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Seroepidemiologic investigation of an outbreak of pandemic influenza A H1N1 2009 aboard a US Navy Vessel-San Diego, 2009.
Khaokham, Christina B. 1; Selent, Monica 1; Loustalot, Fleetwood V. 1; Zarecki, Shauna Mettee 1; Harrington, Douglas 2; Hoke, Eileen 3; Faix, Dennis J. 4; Ortiguerra, Ryan 4; Alvarez, Bryan 2; Almond, Nathaniel 2; McMullen, Kellie 2; Cadwell, Betsy 5; Uyeki, Timothy M. 5; Blair, Patrick J. 4; Waterman, Stephen H. 5
[Article] Influenza & Other Respiratory Viruses. 7(5):791-798, September 2013.

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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[middle dot]2; 95% confidence interval [CI]: 1[middle dot]1-4[middle dot]4), marines versus navy personnel (OR = 1[middle dot]7; 95% CI, 1[middle dot]0-2[middle dot]9), and those aged 19-24 versus >=35 years (OR = 3[middle dot]9; 95% CI, 1[middle dot]2-12[middle dot]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.