Data on Whole-Genome Sequencing Are Insufficient to Rule Out Patient-to-Patient Transmission as a Significant Source of Staphylococcus aureus Acquisition in an Intensive Care Unit

TO THE EDITOR—The recent publication by Price et al [1] uses novel methods to illustrate the complex dynamics involved in nosocomial transmission of Staphylococcus aureus through the use of wholegenome sequencing. However, the claim that patient-to-patient transmission is a rare occurrence is not well justified by the presented data. The authors report 7 of 37 (18.7%) patient-to patient transmission events based on whole-genome sequencing of isolates with available spa typing results. Our calculated confidence intervals for these proportions range from 6.3% to 31.5%. Furthermore, if only methicillinresistant S. aureus transmission events in isolates with available spa types are analyzed, the proportion increases to 5 of 14 (35.7%), with confidence intervals of 10.6%–60.8%. These numbers are more consistent with existing data on rates of horizontal transmission of S. aureus [2]. Although the study clearly shows the limitations of conventional methods of transmission assessment, the data are far from conclusive in exonerating patient-topatient transmission as a major source of in-hospital S. aureus acquisition.