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Estimability and interpretation of vaccine efficacy using frailty mixing models.
The authors consider estimability and interpretation of vaccine efficacy based on time to event data, allowing that some of the population might have a very low probability of acquiring disease, and the rest have partial, possibly continuously distributed, susceptibility. The efficacy parameters of interest in the frailty mixing model include the fraction highly unlikely to acquire the infection or disease due to the vaccine, the degree of partial protection in those still susceptible, and the average protection or summary measure of efficacy under heterogeneity. The efficacy estimates can still be usefully interpreted when the heterogeneity results from heterogeneity in contact patterns, contact rates, or infectiousness of the contacts, as long as these are equal in the vaccinated and unvaccinated groups. A likelihood-based method allows estimation of the efficacy parameters of interest from grouped time to event data. Simulated vaccine studies assuming different levels and distributions of efficacy demonstrate that ignoring heterogeneity in susceptibility or exposure to infection generally results in underestimation of vaccine efficacy as well as incorrect interpretation of the estimates. The approach is also applicable to other covariates affecting susceptibility or exposure to infection in infectious diseases. Exploitation of the dependent happening structure of infectious diseases to obtain a shape for the baseline hazard may help identifiability. The authors recommend fitting several models to time to event data in vaccine studies.
Intraclass correlation estimates in a school-based smoking prevention study. Outcome and mediating variables, by sex and ethnicity.
Most school-based smoking prevention studies employ designs in which schools or classrooms are assigned to different treatment conditions while observations are made on individual students. This design requires that the treatment effect be assessed against the between-school variance. However, the between-school variance is usually larger than the variance that would be obtained if students were individually randomized to different conditions. Consequently, the power of the test for a treatment effect is reduced, and it becomes difficult to detect important treatment effects. To assess the potential loss of power or to calculate appropriate sample sizes, investigators need good estimates of the intraclass correlations for the variables of interest. The authors calculated intraclass correlations for some common outcome variables in a school-based smoking prevention study, using a three-level model-i.e., students nested within classrooms and classrooms nested within schools. The authors present the intraclass correlation estimates for the entire data set, as well as separately by sex and ethnicity. They also illustrate the use of these estimates in the planning of future studies.
Diabetes, body size, and risk of endometrial cancer.
Data from a population-based case-control study of Wisconsin women were used to evaluate the relation of diabetes to the risk of endometrial cancer on the basis of body mass index (BMI). Cases (n=723) were identified from a statewide tumor registry; controls (n=2,291) were selected randomly from population lists. Diabetes status, weight, height, and other factors were ascertained by telephone interview. Subjects were categorized as not overweight (BMI, <29.1), overweight (BMI, 29.1-31.9), or obese (BMI, >31.9) according to the BMI distribution of middle-aged white women in the Second National Health and Nutrition Examination Survey. Joint associations between diabetes status, BMI, and endometrial cancer were evaluated using unconditional logistic regression models that controlled for age, parity, use of hormone replacement therapy, education, and smoking. Compared with persons without diabetes, those with diabetes had an adjusted odds ratio of 1.86 (95% confidence interval (CI) 1.37-2.52) for endometrial cancer. This association was modified by BMI (p interaction=0.04). Compared with nonoverweight nondiabetic subjects, nonoverweight and overweight women who reported diabetes had nonsignificant elevated risks of endometrial cancer (nonoverweight, odds ratio (OR)=1.10, CI 0.66-1.86; overweight, OR=1.58, CI 0.81-3.05). In contrast, elevated risk estimates were observed for obese diabetic women (OR=2.95, CI 1.60-5.46). These data contradict earlier reports and suggest that diabetes confers no additional risk of endometrial cancer in women who are neither overweight nor obese.
Maternal supplemental and dietary zinc intake and the occurrence of neural tube defects in California.
The authors investigated the association between maternal preconceptional supplemental and dietary zinc intake and risk of neural tube defects (NTDs) in a population-based case-control study conducted between 1989 and 1991 in California. Cases were 430 NTD-affected fetuses/infants, and controls were 429 randomly selected non-malformed infants. Mothers reported their preconceptional use of vitamin, mineral, and food supplements, and completed a 98-item food frequency questionnaire. Increased total preconceptional zinc intake was associated with a reduced risk for NTDs (quintile 5 vs. quintile 1, odds ratio (OR) = 0.65, 95% confidence interval (CI) 0.43, 0.99). Phytate intake, a constituent of the diet known to impede zinc absorption, appeared to modify the zinc - NTD association. In addition, increased servings of animal products, the most bioavailable food source of zinc, was associated with a reduced risk for NTDs (quintile 5 vs. quintile 1, OR = 0.49, 95% CI 0.32, 0.76). Risk estimates for zinc intake were changed little after controlling for multiple sociodemographic factors and total folate intake, but were attenuated after controlling for nutrients highly correlated with dietary sources of zinc, such as protein. In sum, the analyses indicate that risk of NTDs in infants and fetuses decreased with increasing maternal preconceptional zinc intake. However, it remains unclear whether increased zinc intake, or another nutrient or combination of nutrients highly correlated with zinc intake in the diet, is causally associated with reduced NTD risk.
Re: "Use of census-based aggregate variables to proxy for socioeconomic group: evidence from national samples".
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Aging and total cholesterol levels: cohort, period, and survivorship effects.
This analysis describes the association of age with the serum total cholesterol level in 5,010 participants in a geriatric health screening program. Cholesterol levels were measured annually in participants monitored for up to 12 years. The association of age with cholesterol level is described via three approaches: cross-sectional analysis, descriptive longitudinal analysis, and longitudinal analysis using statistical modeling. The results were compared to examine the influence of cohort, period, and survivorship effects on the association between age and cholesterol. In cross-sectional analysis, the cholesterol level was fairly constant for the ages of 65 to 75 years, but decreased by 21% over the age range from 75 to 95 years. Descriptive longitudinal analysis suggested that both cohort and period effects were influencing the cross-sectional findings. In longitudinal analysis adjusting for both cohort and period effects, the findings were similar to those from cross-sectional analysis for the ages of 65 to 75 years, but from the ages of 75 to 95 years, cholesterol decreased by only 9%--half as great a decline as that estimated from cross-sectional analysis. When longitudinal data were limited to those with complete follow-up, the predicted decline for the age range from 75 to 95 years was only 6%. Although this flattening of the age trend was suggestive, there was no conclusive evidence that it reflected an association between baseline cholesterol and loss to follow-up.
Outdoor air temperature and mortality in The Netherlands: a time-series analysis.
Death rates become progressively higher when outdoor air temperature rises above or falls below 20-25 degrees C. This study addresses the question of whether this relation is largely attributable to the direct effects of exposure to heat and cold on the human body in general, and on the circulatory system in particular. The association between daily mortality and daily temperatures in the Netherlands in the period 1979-1987 was examined by controlling for influenza incidence, air pollution, and "season"; distinguishing lag periods; examining effect modification by wind speed and relative humidity; and distinguishing causes of death. Important direct effects of exposure to cold and heat on mortality were suggested by the following findings: 1) control for influenza incidence reduced cold-related mortality by only 34% and reduced heat-related mortality by 23% (the role of air pollution and "season" was negligible); 2) 62% of the "unexplained" cold-related mortality, and all heat-related mortality, occurred within 1 week; and 3) effect modification by wind speed was in the expected direction. The finding that 57% of "unexplained" cold-related mortality and 26% of the "unexplained" heat-related mortality was attributable to cardiovascular diseases suggests that direct effects are only in part the result of increased stress on the circulatory system. For heat-related mortality, direct effects on the respiratory system are probably more important. For cold-related mortality, the analysis yielded evidence of an important indirect effect involving increased incidence of influenza and other respiratory infections.
Estimation of design effects and diarrhea clustering within households and villages.
The degree to which diarrheal disease clustered within households and within villages among preschool age children was examined using data from four population-based prevalence surveys undertaken in Malawi, Zambia, Indonesia, and Nepal over the past decade. The design effect for each cluster survey was calculated using the diarrhea prevalence, the cluster sizes, and the magnitude of diarrhea clustering within the sampling unit (villages). A recently developed statistical method, alternating logistic regression, was used to estimate disease associations within households of up to nine preschool age children residing within villages of up to 589 such children. Pairwise odds ratios estimating diarrhea clustering within villages ranged from 1.03 (95% confidence interval (CI) 1.01-1.07) in Zambia to 2.19 (95% CI 1.73-2.78) in Indonesia. The design effects ranged from 2.07 (95% CI 1.26-3.19) in Zambia to 7.93 (95% CI 5.16-11.52) in Indonesia. Design effects were strongly dependent on cluster size. The design effects for clusters of size 50 would have ranged from 1.38 to 4.73. Pairwise odds ratios for diarrhea clustering within households ranged from 1.88 (95% CI 1.61-2.19) in Nepal to 10.05 (95% CI 8.46-11.94) in Indonesia. Household odds ratios were always larger than village odds ratios. The village and household pairwise odds ratios adjusted for age, the type of latrine used by the household, and presence of a market in the village were slightly higher than the unadjusted odds ratios. Alternating logistic regression provided useful estimates of disease clustering within villages and household while allowing for covariate adjustment.
Invited commentary: ecologic biases.
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Efficacy of comprehensive rehabilitation programs and back school for patients with low back pain: a meta-analysis.
BACKGROUND AND PURPOSE: The use of back school as a treatment for low back pain is widespread, but determining the efficacy of this approach is complicated by variations in back schools and study methods across clinical trials. The purpose of this study was to conduct a meta-analysis to synthesize existing evidence on the efficacy of back school as either a primary intervention or a part of a comprehensive rehabilitation program for patients with low back pain. METHODS: The results of 19 prospective randomized controlled trials were evaluated. Quantitative reviewing procedures were used to calculate the effect sizes that compared patients receiving back school with those in a control or comparison group. Effect sizes were computed for 206 hypothesis tests involving 2,373 patients. RESULTS: The average effect size for comprehensive rehabilitation programs that included back school (d = 0.28) was larger than the average effect size for programs that offered back school as the primary intervention (d = -0.14). When effect sizes were stratified by program type and outcome, the comprehensive programs were superior to primary back school programs with respect to pain reduction, increased spinal mobility, and increased strength. Both types of programs showed reasonable success with education/compliance outcomes (d = 0.27-0.28). Lower effect sizes were found among the types of programs for disability and work/vocational outcomes (d < or = 0.20). CONCLUSIONS AND DISCUSSION: Back schools were most efficacious when coupled with a comprehensive rehabilitation program. Efficacy was supported for the treatment of pain and physical impairments and for education/compliance outcomes. Work/vocational and disability outcomes, however, were not improved substantially beyond control levels in comprehensive or primary back school programs. [Di Fabio RP. Efficacy of comprehensive rehabilitation programs and back school for patients with low back pain: a meta-analysis.
The Hawthorne studies-a fable for our times?
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The awareness of being observed changes the patient's psychological well-being in anesthesia.
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More about the Hawthorne effect.
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Tap or bottled water consumption and spontaneous abortion: a 1986 case-control study in California.
To investigate whether drinking tap or bottled water during pregnancy affects the risk of spontaneous abortion, we asked questions about water consumption in a large case-control study (626 cases, 1,300 controls). The study ascertained cases from hospital pathology laboratory reports of pregnancies that began in 1986 and obtained controls from birth certificates. The crude odds ratio for consumption of any vs no cold tapwater at home during the first trimester was 1.2 (95% confidence interval = 1.0-1.5), with no dose-response effect. The crude odds ratio for any bottled water consumption was 0.79 (95% confidence interval = 0.65-0.96), with a downward trend by amount consumed. Adjusting for many potential confounders did not alter these associations appreciably, although some variables appeared to be effect modifiers. The point estimates were stronger among women who were more difficult to contact, suggesting the possibility of bias.
Alcohol consumption as a modifier of the relation between blood lead and blood pressure.
Hypothetically, blood lead may be involved in blood pressure elevations owing to a facilitation of vascular adrenergic stimulation by extrinsic factors like alcohol. We therefore investigated whether self-reported alcohol consumption was a modifier of the blood lead-blood pressure relation in 3,664 men and women, age 28-67 years, from the MONICA Augsburg study in Germany. In women, only heavy drinkers (40 gm alcohol per day or more) showed a strong relation of blood lead with systolic and diastolic blood pressure, whereas little association was observed for abstaining and for moderately drinking women. In men, we found similar alcohol modifications of the lead-pressure relation only for those with rural, rather than urban, places of residence. We have no explanation for this subgroup divergence. Our results may be taken to indicate a possible role of alcohol consumption in modifying the action of lead on blood pressure.
Complexity and the Hawthorne effect in community trials.
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Student perceptions of the effectiveness of a problem-based surgery curriculum.
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