title: Seroepidemiologic investigation of an outbreak of pandemic influenza A H1N1 2009 aboard a US Navy vessel--San Diego, 2009.
abstract: BACKGROUND
  During summer 2009, a US Navy ship experienced an influenza-like illness outbreak with 126 laboratory-confirmed cases of pandemic influenza A (H1N1) 2009 virus among the approximately 2000-person crew.
  
  
  METHODS
  During September 24-October 9, 2009, a retrospective seroepidemiologic investigation was conducted to characterize the outbreak. We administered questionnaires, reviewed medical records, and collected post-outbreak sera from systematically sampled crewmembers. We used real-time reverse transcription-PCR or microneutralization assays to detect evidence of H1N1 virus infection.
  
  
  RESULTS
  Retrospective serologic data demonstrated that the overall H1N1 virus infection attack rate was 32%. Weighted H1N1 virus attack rates were higher among marines (37%), junior-ranking personnel (34%), and persons aged 19-24 years (36%). In multivariable analysis, a higher risk of illness was found for women versus men (odds ratio [OR] = 2.2; 95% confidence interval [CI]: 1.1-4.4), marines versus navy personnel (OR = 1.7; 95% CI, 1.0-2.9), and those aged 19-24 versus ≥ 35 years (OR = 3.9; 95% CI, 1.2-12.8). Fifty-three percent of infected persons did not recall respiratory illness symptoms. Among infected persons, only 35% met criteria for acute respiratory illness and 11% for influenza-like illness.
  
  
  CONCLUSIONS
  Approximately half of H1N1 infections were asymptomatic, and thus, the attack rate was higher than estimated by clinical illness alone. Enhanced infection control measures including pre-embarkation illness screening, improved self-reporting of illness, isolation of ill and quarantine of exposed contacts, and prompt antiviral chemoprophylaxis and treatment might be useful in controlling shipboard influenza outbreaks.
authors.affiliation: Epidemic Intelligence Service, Centers for Disease Control and Prevention, Atlanta, GA, USA. igc9@cdc.gov
authors.forename: Christina B
authors.initials: CB
authors.lastname: Khaokham
authors.forename: Monica
authors.initials: M
authors.lastname: Selent
authors.forename: Fleetwood V
authors.initials: FV
authors.lastname: Loustalot
authors.forename: Shauna Mettee
authors.initials: SM
authors.lastname: Zarecki
authors.forename: Douglas
authors.initials: D
authors.lastname: Harrington
authors.forename: Eileen
authors.initials: E
authors.lastname: Hoke
authors.forename: Dennis J
authors.initials: DJ
authors.lastname: Faix
authors.forename: Ryan
authors.initials: R
authors.lastname: Ortiguerra
authors.forename: Bryan
authors.initials: B
authors.lastname: Alvarez
authors.forename: Nathaniel
authors.initials: N
authors.lastname: Almond
authors.forename: Kellie
authors.initials: K
authors.lastname: McMullen
authors.forename: Betsy
authors.initials: B
authors.lastname: Cadwell
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Uyeki
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Blair
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Waterman
mesh_terms: D000328:Adult; D000914:Antibodies, Viral / Q000097:blood; D002140:California / Q000453:epidemiology; D004196:Disease Outbreaks; D005260:Female; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000097:blood / Q000453:epidemiology* / Q000821:virology; D008297:Male; D008889:Military Personnel / Q000706:statistics & numerical data; D058873:Pandemics; D012189:Retrospective Studies; D016036:Seroepidemiologic Studies; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral
keywords: Disease outbreaks; H1N1 subtype; epidemiology; influenza A virus; military personnel