Missing Data in Marginal Structural Models: A Plasmode Simulation Study Comparing Multiple Imputation and Inverse Probability Weighting.

BACKGROUND: The use of marginal structural models (MSMs) to adjust for time-varying confounding has increased in epidemiologic studies. However, in the setting of MSMs, recommendations for how best to handle missing data are contradictory. We present a plasmode simulation study to compare the validity and precision of MSMs estimates using complete case analysis (CC), multiple imputation (MI), and inverse probability weighting (IPW) in the presence of missing data on time-independent and time-varying confounders. MATERIALS AND METHODS: Simulations were based on a cohort substudy using data from the Osteoarthritis Initiative which estimated the marginal causal effect of intra-articular injection use on yearly changes in knee pain. We simulated 81 scenarios with parameter values varied on missing mechanisms (MCAR, MAR, and MNAR), percentages of missing (10%, 20%, and 30%), type of confounders (time-independent, time-varying, either or both), and analytical approaches (CC, IPW, and MI). The performance of CC, IPW, and MI methods was compared using relative bias, mean squared error of the estimates of interest, and empirical power. RESULTS: Across scenarios defined by missing data mechanism, extent of missing data, and confounder type, MI generally produced less biased estimates (range: 1.2%-6.7%) with better precision (range: 0.17-0.18) compared with IPW (relative bias: -5.3% to 8.0%; precision: 0.19-0.53). Empirical power was constant across the scenarios using MI. CONCLUSIONS: Under simple yet realistically constructed scenarios, MI seems to confer an advantage over IPW in MSMs applications.

Genomic epidemiology of meticillin-resistant Staphylococcus aureus ST22 widespread in communities of the Gaza Strip, 2009.

BackgroundRemarkably high carriage prevalence of a community-associated meticillin-resistant Staphylococcus aureus (MRSA) strain of sequence type (ST) 22 in the Gaza strip was reported in 2012. This strain is linked to the pandemic hospital-associated EMRSA-15. The origin and evolutionary history of ST22 in Gaza communities and the genomic elements contributing to its widespread predominance are unknown. Methods: We generated high-quality draft genomes of 61 ST22 isolates from Gaza communities and, along with 175 ST22 genomes from global sources, reconstructed the ST22 phylogeny and examined genotypes unique to the Gaza isolates. Results: The Gaza isolates do not exhibit a close relationship with hospital-associated ST22 isolates, but rather with a basal population from which EMRSA-15 emerged. There were two separate resistance acquisitions by the same MSSA lineage, followed by diversification of other genetic determinants. Nearly all isolates in the two distinct clades, one characterised by staphylococcal cassette chromosome mec (SCCmec) IVa and the other by SCCmec V and MSSA isolates, contain the toxic shock syndrome toxin-1 gene. Discussion: The genomic diversity of Gaza ST22 isolates is not consistent with recent emergence in the region. The results indicate that two divergent Gaza clones evolved separately from susceptible isolates. Researchers should not assume that isolates identified as ST22 in the community are examples of EMRSA-15 that have escaped their healthcare roots. Future surveillance of MRSA is essential to the understanding of ST22 evolutionary dynamics and to aid efforts to slow the further spread of this lineage.

Genomic Epidemiology of Penicillin-Nonsusceptible Pneumococci with Nonvaccine Serotypes Causing Invasive Disease in the United States.

Conjugate vaccination against seven pneumococcal serotypes (PCV7) reduced disease prevalence due to antibiotic-resistant strains throughout the 2000s. However, diseases caused by resistant nonvaccine type (NVT) strains increased. Some of these emerging strains were derived from vaccine types (VT) that had changed their capsule by recombination. The introduction of a vaccine targeting 13 serotypes (PCV13) in 2010 has led to concern that this scenario will repeat itself. We generated high-quality draft genomes from 265 isolates of NVT pneumococci not susceptible to penicillin (PNSP) in 2009 and compared them with the genomes of 581 isolates from 2012 to 2013 collected by the Active Bacterial Core surveillance (ABCs) of the Centers for Disease Control and Prevention (CDC). Of the seven sequence clusters (SCs) identified, three SCs fell into a single lineage associated with serogroup 23, which had an origin in 1908 as dated by coalescent analysis and included isolates with a divergent 23B capsule locus. Three other SCs represented relatively deep-branching lineages associated with serotypes 35B, 15A, and 15BC. In all cases, the resistant clones originated prior to 2010, indicating that PNSP are at present dominated by descendants of NVT clones present before vaccination. With one exception (15BC/ST3280), these SCs were related to clones identified by the Pneumococcal Molecular Epidemiology Network (PMEN). We conclude that postvaccine diversity in NVT PNSP between 2009 and 2013 was driven mainly by the persistence of preexisting strains rather than through de novo adaptation, with few cases of serotype switching. Future surveillance is essential for documenting the long-term dynamics and resistance of NVT PNSP.

Microbial Genomics of Ancient Plagues and Outbreaks.

The recent use of next-generation sequencing methods to investigate historical disease outbreaks has provided us with an unprecedented ability to address important and long-standing questions in epidemiology, pathogen evolution, and human history. In this review, we present major findings that illustrate how microbial genomics has provided new insights into the nature and etiology of infectious diseases of historical importance, such as plague, tuberculosis, and leprosy. Sequenced isolates collected from archaeological remains also provide evidence for the timing of historical evolutionary events as well as geographic spread of these pathogens. Elucidating the genomic basis of virulence in historical diseases can provide relevant information on how we can effectively understand the emergence and re-emergence of infectious diseases today and in the future.

Impact of Host Heterogeneity on the Efficacy of Interventions to Reduce Staphylococcus aureus Carriage.

BACKGROUND: Staphylococcus aureus is a common cause of bacterial infections worldwide. It is most commonly carried in and transmitted from the anterior nares. Hosts are known to vary in their proclivity for S. aureus nasal carriage and may be divided into persistent carriers, intermittent carriers, and noncarriers, depending on duration of carriage. Mathematical models of S. aureus to predict outcomes of interventions have, however, typically assumed that all individuals are equally susceptible to colonization. OBJECTIVE: To characterize biases created by assuming a homogeneous host population in estimating efficacy of control interventions. DESIGN: Mathematical model. METHODS: We developed a model of S. aureus carriage in the healthcare setting under the homogeneous assumption as well as a heterogeneous model to account for the 3 types of S. aureus carriers. In both models, we calculated the equilibrium carriage prevalence to predict the impact of control measures (reducing contact and decolonization). RESULTS: The homogeneous model almost always underestimates S. aureus transmissibility and overestimates the impact of intervention strategies in lowering carriage prevalence compared to the heterogeneous model. This finding is generally consistent regardless of changes in model setting that vary the proportions of various carriers in the population and the duration of carriage for these carrier types. CONCLUSIONS: Not accounting for host heterogeneity leads to systematic and substantial biases in predictions of the effects of intervention strategies. Further understanding of the clinical impacts of heterogeneity through modeling can help to target control measures and allocate resources more efficiently.

Antibiotics in agriculture and the risk to human health: how worried should we be?

The use of antibiotics in agriculture is routinely described as a major contributor to the clinical problem of resistant disease in human medicine. While a link is plausible, there are no data conclusively showing the magnitude of the threat emerging from agriculture. Here, we define the potential mechanisms by which agricultural antibiotic use could lead to human disease and use case studies to critically assess the potential risk from each. The three mechanisms considered are as follows 1: direct infection with resistant bacteria from an animal source, 2: breaches in the species barrier followed by sustained transmission in humans of resistant strains arising in livestock, and 3: transfer of resistance genes from agriculture into human pathogens. Of these, mechanism 1 is the most readily estimated, while significant is small in comparison with the overall burden of resistant disease. Several cases of mechanism 2 are known, and we discuss the likely livestock origins of resistant clones of Staphylococcus aureus and Enterococcus faecium, but while it is easy to show relatedness the direction of transmission is hard to assess in robust fashion. More difficult yet to study is the contribution of mechanism 3, which may be the most important of all.

Stability of the pneumococcal population structure in Massachusetts as PCV13 was introduced.

BACKGROUND: The success of 7-valent pneumococcal conjugate vaccination (PCV-7) introduced to the US childhood immunization schedule in 2000 was partially offset by increases in invasive pneumococcal disease (IPD) and pneumococcal carriage due to non-vaccine serotypes, in particular 19A, in the years that followed. A 13-valent conjugate vaccine (PCV-13) was introduced in 2010. As part of an ongoing study of the response of the Massachusetts pneumococcal population to conjugate vaccination, we report the findings from the samples collected in 2011, as PCV-13 was introduced. METHODS: We used multilocus sequence typing (MLST) to analyze 367 pneumococcal isolates carried by Massachusetts children (aged 3 months-7 years) collected during the winter of 2010-11 and used eBURST software to compare the pneumococcal population structure with that found in previous years. RESULTS: One hundred and four distinct sequence types (STs) were found, including 24 that had not been previously recorded. Comparison with a similar sample collected in 2009 revealed no significant overall difference in the ST composition (p = 0.39, classification index). However, we describe clonal dynamics within the important replacement serotypes 19A, 15B/C, and 6C, and clonal expansion of ST 433 and ST 432, which are respectively serotype 22F and 21 clones. CONCLUSIONS: While little overall change in serotypes or STs was evident, multiple changes in the frequency of individual STs and or serotypes may plausibly be ascribed to the introduction of PCV-13. This 2011 sample documents the initial impact of PCV-13 and will be important for comparison with future studies of the evolution of the pneumococcal population in Massachusetts.

Epigenomic association analysis identifies smoking-related DNA methylation sites in African Americans.

Cigarette smoking is an environmental risk factor for many chronic diseases, and disease risk can often be managed by smoking control. Smoking can induce cellular and molecular changes, including epigenetic modification, but the short- and long-term epigenetic modifications caused by cigarette smoking at the gene level have not been well understood. Recent studies have identified smoking-related DNA methylation (DNAm) sites in Caucasians. To determine whether the same DNAm sites associate with smoking in African Americans, and to identify novel smoking-related DNAm sites, we conducted a methylome-wide association study of cigarette smoking using a discovery sample of 972 African Americans, and a replication sample of 239 African Americans with two array-based methods. Among 15 DNAm sites significantly associated with smoking after correction for multiple testing in our discovery sample, 5 DNAm sites are replicated in an independent cohort, and 14 sites in the replication sample have effects in the same direction as in the discovery sample. The top two smoking-related DNAm sites in F2RL3 (factor II receptor-like 3) and GPR15 (G-protein-coupled receptor 15) observed in African Americans are consistent with previous findings in Caucasians. The associations between the replicated DNAm sites and smoking remain significant after adjusting for genetic background. Despite the distinct genetic background between African Americans and Caucasians, the DNAm from the two ethnic groups shares common associations with cigarette smoking, which suggests a common molecular mechanism of epigenetic modification influenced by environmental exposure.

Meningococcal disease: changes in epidemiology and prevention.

The human bacterial pathogen Neisseria meningitidis remains a serious worldwide health threat, but progress is being made toward the control of meningococcal infections. This review summarizes current knowledge of the global epidemiology and the pathophysiology of meningococcal disease, as well as recent advances in prevention by new vaccines. Meningococcal disease patterns and incidence can vary dramatically, both geographically and over time in populations, influenced by differences in invasive meningococcal capsular serogroups and specific genotypes designated as ST clonal complexes. Serogroup A (ST-5, ST-7), B (ST-41/44, ST-32, ST-18, ST-269, ST-8, ST-35), C (ST-11), Y (ST-23, ST-167), W-135 (ST-11) and X (ST-181) meningococci currently cause almost all invasive disease. Serogroups B, C, and Y are responsible for the majority of cases in Europe, the Americas, and Oceania; serogroup A has been associated with the highest incidence (up to 1000 per 100,000 cases) and large outbreaks of meningococcal disease in sub-Saharan Africa and previously Asia; and serogroups W-135 and X have emerged to cause major disease outbreaks in sub-Saharan Africa. Significant declines in meningococcal disease have occurred in the last decade in many developed countries. In part, the decline is related to the introduction of new meningococcal vaccines. Serogroup C polysaccharide-protein conjugate vaccines were introduced over a decade ago, first in the UK in a mass vaccination campaign, and are now widely used; multivalent meningococcal conjugate vaccines containing serogroups A, C, W-135, and/or Y were first used for adolescents in the US in 2005 and have now expanded indications for infants and young children, and a new serogroup A conjugate vaccine has recently been introduced in sub-Saharan Africa. The effectiveness of these conjugate vaccines has been enhanced by the prevention of person-to-person transmission and herd immunity. In addition, progress has been made in serogroup B-specific vaccines based on conserved proteins and outer membrane vesicles. However, continued global surveillance is essential in understanding and predicting the dynamic changes in the epidemiology and biological basis of meningococcal disease and to influence the recommendations for current and future vaccines or other prevention strategies.