lifelong protection. The paper goes on to compare predicted patterns of seroconversion that would be observed in clinical trials and in community-wide vaccination campaigns for vaccines that confer the same levels of herd immunity but are imperfect in different ways.

AIDS Vaccines↗
PubMed1993

Exploratory data analysis of three sexual behaviour surveys: implications for HIV-1 transmission in the U.K.

Recently, the number of European and North American heterosexually transmitted AIDS cases has begun to increase. It appears that injecting drug users may be acting as an important infection source for heterosexual transmission, and that the HIV-1 epidemic may be slowly diffusing out into the general population. Therefore, the question arises, as to whether there will be an extensive heterosexually transmitted epidemic in Europe or North America. The possibility of such an epidemic occurring depends upon three factors: sexual mixing patterns, the prevalence and the distribution of specific risk factors, and changes in risk behaviours. To assess the potential for an extensive sustained heterosexually transmitted epidemic it is necessary to estimate these three factors by conducting large-scale linked behavioural and seroprevalence surveys. Unfortunately, such surveys have yet to be conducted in any country. However, a limited amount of sexual behaviour data are available from three small studies that have been conducted in the U.K. Although each of the studies suffers from methodological imperfections, together they may be used as the basis for a preliminary assessment of the potential effects of heterosexual transmission in the U.K. In this paper I discuss the results from an exploratory analysis of these data; the results are organized into five sections: (i) assessment of the prevalence and distribution of specific risk behaviours; (ii) determination of covariates of risk behaviours (and hence characterization of the subgroups that may be at risk); (iii) assessment of age-dependent sexual mixing patterns; (iv) assessment of sexual behaviour changes; and (v) calculation of epidemiological parameters. The validity and the reliability of the data from the three studies are assessed. The analysis suggests that it is possible that a heterosexually transmitted HIV-1 epidemic could occur in the U.K. Hence, it is essential to conduct longitudinal surveys that collect linked seroprevalence and behavioural data.

Adolescent↗
PubMed1993

Sex acts, sex partners, and sex budgets: implications for risk factor analysis and estimation of HIV transmission probabilities.

Many epidemiological studies have identified the number of sex partners as a risk factor for the acquisition of HIV, but few studies have identified the number of sex acts as a risk factor. The seeming lack of importance of the number of sex acts as a risk factor has yet to be explained. In this report we conduct an exploratory data analysis to evaluate the relationship between the number of sex acts and the number of sex partners for heterosexuals. Our results indicate that it may be most appropriate to view sexual activity within a sex budget and resource allocation framework. We use the results (a) to suggest an explanation for why the results from some of the risk factor analysis studies have identified a per partnership but not a per act risk, and (b) to assess the implications of the relationship for the estimation of heterosexual transmission probabilities for HIV.

Adult↗
PubMed1993

Effect of differential mortality on risk behavior change in cohort studies.

Recent theoretical work suggests that reductions in aggregate measures of risk behaviors are to be expected during a human immunodeficiency virus epidemic, because mortality is likely to be differential with respect to the level of the risk behavior. We present and apply a methodology for quantifying the effects of differential mortality on risk behavior changes in closed cohort studies. We demonstrate that differential mortality has caused 21% of the observed reduction in the mean, 29% of the observed reduction in the effective average, and 33% of the observed reduction in the variance of a risk behavior in a cohort of gay men.

Acquired Immunodeficiency Syndrome↗
PubMed1993

Incorrect specification of marker allele frequencies: effects on linkage analysis.

Most current linkage analyses make use of highly polymorphic DNA markers. Assigning correct allele frequencies for these markers may be extremely difficult in particular study populations. Designation of erroneous frequencies may result in false-positive evidence for linkage, as well as in failure to correctly exclude linkage. These effects are most pronounced in small pedigrees with key individuals unavailable for typing. The power to correctly detect true linkage does not appear to be greatly affected by inaccurate allele frequencies. Before linkage analyses are performed for specific pedigrees, it is recommended that simulation analyses be performed, followed by uncertainty and sensitivity analyses.

Alleles↗
PubMed1991

Mixing ecology and epidemiology.

Mixing matrices can be used to describe subgroup interactions in mathematical models which have heterogeneity in population structure. A discussion is presented of two different approaches to the formulation of such mixing matrices. The relation between the two different methods is discussed, using examples based upon models of the transmission dynamics of HIV. There follows a discussion of the application of the mixing matrix approach to other areas in population biology, and a complementary overview of recent advances in theoretical ecology that may have applications in theoretical epidemiology.

Ecology↗
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

The supply and demand dynamics of sexual behavior: implications for heterosexual HIV epidemics.

This article investigates the supply and demand dynamics of sexual behavior in a simple epidemiological model of heterosexual transmission of human immunodeficiency virus (HIV). The supply and demand dynamics of sexual behavior are modeled by specifying implicit sexual behavior change (ISBC) mechanisms. These mechanisms specify how males and females modify their rates of sex partner change in response to changes in the availability of the opposite sex. During an epidemic, the availability of both sexes will change owing to recruitment and mortality; consequently, implicit sexual behavior changes will be induced. These behavior changes will be independent of any explicit sexual behavioral changes that may occur as a result of intervention strategies. We investigate four different ISBC mechanisms: the two extremes of the continuum of possible mechanisms (where only one sex changes behavior), as well as two intermediate possibilities (where both sexes change behavior). The results show that the epidemiological effects of these ISBC mechanisms depend upon the transmission speed of the virus. If HIV transmission is slow, the epidemiological effects of the four ISBC mechanisms cannot be differentiated. If HIV transmission is fast, the four ISBC mechanisms differ considerably in the degree to which they modify the gender-specific rates of sex partner change, but the sexual behavioral changes occur too late to significantly decrease the cumulative number of infected persons. However, if HIV transmission is moderately fast, the four different ISBC mechanisms produce significantly different epidemics. The differences between the epidemics, produced by the four ISBC mechanisms, are magnified as the degree of asymmetry between the heterosexual transmission efficiencies increases. We discuss the implications of our results for both the future number of AIDS cases that will be observed in the "real world" and the number of AIDS cases that will be predicted from mathematical models. We also discuss the implications for studies that evaluate the causation of sexual behavior changes.

Acquired Immunodeficiency Syndrome↗
PubMed1990

Loglinear models, sexual behavior and HIV: epidemiological implications of heterosexual transmission.

An analysis is presented of sexual behaviour data from a "random" sample (n = 780) of the adult population of England and Wales. Sex stratified frequency distributions of the reported number of sex partners/time were analysed for heterosexuals, demographic characteristics associated with the number of sex partners/time were identified, and epidemiological parameters (the basic reproductive rate of HIV and the doubling time of the epidemic) were calculated. These analyses suggest that the size of the group at risk for acquiring the virus by heterosexual transmission may be large and that the age of first sexual intercourse (for males and females) is decreasing in younger cohorts. Members of the potential heterosexual at-risk group may be identified by demographic variables such as marital status (males and females) and age (females only), but not by socioeconomic class. The epidemiological implications of our results for the heterosexual transmission of HIV are discussed.

Adolescent↗
PubMed1985

Adaptation at specific loci. IV. Differential mating success among glycolytic allozyme genotypes of Colias butterflies.

Male mating success as a function of genotype is an important fitness component. It can be studied in wild populations, in species for which a given group of progeny has exactly one father, by determining genotypes of wildcaught mothers and of sufficient numbers of their progeny. Here, we study male mating success as a function of allozyme genotype at two glycolytic loci in Colias butterflies, in which sperm precedence is complete, so that the most recent male to mate fathers all of a female's subsequent progeny.--For the phosphoglucose isomerase, PGI, polymorphism, we predict mating advantage and disadvantage of male genotypes based on evaluation of their biochemical functional differences in the context of thermal-physiological-ecological constraints on the insects' flight activity. As predicted, we find major, significant advantage in mating success for kinetically favored genotypes, compared to the genotype distribution of males active with the sampled females in the wild. These effects are repeatable among samples and on different semispecies' genetic backgrounds.--Initial study of the phosphoglucomutase, PGM, polymorphism in the same samples reveals heterozygote advantage in male-mating success, compared to males active with the females sampled. This contrasts with a lack of correspondence between PGI and PGM genotypes in other fitness index or component differences.--Epistatic interactions in mating success between the two loci are absent.--There is no evidence for segregation distortion associated with the alleles of either primary locus studied, nor is there significant assortative mating.--These results extend our understanding of the specific variation studied and suggest that even loci closely related in function may have distinctive experience of evolutionary forces. Implications of the specificity of the effects seen are briefly discussed.

Adaptation, Physiological↗