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Biomedical subjects

N G Becker

Publications and source records attributed to N G Becker.

At least 37 records · Page 2Linked to original sources
PubMed1996

Preventing epidemics in a community of households.

The occurrence of epidemics of vaccine-preventable diseases, and the immunization coverage required to prevent them, is affected by the presence of households and heterogeneity in the community. We consider a community where individuals live in households and are of different types, according to infectivity and/or susceptibility to infection. We describe a method for computing the critical immunization coverage to prevent epidemics in such communities and discuss the effectiveness of immunization strategies. In a heterogeneous community where individuals live in households several immunization strategies are possible and we examine strategies targeting households, randomly selected individuals, or groups with highly intense transmission, such as school children. We compare estimates of the critical immunization coverage if we assume that disease is spread solely by random mixing with estimates which result if we assume the effects of the household structure. Estimates made under these two sets of assumptions differ. The results provide insights into the community effects of vaccination, and the household structure of the community should be taken into account when designing immunization policies.

Child↗
PubMed1995

Assessment of two-dose vaccination schedules: availability for vaccination and catch-up strategies.

An age-specific transmission model is used to discuss aspects of two-dose vaccination schedules. Motivated by measles, the model allows for a period of maternally acquired immunity and assumes that infection leads to permanent immunity. The model expresses the coverage at the second dose in terms of the availability of individuals for vaccination, where availability depends on whether the individual received the first vaccination and the time between the first and second doses. It is emphasized by illustration that medium-term performance is a more appropriate assessment of vaccination schedules than eventual outcome, particularly since elimination of measles does not seem possible at this time. Failure to take account of individuals' availability for vaccination when designing vaccination schedules can lead to a poor control strategy and to unjustified optimism. When disease transmission is in equilibrium, an instantaneous change to a vaccination schedule with high coverage often results in a period of low incidence followed by a series of substantial epidemics occurring over a long period of time. A gradual increase to the same level of vaccination can dampen these epidemic waves significantly and gives similar numbers of cases in the medium and long terms. It is found that a strategy based on doses offered at ages 1 and 11 years is far from optimal, but the inclusion of a catch-up vaccination at school entry, offered only to previously unvaccinated individuals, can improve this two-dose strategy dramatically.

Age Factors↗
PubMed1995

The effect of household distribution on transmission and control of highly infectious diseases.

Two epidemic threshold parameters are derived for the spread of a highly infectious disease in a community of households, where a household is any group whose members have frequent contacts with each other. It is assumed that the infection of any member of a household results in the infection of all susceptible members of that household. The threshold parameters have simple expressions in terms of the mean household size and the mean and variance of the number of susceptibles per household. They provide a basic reproduction number R0 for the spread of infection from individual to individual and a basic reproduction number RH0 for the spread of infection from household to household. The threshold parameters are used to derive the levels of immunity required for the prevention of major epidemics in the community. They are also used to evaluate various vaccination strategies having the same vaccination coverage. For a community with households of equal size, it is found that random vaccination of individuals is better than immunizing all members of a corresponding fraction of households. In contrast, when households have varying sizes, immunizing all members of large households can be better than a corresponding vaccination coverage of randomly selected individuals. It is illustrated that these threshold parameters can also be used for a community of households with schools or day care centers. In particular, the effectiveness of immunizing all members of a school is quantified.

Child↗
PubMed1995

Threshold parameters for epidemics in different community settings.

Threshold parameters of epidemic models play a central role in the assessment of proposed control strategies for infectious diseases. They have been determined for numerous standard epidemic models. This paper points out, with several examples, that threshold parameters depend on the social setting of the community and the variations in the behavior of the members of the community. Specifically, communities are considered in which individuals have fixed patterns of behavior or random patterns of behavior as well as communities of households with fixed or random patterns of behavior. A threshold parameter is computed for each of the different settings. Some comparisons are made to provide insights into the effects that social settings and behavior changes have on the threshold parameters.

Communicable Diseases↗
PubMed1994

Simultaneous back-projection of AIDS incidence data for two or more groups.

Back-projection of AIDS incidence data is useful for estimating characteristics of the HIV epidemic curve and forms a basis for projections of the AIDS epidemic curve. Its application to subgroups of the population is limited by its imprecision for groups with a small number of cases. Back-projection can be made more precise by pooling data from different groups and linking their HIV infection intensities by a model. Here we propose a method based on proportional infection intensities and study its performance with simulations and applications to AIDS in different States of Australia and haemophiliacs in the U.S.A. This method of simultaneous back-projection is shown to reduce substantially the width of confidence intervals for HIV infection intensities and for total numbers infected.

Algorithms↗
PubMed1994

Waning immunity and its effects on vaccination schedules.

A relatively comprehensive age-specific transmission model is used to determine the effect of various factors on the optimal vaccination ages in one-dose and two-dose vaccination schedules. Motivated by the situation for measles, the model allows the duration of immunity of newborns to depend on the level of immunity of the mother at the time of the birth and allows for waning immunity as well as boosting of immunity by exposure to the disease. It is found that a significant amount of waning of disease-acquired immunity is plausible when boosting occurs but this is not an important factor in determining optimal vaccination schedules. On the other hand, plausible rates of loss of vaccine-induced immunity can have a substantial effect on the optimal vaccination schedule, particularly when there is no boosting of immunity. For two-dose schedules the optimal vaccination ages depend significantly on the level of vaccination coverage achieved. In the presence of plausible rates of loss of vaccine-induced immunity for measles, it is found that the vaccination coverage required to eradicate the disease is substantially higher than previously suggested.

Age Factors↗
PubMed1994

A transmission model for a disease with some fatalities.

A stochastic epidemic model which allows some infected individuals to die from the disease is considered. The model includes the general epidemic and the fatal disease model as particular cases. A set of iterative equations is derived from which the probability distribution of the final size of the epidemic can be computed. It is illustrated that the fatal disease model is distinct in that it results in nearly all or hardly anyone being infected in a large population. An easily computed estimator for a key parameter of the model is derived and it is illustrated that it has good efficiency relative to the maximum likelihood estimator, which is very tedious to compute.

Communicable Diseases↗
PubMed1994

A transmission model for HIV with application to the Australian epidemic.

An age-specific transmission model is proposed to describe the spread of HIV in a homosexual population. The model incorporates developments in the treatment of patients and the change in the surveillance definition of AIDS. The model is applied to the Australian epidemic with the aim of determining the extent of behavioral changes during the epidemic and assessing the extent to which therapy has changed the course of the epidemic. It is found that therapy and the adoption of totally safe sex practices by individuals who have tested HIV positive cannot explain the recent downturn in the rate of increase of observed AIDS incidence. A significant change in behavior within the general homosexual community is indicated.

Acquired Immunodeficiency Syndrome↗
PubMed1993

Smoothed nonparametric back-projection of AIDS incidence data with adjustment for therapy.

Back-projection is a major tool for assessing the extent of the HIV epidemic. Its application relies on a model for the incubation period of AIDS into which the administration and effect of therapy has been incorporated. We propose a compartmental model with proportional transition rates to describe variation and changes in the duration of the incubation period. The model is easily related to available data and offers fast computation. Its detailed and direct reflection of the administration and effect of therapy makes it relatively easy to ensure that therapy has been accommodated to an appropriate extent. The effect of therapy on back-projection estimates of the HIV epidemic curve is demonstrated with an application to Australian AIDS incidence data.

Acquired Immunodeficiency Syndrome↗
PubMed1993

Assessing the extent of the Australian HIV epidemic from AIDS surveillance data.

Current knowledge about human immunodeficiency virus (HIV) disease is used to assess past and future trends in Australian HIV/AIDS (acquired immune deficiency syndrome) incidence, focusing on the precision with which such assessments can be made. The statistical method of back-projection is applied to reconstruct the past pattern of HIV incidence from surveillance data on AIDS incidence to June 1992. The results indicate that HIV incidence rose rapidly in the early 1980s to peak in 1983-1984, followed by a sharp decline. This finding is insensitive to plausible variations from the assumptions made, and is consistent with both success in preventive strategies and high levels of infection in a subgroup having a high probability of exposure. Cumulative HIV incidence to the end of 1987 is estimated with a 90 per cent confidence interval from 9,350 to 10,350. Estimation of the cumulative HIV incidence to June 1992 is less precise, with a 90 per cent confidence interval of 12,900 to 17,800. After adjustment for underreporting the incidence could be as high as 22,000, but only if recent infection rates, which cannot be quantified accurately, were very high. Based on data to June 1992, the estimated trend in AIDS incidence indicates 680 new cases in 1993, rising gradually to 695 in 1995. The estimated rate of increase in AIDS incidence over the recent past and near future is significantly less than that observed earlier in the epidemic. This is a consequence of both the earlier peak in HIV incidence and the effect of therapy.

Acquired Immunodeficiency Syndrome↗
PubMed1993

Martingale methods for the analysis of epidemic data.

After explaining why martingale methods play an important role in statistical inference for parameters of epidemic models, we give a tutorial introduction to these methods in the more familiar context of data on independent and identically distributed survival times. In this simpler setting we introduce requisite results from martingale theory and demonstrate that martingale methods simply lead to well known estimates and their standard errors. We then turn to the context of epidemics, and illustrate how martingale methods can be used to derive a method of inference for the infection potential in a simple model with removal of infectives. The resulting method involves only simple computations and we demonstrate that the method applies under much more general assumptions. There follows a critical review of several applications of martingale methods for the analysis of infectious disease data. It emerges that the approach provides simple methods of inference in some situations where standard methods of inference are not available, or are too cumbersome. The range of applications seems limited, but new applications continue to be found. Little has been done to confirm high efficiency of martingale methods in epidemic applications.

Communicable Diseases↗
PubMed1992

Statistical challenges of AIDS.

"This paper considers questions concerning the incubation period [of HIV infections], the effects of treatments, prediction of AIDS cases, the choice of surrogate end points for the assessment of treatments and design of strategies for screening blood samples. These issues give rise to a broad range of intriguing problems for statisticians. We describe some of these problems, how they have been tackled so far and what remains to be done. The discussion touches on topical statistical methods such as smoothing, bootstrapping, interval censoring and the ill-posed inverse problem, as well as asking fundamental questions for frequentist statistics." The geographical scope is worldwide, with some data for selected developed countries used to illustrate the models.

Acquired Immunodeficiency Syndrome↗
PubMed1991

Routine coronary angiography for effort angina.

OBJECTIVE: To compare the value of exercise testing and coronary angiography in the management of patients with effort angina. DESIGN: A retrospective follow-up study. SETTING: A university teaching hospital. PATIENTS: Four hundred and thirty-four patients with a history of angina, who developed pain or ST segment changes during an exercise test and who were followed-up at up to nine years after their exercise test. INTERVENTIONS: Coronary angiography was offered to patients for disabling angina only. Bypass surgery was generally offered to patients with multiple vessel coronary artery obstruction. MAIN OUTCOME MEASURE: Patient survival. RESULTS: Stepwise regression analysis based on the Cox proportional hazards model indicated that the decision to perform coronary angiography (P less than 0.001) and the amount of ST segment depression on exercise testing (P less than 0.01) were predictors of survival. With ST segment depression set at 1 mm and age set at 53.5 years (both sample median values) the probability of survival at five years was 86% in patients who did not undergo coronary angiography compared with 96% in patients who did undergo angiography. There were eight deaths in 59 patients who did not show any ST segment depression during the exercise test and who were treated medically without coronary angiography. CONCLUSIONS: Coronary angiography is a superior technique to exercise testing for detecting patients with effort angina who are at high risk of premature mortality. Coronary bypass surgery makes a big improvement in the chance of five-year survival in such patients.

Angina Pectoris↗
PubMed1991

A method of non-parametric back-projection and its application to AIDS data.

The method of back-projection has been used to estimate the unobserved past incidence of infection with the human immunodeficiency virus (HIV) and to obtain projections of future AIDS incidence. Here a new approach to back-projection, which avoids parametric assumptions about the form of the HIV infection intensity, is described. This approach gives the data greater opportunity to determine the shape of the estimated intensity function. The method is based on a modification of an EM algorithm for maximum likelihood estimation that incorporates smoothing of the estimated parameters. It is easy to implement on a computer because the computations are based on explicit formulae. The method is illustrated with applications to AIDS data from Australia, U.S.A. and Japanese haemophiliacs.

Acquired Immunodeficiency Syndrome↗
PubMed1990

Sexual transmission of hepatitis C virus: cohort study (1981-9) among European homosexual men.

OBJECTIVE: To determine the prevalence, incidence, and persistence of positivity for antibodies to hepatitis C virus (anti-HCV) and the potential for sexual transmission of the virus. DESIGN: A cohort analysis covering 1981-9 comparing estimated cumulative incidences of and seroconversion rates for anti-HCV with those of hepatitis B core antibody (anti-HBc) and antibodies to the human immunodeficiency virus (anti-HIV). SETTING: Copenhagen and Aarhus, Denmark. SUBJECTS: 259 Male members of a Danish homosexual organisation. MAIN OUTCOME MEASURES: Correlations of prevalence and incidence with a wide range of sexual lifestyle variables. RESULTS: Only four (1.6%) subjects were positive for anti-HCV in 1981. The estimated cumulative incidence of positivity for anti-HCV was 4.1% in 1984 (seroconversion rate during 1981-4 (2.5%)) and remained at 4.1% in 1989 (seroconversion rate nil during 1984-9). In contrast, positivity for anti-HBC rose from 44.0% in 1981 to 52.7% in 1984 (seroconversion rate 15.5%) and 58.8% in 1989 (seroconversion rate 12.9%), and that for anti-HIV rose from 8.8% to 24.0% (seroconversion rate 16.7%) and 30.1% (seroconversion rate 8.0%) respectively. Three anti-HCV positive patients seroreverted three to five years later. None of the anti-HCV positive subjects had had a transfusion and only one gave a past history of intravenous drug use. Variables in sexual lifestyle correlated with the presence of anti-HBc but not with that of anti-HCV. CONCLUSIONS: In contrast with hepatitis B virus and HIV, sexual transmission of hepatitis C virus seems to be a rare event. Furthermore, antibodies to the virus may become undetectable after several years.

Adult↗
PubMed1983

Assessing the heterogeneity of disease spread through a community.

Care needs to be exercised in attempts at obtaining a description of the spread of disease merely by fitting a mathematical model to infectious disease data and adjusting the model until it adequately fits the observed epidemic curve. It is always necessary to perform separate statistical tests of the underlying assumptions of an epidemic model before attempting to use such a model to obtain epidemiologically meaningful insights into the mechanism of disease spread. Methods for such tests are presented and illustrated with reference to epidemics of respiratory diseases that occurred on the island of Tristan da Cunha in the South Atlantic between 1964 and 1968.

Adolescent↗