title: A "pre-seasonal" hospital outbreak of influenza pneumonia caused by the drift variant A/Victoria/361/2011-like H3N2 viruses, Hong Kong, 2011.
abstract: BACKGROUND
  Beginning from late 2011 and early 2012, increasing circulation of antigenically drifted influenza A/Victoria/361/2011-like H3N2 viruses within genotype 3 of the A/Victoria/208/2009 clade have been reported in multiple European countries and elsewhere. Whether these emerging viruses are associated with increased disease severity is unclear.
  
  
  OBJECTIVE
  To report the clinical and virological findings of a moderately severe hospital outbreak of A/Victoria/361/2011-like viruses that occurred in November 2011 in Hong Kong.
  
  
  METHODS
  Clinical and virological hospital outbreak investigation.
  
  
  RESULTS
  The outbreak occurred in an adult psychiatric ward in November 2011, a time well before the usual local seasonal influenza winter peak. Altogether, 7 patients and 1 healthcare-worker were affected (mean age, 47 [range, 34-61] years). The attack rates among patients and healthcare-workers were 33% (7/21) and 7% (1/15), respectively. Pneumonia developed in 38% (3/8) of cases; none had underlying immunocompromised conditions. High nasopharyngeal viral loads were detected. All cases responded to antiviral treatment. Multiple amino acid mutations with reference to earlier A(H3N2) vaccine strains were mapped to key antigenic sites on hemagglutinin; however, no critical mutations on receptor binding sites were detected. Viral sequence variations jeopardized the performance of molecular diagnostic assays.
  
  
  CONCLUSIONS
  Severe disease and pneumonia occurred in a substantial proportion of non-immunocompromised adults in a hospital outbreak attributed to the emerging antigenically drifted A/Victoria/361/2011-like H3N2 viruses. Close monitoring of the transmission of this drift variant is required. Further studies are also necessary to determine virus virulence.
authors.affiliation: Department of Microbiology, The Chinese University of Hong Kong, Prince of Wales Hospital, Hong Kong Special Administrative Region, People's Republic of China. martin.chan@cuhk.edu.hk
authors.forename: Martin C W
authors.initials: MC
authors.lastname: Chan
authors.forename: Nelson
authors.initials: N
authors.lastname: Lee
authors.forename: Karry L K
authors.initials: KL
authors.lastname: Ngai
authors.forename: Bonnie C K
authors.initials: BC
authors.lastname: Wong
authors.forename: May K P
authors.initials: MK
authors.lastname: Lee
authors.forename: K W
authors.initials: KW
authors.lastname: Choi
authors.forename: Raymond W M
authors.initials: RW
authors.lastname: Lai
authors.forename: Paul K S
authors.initials: PK
authors.lastname: Chan
mesh_terms: D000328:Adult; D003428:Cross Infection / Q000453:epidemiology* / Q000821:virology; D004196:Disease Outbreaks; D040961:Genetic Drift; D005838:Genotype; D006723:Hong Kong / Q000453:epidemiology; D006761:Hospitals; D006801:Humans; D053122:Influenza A Virus, H3N2 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology; D008297:Male; D008875:Middle Aged; D009154:Mutation; D009305:Nasopharynx / Q000821:virology; D011024:Pneumonia, Viral / Q000453:epidemiology* / Q000821:virology; D019562:Viral Load