Retrospective Investigation of an Influenza A/H1N1pdm Outbreak in an Italian Military Ship Cruising in the Mediterranean Sea, May-September 2009

Mario Tarabbo,Riccardo Guarducci,Alessandro Emanuele,Enrico Girardi,Maria R Capobianchi,Giuditta Lucà,Daniele Lapa,Concetta Castilletti,Giuseppe Ippolito,Vincenzo F P La Gioia,Onofrio Zaccaria,Pietro Tommaselli,Malcolm Gracie Semple

doi:10.1371/journal.pone.0015933

Abstract

BackgroundClinical surveillance may have underestimated the real extent of the spread of the new strain of influenza A/H1N1, which surfaced in April 2009 originating the first influenza pandemic of the 21st century. Here we report a serological investigation on an influenza A/H1N1pdm outbreak in an Italian military ship while cruising in the Mediterranean Sea (May 24-September 6, 2009).MethodsThe contemporary presence of HAI and CF antibodies was used to retrospectively estimate the extent of influenza A/H1N1pdm spread across the crew members (median age: 29 years).FindingsDuring the cruise, 2 crew members fulfilled the surveillance case definition for influenza, but only one was laboratory confirmed by influenza A/H1N1pdm-specific RT-PCR; 52 reported acute respiratory illness (ARI) episodes, and 183 reported no ARI episodes. Overall, among the 211 crew member for whom a valid serological result was available, 39.3% tested seropositive for influenza A/H1N1pdm. The proportion of seropositives was significantly associated with more crowded living quarters and tended to be higher in those aged <40 and in those reporting ARI or suspected/confirmed influenza A/H1N1pdm compared to the asymptomatic individuals. No association was found with previous seasonal influenza vaccination.ConclusionsThese findings underline the risk for rapid spread of novel strains of influenza A in confined environment, such as military ships, where crowding, rigorous working environment, physiologic stress occur. The high proportion of asymptomatic infections in this ship-borne outbreak supports the concept that serological surveillance in such semi-closed communities is essential to appreciate the real extent of influenza A/H1N1pdm spread and can constitute, since the early stage of a pandemic, an useful model to predict the public health impact of pandemic influenza and to establish proportionate and effective countermeasures.

Highlights

During April-May 2009, a new strain of influenza A rapidly spreading from Mexico all around the world, originated the first influenza pandemic of the 21st century
These findings underline the risk for rapid spread of novel strains of influenza A in confined environment, such as military ships, where crowding, rigorous working environment, physiologic stress occur
The high proportion of asymptomatic infections in this ship-borne outbreak supports the concept that serological surveillance in such semi-closed communities is essential to appreciate the real extent of influenza A/H1N1pdm spread and can constitute, since the early stage of a pandemic, an useful model to predict the public health impact of pandemic influenza and to establish proportionate and effective countermeasures

Summary

Introduction

During April-May 2009, a new strain of influenza A rapidly spreading from Mexico all around the world, originated the first influenza pandemic of the 21st century. Data from Mexico suggested that this new pandemic virus strain (A/H1N1pdm) had a high infection rate in younger age groups, and high case-fatality ratio, at least in some particular risk groups [1,2,3]. Most of these inferences derived from confirmed cases that rely on laboratory results showing the presence of influenza virus genome in the respiratory tract of the affected individuals, selected on the basis of a strict case definition that tended to exclude less severe cases. We report a serological investigation on an influenza A/H1N1pdm outbreak in an Italian military ship while cruising in the Mediterranean Sea (May 24-September 6, 2009)

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