Epidemiology

Selection bias is a common concern in causal inference. Two mechanisms can give rise to selection bias: collider bias or stratifying on an effect modifier of the exposure-outcome causal effect. Research into the impact of selection bias on instrumental variable (IV) analyses has focused primarily on collider bias. Here, we show that IV estimates can be biased if selection is an effect modifier (or caused by an effect modifier) of the exposure-outcome effect. In a simulation study, we investigate the size and direction of the bias when estimating effects on the mean difference or odds ratio scale. Bias can also occur due to heterogeneity in the instrument-exposure relationship, although its magnitude is typically smaller. Finally, we discuss methods to deal with this bias in practice, including inverse probability weighting (IPW).

Introduction Health care proxies serve as surrogate decision-makers for patients with limited decisional capacity, yet only one in three U.S. adults have designated one. This trial will evaluate low-cost, scalable behavioral interventions embedded in previsit mailed outreach to increase health care proxy documentation. Methods and analysis In this 2 x 2 + 1 factorial superiority trial, approximately 20,000 patients with an upcoming in-person primary care visit at a Northern California health system will be randomized over a 12-month period into one of four intervention mailers encouraging designation of a health care proxy, or to no mailer under usual care. Eligible patients will have no documentation of a health care proxy in the electronic health record (EHR) and will be aged 55 years or older, or 18 years or older with cancer, congestive heart failure, or chronic kidney disease. The four mailers differ by the presence or absence of a pre-commitment prompt, and whether the mailer is sent from the patient's primary care provider or the health system. The primary outcome is EHR-documentation of a health care proxy by 30 days after the index scheduled visit date. Secondary outcomes include proxy documentation through 90 days, legal advance care planning documentation through 30 and 90 days, and visit attendance. We will use logistic regression to estimate the intention-to-treat effect of receiving any intervention mailer versus no mailer, a pre-commitment mailer versus a mailer without pre-commitment, and a provider-signed versus health system-signed mailer. Ethics and dissemination The University of Southern California Institutional Review Board approved the protocol and granted a waiver of informed consent for this minimal-risk trial. Results will be posted on ClinicalTrials.gov within one year of the primary completion date. Trial Registration The trial was prospectively registered on December 12, 2025 at ClinicalTrials.gov (NCT07290478).

Background: Waning vaccine protection and variable epidemic timing create a tradeoff between vaccinating before influenza circulation and preserving protection when disease burden is greatest. Existing guidance does not quantify the week that best balances this tradeoff. Objective: To estimate the optimal week for influenza vaccination while accounting for uncertainty in epidemic timing and vaccine protection. Design: Simulation-based probabilistic decision analysis. Setting: United States outpatient influenza surveillance from the 50 states and District of Columbia during the 2010/11 through 2018/19 and 2023/24 through 2025/26 seasons. Participants: People in the United States who would receive 1 influenza vaccine dose. Interventions: Vaccination during Morbidity and Mortality Weekly Report (MMWR) weeks 36 through 12. Measurements: We combined influenza-like illness (ILI) syndromic surveillance and influenza-specific data with uncertainty in epidemic timing, initial vaccine effectiveness, waning, and immune-response lag. For each candidate week, the model estimated direct protection against outpatient surveillance burden. Results: The primary model identified the optimal time to vaccinate as week 47, with weeks 47-48 being of similar utility. Our influenza-weighted outpatient analysis (ILI+) selected week 48. Compared with week 44, near the end of the current September-October guidance window, week 48 reduced modeled influenza-weighted outpatient burden by 50 visit equivalents per 10,000 weekly encounters. Limitations: The model assumed vaccination would occur, used outpatient surveillance burden rather than severe outcomes, and did not incorporate transmission effects. Conclusion: Under the primary waning model, these findings favor vaccinating during weeks 47-48, approximately late November through early December, for most one-dose recipients.

Background: Doxycycline post-exposure prophylaxis (doxy-PEP) reduces the incidence of bacterial sexually transmitted infections (STIs) among men who have sex with men and transgender women (MSMTW), but it may select for antimicrobial resistance (AMR). AMR development will depend in part how doxy-PEP changes rates of antibiotic use. Methods: We conducted a retrospective electronic medical record review of antibiotic prescriptions received by patients at the Massachusetts General Hospital Sexual Health Clinic from January 1, 2023, to December 27, 2025. Using a Bayesian negative binomial regression, we assessed the direct, indirect, and combined effects of doxy-PEP implementation on antibiotic prescription rates among doxy-PEP-eligible MSMTW who were receiving HIV pre-exposure prophylaxis. Results: Controlling for direct effects, the cohort's total antibiotic prescription rate decreased by 10% (0.90, 95% CI 0.87 - 0.94) for every 100 doxy-PEP starts. Doxy-PEP users were prescribed antibiotics at double the rate predicted in the absence of doxy-PEP implementation, and, controlling for indirect effects, received 3.40 (95% CI 2.95 - 3.91) times the antibiotic prescriptions of patients not using doxy-PEP. Doxy-PEP non-users were prescribed antibiotics at less than half the rate predicted in the absence of doxy-PEP. The full cohort's overall antibiotic prescription rate increased by 1.4 times after doxy-PEP implementation. Conclusions: Individuals who are taking doxy-PEP have higher antibiotic prescription rates, increasing selection for antibiotic-resistant bacteria in these individuals. However, doxy-PEP-driven decreases in the overall incidence of bacterial STIs have the potential to decrease selective pressure for resistant organisms in those not using doxy-PEP.

Background. Mendelian randomization (MR) studies often evaluate exposures such as LDL cholesterol (LDL-C). The widespread use of lipid lowering medications complicates the interpretation and the validity of MR estimates. Methods. We describe two causal estimands in populations with medication use: a lifetime effect, and a lifetime effect under no medication use. In simulations, we compared the common approach of excluding medication users with estimation based on inverse probability (IP) weighting to adjust for medication use. We applied both approaches to a MR analysis of LDL-C and coronary artery disease in the Million Veteran Program (MVP), a large prospective cohort of U.S. veterans with linked electronic health record and genetic data. Results. In simulations, MR analyses that did not adjust for medication use estimated a lifetime effect that reflected a valid estimate of a total effect that included both the harms of higher LDL-C and the benefits of statins. To estimate the effect under no medication use, excluding statin users introduced selection bias. Alternatively, IP weighting could address bias from incident statin users, but could not address bias related to prevalent medication use. Estimates from MVP data varied considerably, reflecting the importance of these analytic choices. Conclusions. Different medication adjustment strategies in MR studies implicitly target different causal estimands and are subject to distinct biases. Transparent analytical choices and careful interpretation are essential for informative MR results in the context of widespread medication use.

Importance. Observational studies dominate evidence in cardiovascular outcomes research, yet no widely adopted reporting standard requires authors to disclose how much of an observed association is attributable to systematic error. The negative-control calibration framework of the Observational Health Data Sciences and Informatics consortium returns a binary answer: a calibrated P-value either crosses 0.05 or it does not. That answer conflates "is there a signal" with "is the signal credible." Objective. To define the Bias Attribution Fraction (BAF), a continuous estimator in [0, 1] that quantifies the fraction of an observed log relative risk that is attributable to systematic bias, together with its 95% credible interval, and to evaluate whether reporting calibrated P-value plus BAF plus interval width produces more defensible decisions than reporting the calibrated P-value alone. Design. Three-part study: (i) metric definition and interval construction; (ii) Monte Carlo simulation across 1920 conditions by 1000 repetitions = 1,920,000 simulated target estimates (a primary grid plus a sign-flipped companion grid of identical size, pooled for all analyses), with truth known by construction; (iii) two-arm beta-blocker target trial emulation in MIMIC IV, adhering to the TARGET guideline, with targeted maximum likelihood estimation (TMLE) and 12 negative-control outcomes. Setting. MIMIC IV (critical care) and MIMIC IV ECG, 2008 to 2019. Participants. Adults with a first intensive care unit stay and a recorded 12-lead ECG. Exposures. Two clinical questions are evaluated: (1) reaching at least 50% of guideline-recommended target dose of an index beta-blocker within 7 days of admission, sustained to 12 months, versus lower or no exposure; (2) receiving guideline-directed medical therapy (GDMT) with at least two of three foundational drug classes at adequate dose for 12 months versus fewer. Main Outcomes and Measures. One-year all-cause mortality. Primary analytic outputs: (i) the calibrated P-value from a negative-control empirical null distribution with 12 outcomes; (ii) BAF-hat with a Markov chain Monte Carlo 95% credible interval; (iii) a verdict drawn from a truth-calibrated lookup table mapping the BAF-hat by interval-width cell to a probability of being bias-dominated. Results. Across 1,920,000 simulated repetitions, the calibrated P-value alone yielded a misuse rate (declared usable while truth was bias-dominated) of 36.02%. Adding the BAF point estimate reduced misuse to 16.48%, and requiring the whole credible interval of BAF to lie below 0.5, clear of the bias-dominated zone, reduced misuse to 10.16%. Requiring in addition a calibrated probability of bias dominance below 15% reduced misuse to 3.02% at the cost of declaring 22.1% of estimates usable (versus 61.8% for the calibrated P-value alone). Read from the bias-dominated view, meaning that withholding a repetition is a positive call and being truly bias-dominated is a positive truth, the BAF estimator achieved an area under the receiver operating characteristic curve of 0.922, compared with 0.787 for the calibrated P-value. In MIMIC IV, question 1 produced a calibrated P-value of 0.894 and BAF-hat = 0.875 (95% credible interval 0.385 to 0.992); the apparent one-year mortality reduction of 16.9% was entirely attributable to systematic bias and the association was not usable as effect evidence. Question 2 also failed layer 1, with a calibrated P-value of 0.1026, and BAF-hat = 0.460 placed it in the "competitive, no verdict" zone. Conclusions and Relevance. Pairing the calibrated P-value with BAF and its 95% credible interval turns a binary credibility judgement into a three-channel report (signal existence, effect size, and bias fraction). Despite TARGET-adherent design, doubly robust TMLE, and 12 negative controls, both clinical questions in this study still failed to qualify as effect evidence, with question 1 demonstrating complete bias domination and BAF quantifying the systematic error that remained. The reporting standard proposed here can be applied to any observational study that includes negative controls, without requiring additional data.

In competing risks settings, covariate effects and group comparisons are usually assessed one event at a time - through log-rank or Cox tests on the cause-specific hazards, or Gray's test or Fine-Gray regression on a cumulative incidence function (CIF). This can obscure a clinically important quantity: the ratio between the event of interest and the competing event, since groups may differ little on the individual events yet differ sharply in their ratio. The generalized competing event (GCE) framework makes this ratio the object of inference; on the cause-specific scale the hazard ratio omega+(t) = lambda_1(t)/lambda_2(t) is estimated efficiently from a single stacked (Lunn-McNeil) model. We extend the framework to two scales that describe realized incidence. The subdistribution hazard ratio omega-tilde+(t) = lambda-tilde_1(t)/lambda-tilde_2(t) is estimated by a stacked, risk-set-weighted extension of the Lunn-McNeil construction; the cumulative-incidence ratio rho(t) = F_1(t)/F_2(t) - the odds that a subject's realized event by time t is the event of interest - by jackknife pseudo-observation regression of the Aalen-Johansen estimator. We relate the three contrasts: rho equals omega+ exactly under proportional cause-specific hazards, and equals omega-tilde+ only in the small-time limit under proportional subdistribution hazards, drifting toward 1 thereafter. The orthogonality that makes omega+ efficient is lost on both cumulative-incidence scales - omega tilde+ through overlapping weighted risk sets and shared censoring weights, rho through the shared all-cause survivor - so each carries a covariance term that must be handled and that bounds efficiency relative to the hazard-scale test. We derive the corresponding variances, study operating characteristics by simulation, illustrate on hypothetical prostate and head-and-neck cohorts, and provide an implementation in the gcemod R package.

Research replication is essential to build trust in evidence produced from real-world data. However, methods for conducting and reporting these studies are lacking, particularly related to data quality and fitness assessments. We replicated a single-center study from Children's Hospital of Atlanta in a multi-institutional learning network (PEDSnet) to evaluate the long-term effects of hydroxyurea in children with severe sickle cell disease (SS/S{beta}0 genotype). An AS-IS arm applied the original study's criteria with no major data quality adjustments, while a Data Fitness Enhanced (DFE) arm used systematic data fitness assessment to inform adjustments to cohort inclusion criteria and variable definitions; both arms then replicated the original study's primary analyses. Data quality checks in the DFE arm refined cohort criteria and improved hydroxyurea capture, drug era computation, and hematology specialist mapping. The DFE cohort produced average treatment effects with higher face validity and greater concordance with the original study (e.g., change in ED visits: -0.44 (CI -0.60, -0.26) versus -0.36 (CI -0.57, -0.16) in the original study) than the AS-IS cohort (-0.08 (CI -0.26, 0.09)), which yielded several implausible results. These findings show that superficially plausible cohort characteristics do not guarantee valid results without transparent, systematic data fitness assessment.

Background: The clone-censor-weight (CCW) method is increasingly finding application in target trial emulation to compare treatment strategies involving grace periods. However, its performance has not been systematically evaluated against the ground truth. Many studies utilising CCW ignore time-varying covariates and informative pre-existing censoring. Heavy-tailed inverse probability of censoring weights (IPCW) may yield biased estimates and under-coverage. Methods: Simulation Part 1 evaluated the bias, root mean squared error, and coverage probability of CCW analysis under correctly specified and misspecified IPCW models across scenarios with or without time-varying covariates and informative pre-existing censoring. Part 2 varied the confounding strength, assessing whether inference failures could be detected using an IPCW tail-heaviness index (a weighted Hill estimator-derived Pareto-type tail index) and the estimator's standard deviation, the target of bootstrap standard error. Results: Under correct specification, all bias estimates were below 0.005; coverage approached the nominal level of 0.95 with increasing sample size. Omitting the time-varying covariate or mishandling pre-existing censoring yielded bias up to 0.069 and coverage substantially below 0.95. Under strong confounding, confidence intervals failed to attain nominal coverage, even with correct specification and large sample sizes, when the oracle tail-heaviness index was <=1. When the index was >1, the coverage approached 0.95 as the estimator's standard deviation decreased. Conclusions: CCW analysis yields accurate estimates and confidence intervals with nominal coverage when implemented correctly, and confounding is not excessively strong. Investigators should account for time-varying covariates and pre-existing censoring and evaluate the tail behaviour of IPCW distribution and estimator's bootstrap standard error.

Mendelian Randomization (MR) is a popular tool for inferring causal relationships between traits using genetic variants as instrumental variables. These methods have been extended to also determine the direction of causality. However, causal direction cannot be inferred from a statistical test or estimation procedure (i.e. from data alone) without further assumptions and the methods operating characteristics and relative performances are not well understood. We conducted a comprehensive simulation study to illustrate this issue by evaluating type I error and power of 17 summary-based MR methods for inferring the effect direction. These methods fall within three methodological families: MR Steiger, Causal Direction (CD), and bidirectional MR approaches, with scenarios ranging across combinations of horizontal pleiotropy, unmeasured confounding, measurement error, longitudinal feedback, and varying sample sizes. While most methods achieved sufficient power levels under the alternative hypothesis in most scenarios, we found that every method was susceptible to inferring the wrong causal direction or under powered, and no method consistently maintained both correct type 1 error control and high power. In our applications, we evaluated the effect direction between the trait pairs body mass index (BMI) and major depressive disorder (MDD) and between BMI and asthma. To help researchers to evaluate the 17 methods to infer the effect direction and consider these challenges in their own data, we have developed MRdirection, an R package that runs the simulation studies examining the 17 directional MR methods across different user-defined scenarios. Our study, together with the accompanying R package, provides researchers with a tool for examining directional MR methods given different underlying assumptions.

Researchers must often rely on observational data to draw conclusions about the causal relationship between an exposure and an outcome. This is common in environmental epidemiology research, where the assignment to an exposure can be unethical or impossible. Consider a setting in which we seek to estimate excess mortality during heat waves, but are unable to randomize individuals to moderate or extreme heat. We propose to follow a causal framework for non-randomized time series that uses what we call historical time series matching to construct hypothetical randomized experiments, in which background covariates are balanced. We applied this framework and assessed the following causal question: Are daily mortality counts among elderly Americans higher during hot versus warm summer time periods? We addressed this causal question by drawing frequentist, Fisherian, and Bayesian inferences for five American cities. We estimated the average treatment effect of heat on three-day mortality with 95% Fisherian intervals to be 0.5 [-3.6, 4.4] deaths in Pittsburgh, 3.7 [0.7, 6.6] deaths in Seattle, 12.5 [6.1, 18.9] deaths in Chicago, 8.5 [1.4, 15.6] deaths in Los Angeles, and 20.3 [11.2, 29.5] deaths in New York.

Longitudinal household studies, combined with mathematical modeling, are widely used to characterize the drivers of respiratory pathogen transmission, including the effects of age and symptoms. In practice, household recruitment protocols vary across studies, potentially introducing biases into observed data. However, these biases are typically overlooked in statistical inference, and their impact on parameter estimates remains unknown. Here, we use synthetic household outbreak data simulated under different recruitment protocols to evaluate how recruiting through infected children affects estimates of age-specific infectiousness and susceptibility. We show that, under child-based recruitment, the standard likelihood, which accounts only for transmission dynamics, leads to underestimating child infectiousness and overestimating child susceptibility by more than 30%. We then propose a novel estimation framework that explicitly incorporates the household recruitment process into the likelihood and show that it substantially reduces these biases. Applying this new approach to a French household study conducted during the COVID-19 pandemic, we estimated that children under 6 had 49% lower infectiousness than teenagers and adults during the Alpha wave, whereas no difference was observed during the Omicron wave. This study demonstrates that ignoring recruitment protocols can bias key epidemiological parameter estimates and highlights the importance of accounting for study design.

Abstract Routine blood tests can be used to calculate several inflammation-related ratios, including the neutrophil-to-lymphocyte ratio (NLR), systemic immune-inflammation index (SII), and platelet-to-lymphocyte ratio (PLR). These indices are widely reported as markers of cardiovascular risk, but it is unclear whether they provide comparable information or whether SII, which is derived from platelet count and NLR, adds anything beyond NLR. We analyzed two independent samples from the National Health and Nutrition Examination Survey (NHANES): 15,141 adults from 2005-2010 (development cohort) and 15,392 adults from 2011-2016 (validation cohort), each with complete blood count data and linked mortality follow-up. Survey-weighted Cox models, restricted cubic splines, multiple imputation, and model comparison were used, and the development model was applied to the validation cohort. During a median follow-up of 11.4 years, 795 cardiovascular deaths occurred in the development cohort; the validation cohort recorded 364 cardiovascular deaths over 5.75 years. In the development cohort, hazard ratios per 1-SD increase were 1.32 (95% CI 1.21-1.44) for NLR, 1.21 (1.12-1.32) for SII, and 1.13 (1.04-1.22) for PLR. When NLR and SII were entered together, only NLR remained associated (HR 1.49, 1.23-1.81). The validation cohort showed the same pattern: NLR was strongest (HR 1.35, 1.22-1.49), and neither SII nor PLR remained significant after mutual adjustment. The frozen model showed good discrimination (C-index 0.8991 to 0.9023) and calibration (slope 0.96). Adding NLR produced only a small improvement in discrimination, and reclassification results were inconsistent between cohorts.

In systematic review, the measures reported by the primary studies may not always be the measures sought by the systematic review investigators. Motivated by an systematic review of interventions to reduce meat consumption, we discuss here methods to infer relative risks and confidence intervals from randomized controlled trials that reported differences, possibly in the presence of repeated measures or multiple interventions.

Background: Quantifying behavioral drivers of protective action during infectious disease outbreaks remains challenging. App-based experimental epidemic games (epigames) provide a framework for studying these decisions in naturalistic settings, but correspondence between in-game measures and real-life beliefs or behaviors requires evaluation. We examined whether an activity-based opportunity cost influenced voluntary quarantine decisions within an epigame and whether real-life health beliefs transferred into the game and predicted observed choices. Methods: We conducted a two-week randomized controlled trial from May 4 to May 17, 2026, at VinUniversity (VinUni) in Hanoi, Vietnam. Participants using the Epigames app installed in their smartphone were randomized to a low-barrier group, earning 1 point per qualifying contact with other participant when remaining active, or a high-barrier group, earning 2 points per contact. Of 439 enrolled participants, 283 were successfully randomized to one of the two groups, made at least one valid daily quarantine decision, and their data was used in the analysis. Real-life (S1) and in-game (S2) beliefs measured susceptibility, severity, quarantine self-efficacy, and perceived benefits. Belief correspondence was assessed using Holm-adjusted Spearman correlations and prospective cumulative-logit models. Quarantine choices were analyzed using participant-day binomial generalized estimating equations (GEE) and participant-level quasi-binomial models. Results: Participants made 1,823 valid daily decisions and chose quarantine on 458 participant-days (25.1%). Quarantine rates were 30.1% in the low-barrier group and 20.2% in the high-barrier group. The high barrier was associated with 41% lower population-averaged odds of quarantine (GEE odds ratio [OR] 0.59, 95% confidence interval [CI] 0.40-0.86, = 0.0059); the quasibinomial estimate was identical (OR 0.59, 95% CI 0.40-0.86, = 0.0055). Among 251 participants with matched S1-S2 data, all four corresponding beliefs were positively associated (Spearman's = 0.259-0.418; all Holm-adjusted < 0.001). Prospective ordinal models confirmed positive matched-domain associations (ORs 1.39-2.18 per one-point S1 increase). Beliefs showed little robust association with quarantine behavior, and neither S1 nor S2 beliefs jointly modified the randomized effect. Self-efficacy, network degree, and the preregistered gender main effect were also unsupported. Conclusion: A higher activity-based opportunity cost reduced voluntary quarantine within the VinUni epigame, while participants carried relative differences in stated health beliefs from real-life to in-game framing. These beliefs did not reliably predict observed quarantine choices within the game framing. Epigames can support controlled study of incentives and belief correspondence in naturalistic settings such as college campuses, but behavioral validity or real-world generalizability requires further work.

The efficacy of current antiretroviral interventions for prevention of vertical HIV transmission is both a triumph and a challenge for future efficacy studies of novel agents, including long-acting antiretroviral agents. Given the effectiveness of current antiretroviral interventions, standard randomized controlled trials to assess novel interventions targeted towards further reducing vertical HIV transmission may be infeasible due to required sample sizes. To address this study design challenge, we propose a novel augmented clinical trial design, where a randomized trial with an active comparator is augmented with a concurrent observational cohort to supplement the standard-of-care arm. The proposed design and analysis can overcome sampling biases, and an associated diagnostic can evaluate potential biases from unmeasured covariates. The proposed trial design is illustrated with an example and studied further using a simulation study. In the illustrative example, an augmented trial with a 3:1 allocation ratio in the trial component yielded an unbiased treatment effect estimate, as compared to the trial component alone, and the standard error was reduced substantially. Although the total sample size of the augmented trial was larger than a similarly powered 1:1 standard trial, the sample size of the augmented trial's trial component was half the size of the standard trial and within the range of historically similar trials, thereby providing both feasibility and potential cost advantages. Although limitations with this design remain, many of these limitations can be mitigated by careful study planning and coordination. Augmented trials provide a potentially applicable solution to studying the efficacy of long-acting antiretrovirals for infant postnatal prophylaxis against HIV.

Estimating seroprevalence for an infection -- the proportion of individuals with antibodies -- and its variability between subpopulations is a key first step for research and the allocation of resources for treatment and prevention. The relevant raw data is often serological assay measurements that serve as a proxy for antibody level, such as optical density values from enzyme-linked immunosorbent assays (ELISA). Analysis pipelines typically proceed through sequential steps of fitting calibration data independently in each separate batch, transforming assay measurements to antibody levels via each fitted calibration relationship, classifying antibody levels into discrete serostatus by comparison to a threshold, and finally comparing seropositive proportions between subpopulations. Such approaches have numerous limitations including loss of information, overconfidence (discarding uncertainty), and inappropriate choice of threshold. We developed a Bayesian statistical model that replaces the sequential steps of the typical pipeline by multiple levels within a single hierarchical model, treating each sample's antibody level as a model parameter rather than directly observed data. This integrates the connections from the raw assay measurements all the way through to subpopulation variability in seroprevalence, allowing partial pooling of information between related variables and the propagation of uncertainty from each part of the model throughout the whole of the rest of the model. We calculate observation probabilities for all assay measurements from both samples and calibration data, allowing easy identification of outlying calibration or sample measurements. We replace a single hard classification threshold for disease status by continuous probabilities that are adapted to each subpopulation. We allow a flexible multivariate random-effects logistic regression to capture variability in seroprevalence between subpopulations. Using simulated data we found that the typical stepwise approach gave prevalence estimates far from the true values with narrow confidence intervals. Our method, dvsb, had markedly greater accuracy. We report the run time and convergence of dvsb when applied to a real dataset for Lassa fever IgG antibodies measured with ELISA, comprising 72,863 measurements for 21,391 unique samples (results reported elsewhere). dvsb is available at https://github.com/BDI-pathogens/dvsb.

Background and Objective. Missing data frequently occurs in health databases and can bias analyses if not correctly dealt with. Using real-world data, we compared complete-case and multiple-imputation methods for recovering true parameters of a multivariable logistic regression model for the association between maternal glucose levels during pregnancy and child excess weight at preschool age, where missing values were present in as much as 30% of our sample. Methods. This study utilized a cohort of 130,424 children with complete preschool-age body mass index (BMI) measurements from the Calgary and Edmonton health regions of Alberta, Canada. In the complete BMI data, we introduced missingness through deletion following three distinct mechanisms: missing completely at random (MCAR), at random (MAR), and not at random (MNAR). To handle the missing data created, we employed complete-case and multiple-imputation methods. Maternal glucose levels during pregnancy were categorized into five groups and its association with child excess weight at pre-school age was determined based on a logistic regression model using the full observed data (yielding true values), observed data that was not deleted (complete-case estimates), and imputed data (multiple-imputation estimates). The accuracy of complete-case and multiple-imputation estimates were evaluated against the true values. Finally, we conducted a sensitivity analysis for the MNAR mechanism using pattern-mixture models with an additive shift. Results. Under MCAR and MAR, multiple-imputation generally outperformed complete-case, yielding smaller absolute and relative bias. Both methods achieved high significance ([&ge;] 0.96) for most effects. Mean squared errors for multiple-imputation and complete-case were similar missing completely at random, missing at random, and coverage was consistently high ([&ge;] 0.99). Under MNAR, both complete-case and multiple-imputation showed poor performance regarding bias and statistical significance. Sensitivity analysis using pattern-mixture models indicated performance varied by specific effect. Conclusions. Under MCAR and MAR, multiple-imputation introduced higher bias but demonstrated superior overall performance based on mean squared error and restored statistical power. Conversely, both methods failed under MNAR, where pattern-mixture modeling sensitivity analyses revealed highly variable, effect-specific performance due to unverifiable shift assumptions. When faced with missing data, researchers should assess missingness mechanisms, report both complete-case and multiple-imputation estimates under MCAR/MAR while accounting for power-versus-bias tradeoffs, and employ pattern-mixture sensitivity analyses to test robustness when MNAR is plausible.

Age is a primary determinant of symptom severity and transmission patterns for many infectious diseases, motivating the use of age-stratified models parameterized by contact matrices. In the United States, the absence of direct contact surveys has required estimating synthetic contact matrices from demographic data on household size, school attendance, and workforce participation. However, this likely underestimates contacts among children under age 5, who often attend group childcare missing from censuses. The goal of this study was to use nationally-representative data on childcare arrangements (the Early Childhood Program Participation Survey) to reconstruct daily contacts occurring in childcare settings, and augment existing all-age contact matrices. For infants under 1 year of age, we estimated 0.2 daily contacts with other infants, increasing to 0.7 daily contacts with same-age peers for 1- or 2-year-olds, 1.3 for 3-year-olds, and 3.5 for 4-year-olds. Including childcare settings increases estimated contacts among young children by up to six fold. Using simulations of measles outbreaks in inadequately vaccinated populations, we show that prior contact matrices significantly underestimated the outbreak frequency, size, and impact on preschool age groups. Our findings highlight the need for targeted data collection on childcare contacts to improve model-based evaluation of interventions particularly for young children.

Estimating the correlation between subject-specific rates of change is a common objective in longitudinal studies. A natural approach is to fit ordinary least squares regressions per subject and correlate the resulting slope estimates directly. Under complete data, this slopes-first estimator is algebraically equivalent to a contrast-based plug-in estimator derived from the joint variance-covariance matrix of the two signals across time. Under independent dropout, however, the slopes-first approach implicitly applies heterogeneous weights across missing-data patterns, introducing bias whenever the cross-covariance between the two signals is not constant over time. We formalise this result and propose an unbiased alternative that estimates the joint covariance matrix before applying the contrast. We further extend our results to include covariates as well. Simulations confirm that the slopes-first estimator is biased under non-constant cross-covariance while the proposed alternative remains unbiased. Further, we show in a real-world example that the two estimators can lead to different scientific conclusions and recommend that researchers working with longitudinal data inspect the missing data pattern to make an informed decision about the estimator of choice.