Seroepidemiologic investigation of an outbreak of pandemic influenza A H1N1 2009 aboard a US Navy Vessel—San Diego, 2009

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.