Cohort effects in dynamic models and their impact on vaccination programmes: an example from hepatitis A.
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BACKGROUND: Infection rates for many infectious diseases have declined over the past century. This has created a cohort effect, whereby older individuals experienced a higher infection rate in their past than younger individuals do now. As a result, age-stratified seroprevalence profiles often differ from what would be expected from constant infection rates. METHODS: Here, we account for the cohort effect by fitting an age-structured compartmental model with declining transmission rates to Hepatitis A seroprevalence data for Canadian-born individuals. We compare the predicted impact of universal vaccination with and without including the cohort effect in the dynamic model. RESULTS: We find that Hepatitis A transmissibility has declined by a factor of 2.8 since the early twentieth century. When the cohort effect is not included in the model, incidence and mortality both with and without vaccination are significantly over-predicted. Incidence (respectively mortality) over a 20 year period of universal vaccination is 34% (respectively 90%) higher than if the cohort effect is included. The percentage reduction in incidence and mortality due to vaccination are also over-predicted when the cohort effect is not included. Similar effects are likely for many other infectious diseases where infection rates have declined significantly over past decades and where immunity is lifelong. CONCLUSION: Failure to account for cohort effects has implications for interpreting seroprevalence data and predicting the impact of vaccination programmes with dynamic models. Cohort effects should be included in dynamic modelling studies whenever applicable. (+info)
Behavioural risk factors in two generations of non-Western migrants: do trends converge towards the host population?
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Migrant mortality does not conform to a single pattern of convergence towards prevalence rates in the host population. To understand better how migrant mortality develops, it is necessary to further investigate how the underlying behavioural determinants change following migration. We studied whether the prevalence of behavioural risk factors over two generations of Turkish and Moroccan migrants converge towards the prevalence rates in the Dutch population. From a random sample from the population register of Amsterdam, 291 Moroccan and 505 Turkish migrants, aged 15-30, participated in a structured interview that included questions on smoking, alcohol consumption, physical inactivity and weight/height. Data from the Dutch population were available from Statistics Netherlands. By calculating age-adjusted Odds Ratio's, prevalence rates among both generations were compared with prevalence rates in the host population for men and women separately. We found indications of convergence across generations towards the prevalence rates in the host population for smoking in Turkish men, for overweight in Turkish and Moroccan women and for physical inactivity in Turkish women. Alcohol consumption, however, remained low in all subgroups and did not converge towards the higher rates in the host population. In addition, we found a reversed trend among Turkish women regarding smoking: the second generation smoked significantly more, while the first generation did not differ from ethnic Dutch. In general, behavioural risk factors in two generations of non-Western migrants in the Netherlands seem to converge towards the prevalence rates in the Dutch population. However, some subgroups and risk factors showed a different pattern. (+info)
How generation intervals shape the relationship between growth rates and reproductive numbers.
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Mathematical models of transmission have become invaluable management tools in planning for the control of emerging infectious diseases. A key variable in such models is the reproductive number R. For new emerging infectious diseases, the value of the reproductive number can only be inferred indirectly from the observed exponential epidemic growth rate r. Such inference is ambiguous as several different equations exist that relate the reproductive number to the growth rate, and it is unclear which of these equations might apply to a new infection. Here, we show that these different equations differ only with respect to their assumed shape of the generation interval distribution. Therefore, the shape of the generation interval distribution determines which equation is appropriate for inferring the reproductive number from the observed growth rate. We show that by assuming all generation intervals to be equal to the mean, we obtain an upper bound to the range of possible values that the reproductive number may attain for a given growth rate. Furthermore, we show that by taking the generation interval distribution equal to the observed distribution, it is possible to obtain an empirical estimate of the reproductive number. (+info)
Making academic dentistry more attractive to new teacher-scholars.
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This perspectives article written under the sponsorship of the Commission on Change and Innovation in Dental Education (CCI) of the American Dental Education Association (ADEA) summarizes data on the numbers of women and persons of color earning the D.D.S./D.M.D. degrees and entering the U.S. dentistry profession in the first decade of the twenty-first century and examines job factors of importance to recent graduates of doctoral programs in other academic disciplines that may have relevance for planning recruitment and retention strategies within academic dentistry. The characteristics and expectations of Generation X faculty are explored: who are they and what do they want from the academic workplace? The article describes the culture clash that often occurs when Gen Xers encounter policies and practices that were designed by and for prior generations (e.g., Traditionalists and Boomers) who filled the ranks of dental school faculty in the 1970s, 1980s, and 1990s. Recommendations for rethinking academic employment systems in ways that might make the university workplace more attractive to Generation X are described. (+info)
Comparison of longitudinally and cross-sectionally determined age-related decline in spirometric measurements.
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In order to compare the longitudinally and cross-sectionally determined annual decline of spirometric measurements, we measured spirograms from 326 male adults four times over five years. Acceptable results were obtained three times or more in 269 subjects aged 30 to 55 at the initial survey. Longitudinal annual changes in the height-squared proportional values of forced vital capacity (CFVC), forced expiratory volume in one second (CFEV1), and maximal expiratory flow at 50% and 25% of FVC (CVmax 50 and CVmax 25), was estimated by the model in which the effect of each individual's level was included as an explanatory variable. The cross-sectional annual change in those indices was determined by including the effect of each survey's level in the model. The longitudinal estimate of annual decline was significantly smaller than the cross-sectional estimate for all indices except CVmax 25. No evidence suggested that systemic error of measurement or a learning effect caused significant bias in the data. The discrepancy in the estimated annual changes seemed to be caused by the cohort effect in our subjects, since the cross-sectional analysis is primarily sensitive to harmful factors operating in the past. We concluded that the longitudinal data for individuals or groups should not be compared with any reference value based on a cross-sectional analysis. (+info)
Secular trends in the lifetime prevalence of alcohol dependence in the United States: a re-evaluation.
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Immigration generation status and its association with suicide attempts, substance use, and depressive symptoms among latino adolescents in the USA.
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