Sarah A. Nowak

Dynamic boundaries in asymmetric exclusion processes.

We investigate the dynamics of a one-dimensional asymmetric exclusion process with Langmuir kinetics and a fluctuating wall. At the left-hand boundary, particles are injected onto the lattice; from there, the particles hop to the right. Along the lattice, particles can adsorb or desorb, and the right-hand boundary is defined by a wall particle. The confining wall particle has intrinsic forward and backward hopping, a net leftward drift, and cannot desorb. Performing Monte Carlo simulations and using a moving-frame finite segment approach coupled to mean field theory, we find the parameter regimes in which the wall acquires a steady-state position. In other regimes, the wall will either drift to the left and fall off the lattice at the injection site, or drift indefinitely to the right. Our results are discussed in the context of nonequilibrium phases of the system, fluctuating boundary layers, and particle densities in the laboratory frame versus the frame of the fluctuating wall.

Mechanisms of Receptor/Coreceptor-Mediated Entry of Enveloped Viruses

Enveloped viruses enter host cells either through endocytosis, or by direct fusion of the viral envelope and the membrane of the host cell. However, some viruses, such as HIV-1, HSV-1, and Epstein-Barr can enter a cell through either mechanism, with the choice of pathway often a function of the ambient physical chemical conditions, such as temperature and pH. We develop a stochastic model that describes the entry process at the level of binding of viral glycoprotein spikes to cell membrane receptors and coreceptors. In our model, receptors attach the cell membrane to the viral membrane, while subsequent binding of coreceptors enables fusion. The model quantifies the competition between fusion and endocytotic entry pathways. Relative probabilities for each pathway are computed numerically, as well as analytically in the high viral spike density limit. We delineate parameter regimes in which fusion or endocytosis is dominant. These parameters are related to measurable and potentially controllable quantities such as membrane bending rigidity and receptor, coreceptor, and viral spike densities. Experimental implications of our mechanistic hypotheses are proposed and discussed.

Models of dynamic extraction of lipid tethers from cell membranes.

When a ligand that is bound to an integral membrane receptor is pulled, the membrane and the underlying cytoskeleton can deform before either the membrane delaminates from the cytoskeleton or the ligand detaches from the receptor. If the membrane delaminates from the cytoskeleton, it may be further extruded and form a membrane tether. We develop a phenomenological model for this process by assuming that deformations obey Hookes law up to a critical force at which the cell membrane locally detaches from the cytoskeleton and a membrane tether forms. We compute the probability of tether formation and show that tethers can be extruded only within an intermediate range of force loading rates and pulling velocities. The mean tether length that arises at the moment of ligand detachment is computed as are the force loading rates and pulling velocities that yield the longest tethers.

Social Network Effects of Nonlifesaving Early-Stage Breast Cancer Detection on Mammography Rates

OBJECTIVES We estimated the effect of anecdotes of early-stage, screen-detected cancer for which screening was not lifesaving on the demand for mammography. METHODS We constructed an agent-based model of mammography decisions, in which 10 000 agents that represent women aged 40 to 100 years were linked together on a social network, which was parameterized with a survey of 716 women conducted through the RAND American Life Panel. Our model represents a population in equilibrium, with demographics reflecting the current US population based on the most recent available census data. RESULTS The aggregate effect of women learning about 1 category of cancers-those that would be detected but would not be lethal in the absence of screening-was a 13.8 percentage point increase in annual screening rates. CONCLUSIONS Anecdotes of detection of early-stage cancers relayed through social networks may substantially increase demand for a screening test even when the detection through screening was nonlifesaving.

An Assessment of the New York Health Act: A Single-Payer Option for New York State

The New York State legislature is considering single-payer legislation — the New York Health Act — that would create a state-sponsored single-payer health program called New York Health to provide coverage to all residents of the state. This reports authors assessed how the plan would affect health care utilization and costs in the state, estimating its effects on key outcomes and comparing them with future outcomes under the status quo.

Association Between Physicians' Experiences With Members of Their Social Network and Efforts to Reduce Breast Cancer Screening

Helping clinicians reflect on how their experiences influence their current screening patterns may be an important approach to improving their adherence to revised breast cancer screening guidelines.

A General Agent-Based Model of Social Learning

When engaging in behaviors that may entail risks or outcomes that are unknown or uncertain, individuals often look beyond their own experiences (including past behaviors and subsequent outcomes) to consider the experiences of others in their immediate social networks. This social influence at the micro-scale (i.e., the way in which individuals are influenced by their immediate social networks) can affect change in the greater social web in such a way that social networks may have profound effects on decisionmaking at the population level. Such micro-level social influence is central to many theories of individual decisionmaking and behavior. Observations of population-level dynamics at the macro-level demonstrate the end result of these processes-for example, over time, peoples behavior tends to look more like that of their peers. This article describes a general agent-based model (ABM) for studying social influence, and uses that general ABM to explore the relationship between micro-influence and macro-dynamics for broad classes of problems. We also describe an approach to tailor the general ABM to model a specific behavior influenced by social learning, which we illustrate using surveys designed to inform the ABM. The framework we developed could be useful for studying any system in which social learning may occur. But while our general ABM can produce dynamics reminiscent of those that might result from many different types of behaviors, it will typically need to be tailored when used to model any particular behavior.

Linking Reminders and Physician Breast Cancer Screening Recommendations: Results From a National Survey

PURPOSE Physician reminders have successfully increased rates of mammography. However, considering recent changes to breast cancer screening guidelines that disagree on the optimal age to start and stop mammography screening, we sought to examine the extent to which reminders have been deployed for breast cancer screening targeting younger and older patients. METHODS A mailed survey was sent to a national sample of 2,000 primary care physicians between May and September 2016. Physicians were asked whether they received reminders to screen women in various age groups (40 to 44, 45 to 49, and ≥ 75 years), the organizational screening guidelines they trusted most, and whether they recommended routine breast cancer screening to average-risk women in the different age groups. Using regression models, we assessed the association between reminders and physician screening recommendations, controlling for physician and practice characteristics, and evaluated whether the association varied by the guidelines they trusted. RESULTS A total of 871 physicians responded (adjusted response rate, 52.3%). Overall, 28.9% of physicians reported receiving reminders for patient ages 40 to 44 years, 32.5% for patient ages 45 to 49 years, and 16.5% for patient ages ≥ 75 years. Receiving reminders significantly increased the likelihood of physicians recommending mammography screening. In adjusted analyses, 84% (95% CI, 77% to 90%) of physicians who received reminders recommended screening for women ages ≥ 75 versus 65% (95% CI, 62% to 69%) of those who did not receive reminders. The associations between reminders and screening recommendations remained consistent regardless of which guidelines physicians reported trusting. CONCLUSION Reminders were significantly associated with increases in physician screening recommendations for mammography, underscoring the need for careful implementation in scenarios where guidelines are discordant.

The RAND Health Care Payment and Delivery Simulation Model (PADSIM): Concepts, Methods, and Examples

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