Relationships between the importation, transmission, and nosocomial infections of methicillin-resistant Staphylococcus aureus: an observational study of 112 Veterans Affairs Medical Centers.

Background: The study of hospital methicillin-resistant Staphylococcus aureus (MRSA) epidemiology is complicated by its transmissibility. Our objective was to understand how MRSA importation and transmission influence MRSA nosocomial infections in Veterans Affairs Medical Centers (VAMCs).
Methods: We performed hospital-level analyses of acute-care MRSA admission prevalence, acquisition rates, and incident nosocomial clinical culture (INCC) rates, each a surrogate measure of importation, transmission, and nosocomial infection, respectively. We studied 112 VAMCs from October 2007 through September 2010, after the start of a bundled intervention including active surveillance for MRSA. We analyzed data using generalized linear mixed models.
Results: A total of 2.9 million surveillance tests were collected from 1.4 million patient admissions. Overall MRSA admission prevalence was 11.4%, acquisition was 5.2 per 1000 patient-days at risk, and INCC was 1.8 per 1000 patient-days at risk. A 10% increase in a hospital's average admission prevalence was associated with a 9.7% increase in its weekly acquisition rates (P < .001) and a 9.8% increase in its weekly INCC rates (P < .001). Significant decreases were observed in all 3 measures during the study period (P < .001). When INCC rates were stratified by nasal MRSA carriage at admission, a significant downward trend was observed only among those initially negative.
Conclusions: Measured associations between MRSA admission prevalence, acquisition rate, and INCC rate were consistent with the hypothesis that decreased acquisition led to decreased importation, which in turn further abated acquisition. The downward trend in INCC rate specifically among individuals with negative admission surveillance tests suggests that decreasing transmission contributed to lower rates of nosocomial MRSA infection.