title: Nosocomial transmission of norovirus is mainly caused by symptomatic cases.
abstract: BACKGROUND
  Nosocomial norovirus (NoV) infection is common and may increase the burden of disease in healthcare settings, particularly in vulnerable hospitalized patients. Implementing effective infection control during and after admission may limit further spread, but evidence-based measures are lacking.
  
  
  METHODS
  In this study, we performed a systematic evaluation of sources and modes of transmission during NoV outbreaks within 2 types of healthcare facilities. An outbreak protocol was developed to sample all patients and healthcare workers (HCWs) with and without symptoms on wards involved in outbreaks. Data on clinical history and possible high-risk exposures were collected. Five outbreaks were investigated, involving 28 patients with recognized symptomatic NoV infection.
  
  
  RESULTS
  Enhanced sampling, however, yielded 65 additional cases, of whom 14% (n = 9) were asymptomatic patients, 57% (n = 37) were symptomatic HCWs, and 17% (n = 11) were asymptomatic HCWs. For 12% (n = 8), clinical data were not provided (2 HCWs and 6 patients). On the basis of the shedding kinetics, the onset of infection was estimated for each case. The generation interval was then used to construct plausible transmission pathways and reproduction numbers for symptomatic and asymptomatic patients and HCWs.
  
  
  CONCLUSIONS
  We found that symptomatic patients and HCWs were more often involved in transmission events than asymptomatic shedders. Asymptomatic HCWs rarely contributed to transmission, despite high levels of fecal virus shedding.
authors.affiliation: Laboratory for Infectious Diseases and Perinatal Screening, Centre for Infectious Disease Control (RIVM), Bilthoven, The Netherlands. faizel.sukhrie@rivm.nl
authors.forename: Faizel H A
authors.initials: FH
authors.lastname: Sukhrie
authors.forename: Peter
authors.initials: P
authors.lastname: Teunis
authors.forename: Harry
authors.initials: H
authors.lastname: Vennema
authors.forename: Cedrick
authors.initials: C
authors.lastname: Copra
authors.forename: Matthias F C
authors.initials: MF
authors.lastname: Thijs Beersma
authors.forename: Jolanda
authors.initials: J
authors.lastname: Bogerman
authors.forename: Marion
authors.initials: M
authors.lastname: Koopmans
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D017250:Caliciviridae Infections / Q000473:pathology / Q000635:transmission* / Q000821:virology*; D003428:Cross Infection / Q000473:pathology / Q000635:transmission* / Q000821:virology*; D004196:Disease Outbreaks; D005243:Feces / Q000821:virology; D005260:Female; D005759:Gastroenteritis / Q000473:pathology / Q000821:virology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D029322:Norovirus / Q000302:isolation & purification; D055815:Young Adult