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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.
To characterize biases created by assuming a homogeneous host population in estimating efficacy of control interventions.
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).
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.
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.
Infect. Control Hosp. Epidemiol. 2016;37(2):197–204
Assessments of infectious disease spread in hospitals seldom account for interfacility patient sharing. This is particularly important for pathogens with prolonged incubation periods or carrier states.
We quantified patient sharing among all 32 hospitals in Orange County (OC), California, using hospital discharge data. Same-day transfers between hospitals were considered “direct” transfers, and events in which patients were shared between hospitals after an intervening stay at home or elsewhere were considered “indirect” patient-sharing events. We assessed the frequency of readmissions to another OC hospital within various time points from discharge and examined interhospital sharing of patients with Clostridium difficile infection.
In 2005, OC hospitals had 319,918 admissions. Twenty-nine percent of patients were admitted at least twice, with a median interval between discharge and readmission of 53 days. Of the patients with 2 or more admissions, 75% were admitted to more than 1 hospital. Ninety-four percent of interhospital patient sharing occurred indirectly. When we used 10 shared patients as a measure of potential interhospital exposure, 6 (19%) of 32 hospitals “exposed” more than 50% of all OC hospitals within 6 months, and 17 (53%) exposed more than 50% within 12 months. Hospitals shared 1 or more patient with a median of 28 other hospitals. When we evaluated patients with C. difficile infection, 25% were readmitted within 12 weeks; 41% were readmitted to different hospitals, and less than 30% of these readmissions were direct transfers.
In a large metropolitan county, interhospital patient sharing was a potential avenue for transmission of infectious agents. Indirect sharing with an intervening stay at home or elsewhere composed the bulk of potential exposures and occurred unbeknownst to hospitals.
Vaccination has been an undisputed success in the control of many infectious diseases, both viral and bacterial. In recent decades, researchers have attempted to extend these successes to the development of vaccines against a variety of other infectious agents, ranging from long-standing public health threats like typhoid, gonorrhea, and malaria, to newly emerging or newly discovered organisms, such as human immunodeficiency virus (HIV) and hepatitis B virus. While some of these vaccine development efforts have succeeded quite rapidly – the hepatitis B vaccine is a good example – many have not yet produced highly effective vaccines. The presence of substantial antigenic diversity is a common feature that characterizes many of the infections for which vaccines have proved elusive. This diversity can take either, or both, of two forms. Within a single infected host, the expression of particular antigens may change during the course of an infection by a variety of mechanisms, including intragenomic recombination, phase variation through changes in the lengths of oligonucleotide repeats, and simple mutation. This process of antigenic variation may disrupt antigen-specific immune responses, with important consequences for the maintenance of infection and pathogen virulence. Antigenic diversity can also occur at the population level; in this case, the pathogens of a particular species circulating in the host population are characterized by polymorphism in one or more antigens. Each of these forms of polymorphism may increase the number of antigenic variants that a vaccine must “cover” to give strong protection, thereby increasing the difficulty of vaccine development.
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