Abstract
BackgroundThe aim of this study was to assess the disease burden of the 2009 pandemic influenza A(H1N1) in Greece.Methodology/Principal FindingsData on influenza-like illness (ILI), collected through cross-sectional nationwide telephone surveys of 1,000 households in Greece repeated for 25 consecutive weeks, were combined with data from H1N1 virologic surveillance to estimate the incidence and the clinical attack rate (CAR) of influenza A(H1N1). Alternative definitions of ILI (cough or sore throat and fever>38°C [ILI-38] or fever 37.1–38°C [ILI-37]) were used to estimate the number of symptomatic infections. The infection attack rate (IAR) was approximated using estimates from published studies on the frequency of fever in infected individuals. Data on H1N1 morbidity and mortality were used to estimate ICU admission and case fatality (CFR) rates. The epidemic peaked on week 48/2009 with approximately 750–1,500 new cases/100,000 population per week, depending on ILI-38 or ILI-37 case definition, respectively. By week 6/2010, 7.1%–15.6% of the population in Greece was estimated to be symptomatically infected with H1N1. Children 5–19 years represented the most affected population group (CAR:27%–54%), whereas individuals older than 64 years were the least affected (CAR:0.6%–2.2%). The IAR (95% CI) of influenza A(H1N1) was estimated to be 19.7% (13.3%, 26.1%). Per 1,000 symptomatic cases, based on ILI-38 case definition, 416 attended health services, 108 visited hospital emergency departments and 15 were admitted to hospitals. ICU admission rate and CFR were 37 and 17.5 per 100,000 symptomatic cases or 13.4 and 6.3 per 100,000 infections, respectively.Conclusions/SignificanceInfluenza A(H1N1) infected one fifth and caused symptomatic infection in up to 15% of the Greek population. Although individuals older than 65 years were the least affected age group in terms of attack rate, they had 55 and 185 times higher risk of ICU admission and CFR, respectively.
Highlights
Soon after the identification of the first cases infected with the new influenza A(H1N1) strain in Mexico and USA in March-April 2009, the virus spread rapidly around the world
Based on the data collected from repeated cross-sectional households surveys for 25 consecutive weeks during the autumnearly winter wave of influenza A(H1N1) in Greece in 2009–2010, it was possible to reconstruct the incidence of influenza-like illness (ILI) and influenza A(H1N1) cases over time in the different age population groups
It was estimated that 7%–16% of the Greek population experienced symptomatic influenza A(H1N1), depending on ILI case definition, and that approximately one fifth of the population was infected, either symptomatically or asymptomatically
Summary
Soon after the identification of the first cases infected with the new influenza A(H1N1) strain in Mexico and USA in March-April 2009, the virus spread rapidly around the world. A major challenge in influenza pandemics is to estimate the proportion of the population that was symptomatically infected and to draw conclusions concerning the age-specific severity of the infection. Estimating the attack rate of an influenza pandemic using existing influenza surveillance systems is challenging since only a portion of symptomatic cases seek medical care and only a small number of those seeking medical care are usually tested. During the 2009 influenza A(H1N1) pandemic, several reports have attempted an assessment of the associated burden and provided estimates of hospitalisations, ICU admissions or death rates using the number of laboratory-confirmed cases or the size of the general population as denominator [3,4,5,6]. The aim of this study was to assess the disease burden of the 2009 pandemic influenza A(H1N1) in Greece
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