Hugonnet, Stéphane

Legros, Dominique

Pessoa-Silva, Carmem Lucia

Roth, Cathy

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CORRESPONDENCE • CID 2007:45 (15 December) • 1651demiologic analysis and genotypic character-ization of a nosocomial outbreak of vanco-mycin-resistant Enterococci. J Clin Microbiol1993; 31:1280–5.8. Carmeli Y, Eliopoulos GM, Samore MH. An-tecedent treatment with different antibioticagents as a risk factor for vancomycin-resistantEnterococcus. Emerg Infect Dis 2002; 8:802–7.9. Donskey CJ, Chowdhry TK, Hecker MT, et al.Effect of antibiotic therapy on the density ofvancomycin-resistant Enterococci in the stoolof colonized patients. N Engl J Med 2000; 343:1925–32.10. Teasley DG, Gerding DN, Olson MM, et al.Prospective randomized trial of metronidazoleversus vancomycin for Clostridium diffi-cile–associated diarrhea and colitis. Lancet1983; 2:1043–6.11. Wenisch C, Parschalk B, Hasenhundl M,Hirschl AM, Graninger W. Comparison ofvancomycin, teicoplanin, metronidazole, andfusidic acid for the treatment of Clostridiumdifficile–associated diarrhea. Clin Infect Dis1996; 22:813–8.Reprints or correspondence: Dr. Fred Arthur Zar, Dept. ofMedicine (M/C 718), University of Illinois at Chicago, Rm.440 CSN, 840 S. Wood St., Chicago, IL 60612-7323(fazar@uic.edu).Clinical Infectious Diseases 2007; 45:1649–51 2007 by the Infectious Diseases Society of America. Allrights reserved. 1058-4838/2007/4512-0022$15.00DOI: 10.1086/523717Nosocomial Transmissionof Severe Acute RespiratorySyndrome: Better Qualityof Evidence Is NeededTo the Editor—We would like to com-ment on the article by Yu et al. [1] re-garding the risk factors for nosocomialoutbreaks of severe acute respiratory syn-drome that occurred in some hospitalwards and not in others. The authors per-formed a retrospective case-control studyat the ward level and found that severalenvironmental or administrative factorsand host factors, such as oxygen therapyor bilevel positive airway pressure venti-lation, were associated with the occurrenceof these outbreaks. We wish to discuss thestudy’s most important methodologicalflaws, which may invalidate the results,and to emphasize some points that shouldbe taken into consideration in futurestudies.First, the definition of a superspreadingevent is questionable. The authors madethe strong assumption that the clusteredcases were all secondary to a single iden-tified (or unknown) index case and thattransmissions occurred within the ward.The assessment of the transmission chainseems to be based solely on the timing ofevents, although it is possible that all trans-missions within a “superspreading” eventwere neither nosocomial nor related to theputative index case.Second, the measure of the exposuressuffers from very important limitations.Data collection was performed 1–3 yearsafter the events, leading to an importantrecall bias. Some exposures were of eco-logical nature (applying similarly to all pa-tients in the ward at a given time), thusmaking it impossible to be sure that agiven patient was really exposed to the riskfactor under consideration. Finally, someexposures were measured during a 10-daywindow period after the index patient’shospital admission (or the first case of thecluster); consequently, some exposureswere measured after the transmissionevent, violating the basic principle that therisk factor needs to precede the disease.Finally, it is possible that some wardsexperienced 11 outbreak of infection orthat several patients with severe acute re-spiratory syndrome were admitted, but theauthors failed to explain how they dealtwith this type of situation and the selectionprocedure. Because transmission wasmore intense at the beginning of the out-break of severe acute respiratory syn-drome, a selection bias may have playeda role, resulting in most of the case wardsbeing selected at the beginning of the out-break and most of the control wards beingselected towards the end of the outbreakand, therefore, jeopardizing any com-parisons.Study of this topic poses difficult chal-lenges and relies on observational and ret-rospective studies. However, interventionsto reduce infection risk are fundamentalto promote safety in the workplace, andhealth policy makers face the difficult di-lemma of balancing needs and availableresources. Although there is little doubtthat some procedures may increase trans-mission [2], the risk associated with sev-eral respiratory support techniques ob-served in the study by Yu et al. [1], inaddition to the aforementioned limita-tions, cannot be interpreted without in-formation about individual compliancewith standard infection-control measuresand use of personal protective equipment[3, 4]. Robust data are critical for policymaking, and sound recommendations re-quire a better quality of evidence.AcknowledgmentsPotential conflicts of interest. All authors: noconflictsSte´phane Hugonnet,1 Dominique Legros,2Cathy Roth,2 and Carmem Lucia Pessoa-Silva21Infection Control Program, University of GenevaHospitals, and 2Department of Epidemicand Pandemic Alert and Response, World HealthOrganization, Geneva, SwitzerlandReferences1. Yu IT, Xie ZH, Tsoi KK, et al. Why did out-breaks of severe acute respiratory syndrome oc-cur in some hospital wards but not in others?Clin Infect Dis 2007; 44:1017–25.2. Fowler RA, Guest CB, Lapinsky SE, et al. Trans-mission of severe acute respiratory syndromeduring intubation and mechanical ventilation.Am J Respir Crit Care Med 2004; 169:1198–202.3. Loeb M, McGeer A, Henry B, et al. SARSamong critical care nurses, Toronto. Emerg In-fect Dis 2004; 10:251–5.4. Hugonnet S, Pittet D. Transmission of severeacute respiratory syndrome in critical care: dowe need a change? Am J Respir Crit Care Med2004; 169:1177–8.The authors alone are responsible for the views expressedin this publication, and they do not necessarily represent thedecisions or the stated policy of the World HealthOrganization.Reprints or correspondence: Dr. Ste´phane Hugonnet, InfectionControl Program, University of Geneva Hospitals, Rue Micheli-du-Crest 24, 1211 Geneva 14, Switzerland (stephane.hugonnet@hcuge.ch).Clinical Infectious Diseases 2007; 45:1651 2007 by the Infectious Diseases Society of America. Allrights reserved. 1058-4838/2007/4512-0023$15.00DOI: 10.1086/523725

Nosocomial Transmission of Severe Acute Respiratory Syndrome: Better Quality of Evidence Is Needed

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