title: Transmission and effect of multiple clusters of seasonal influenza in a Swiss geriatric hospital.
abstract: OBJECTIVE
  To investigate a nosocomial outbreak of influenza.
  
  
  METHODS
  Prospective outbreak investigation with active case finding and molecular typing.
  
  
  METHODS
  A large academic geriatric hospital in Switzerland.
  
  
  METHODS
  Elderly hospitalized adults.
  
  
  METHODS
  Based on syndromic surveillance, a nosocomial influenza outbreak was suspected in February 2012. All suspected cases were screened for respiratory viruses using real-time reverse transcription polymerase chain reaction of nasopharyngeal swabs. Infection control procedures (droplet precautions with single room isolation whenever possible) were implemented for all suspected or confirmed cases. Specimens positive for influenza viruses were processed and sequenced whenever possible to track transmission dynamics.
  
  
  RESULTS
  Respiratory samples from 155 suspected cases were analyzed during the outbreak period, of which 69 (44%) were positive for influenza virus, 26 (17%) were positive for other respiratory viruses, and 60 (39%) were negative. Three other cases fulfilled clinical criteria for influenza infection but were not sampled, and one individual was admitted with an already positive test, resulting in a total of 73 influenza cases, of which 62 (85%) were classified as nosocomial. Five distinct clusters of nosocomial transmission were identified using viral sequencing, with epidemiologically unexpected in-hospital transmission dynamics. Seven of 23 patients who experienced influenza complications died. Sixteen healthcare workers experienced an influenza-like illness (overall vaccination rate, 36%).
  
  
  CONCLUSIONS
  Nosocomial influenza transmission caused more secondary cases than repeated community importation during this polyclonal outbreak. Molecular tools revealed complex transmission dynamics. Low healthcare worker vaccination rates and gaps in recommended infection control procedures are likely to have contributed to nosocomial spread of influenza, which remains a potentially life-threatening disease in elderly adults.
authors.affiliation: Infectious Diseases Unit, Bolzano Central Hospital, Bolzano, Italy; Infection Control Program, University of Geneva Hospitals and Faculty of Medicine, Geneva, Switzerland.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Pagani
authors.forename: Yves
authors.initials: Y
authors.lastname: Thomas
authors.forename: Benedikt
authors.initials: B
authors.lastname: Huttner
authors.forename: Valérie
authors.initials: V
authors.lastname: Sauvan
authors.forename: Grigorios
authors.initials: G
authors.lastname: Notaridis
authors.forename: Laurent
authors.initials: L
authors.lastname: Kaiser
authors.forename: Anne
authors.initials: A
authors.lastname: Iten
authors.forename: Didier
authors.initials: D
authors.lastname: Pittet
authors.forename: Stephan
authors.initials: S
authors.lastname: Harbarth
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D016000:Cluster Analysis; D003428:Cross Infection / Q000453:epidemiology* / Q000517:prevention & control; D004196:Disease Outbreaks; D062665:Epidemiological Monitoring; D005260:Female; D006782:Hospitals, Special; D006801:Humans; D053122:Influenza A Virus, H3N2 Subtype / Q000145:classification / Q000235:genetics; D007252:Influenza Vaccines / Q000627:therapeutic use; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control / Q000635:transmission* / Q000821:virology; D011446:Prospective Studies; D017421:Sequence Analysis; D013557:Switzerland / Q000453:epidemiology; D014611:Vaccination
chemical_list: D007252:Influenza Vaccines
keywords: elderly; genome sequencing; healthcare-associated infection; transmission; viral disease