Burden of the 1999-2008 seasonal influenza epidemics in Italy: comparison with the H1N1v (A/California/07/09) pandemic.

Despite preventive efforts, seasonal influenza epidemics are responsible for substantial morbidity and mortality every year worldwide, including developed countries. The A/H1N1v pandemic imposed a considerable healthcare and economic burden. In order to obtain an accurate estimate of the economic burden of influenza, and hence to guide policymakers effectively, systematic studies are necessary. To this end, data from epidemiological surveillance are essential. To estimate the impact of the 1999-2008 seasonal influenza epidemics and the H1N1v pandemic, we analyzed data from the Italian Influenza Surveillance System (CIRI NET). In the period 1999-2008, the Italian surveillance network consisted of sentinel general practitioners and pediatricians, who reported cases of Influenza-Like Illness (ILI) and Acute Respiratory Infections (ARI ) observed during their clinical practice from mid-October to late April each year; reports were sent to the Center for Research on Influenza and other Viral Infections (CIRI -IV). CIRI -IV receives data from 9 of the 20 Italian regions: Liguria, Abruzzo, Calabria, Friuli Venezia Giulia, Lombardy, Puglia, Sicily, Tuscany and Umbria. Previous estimates of influenza case costs were used in economic evaluations. Clinical-epidemiological and virological surveillance of the seasonal epidemics from 1999-2008 showed that the highest epidemic period was 2004-2005, when a new variant of the H3N2 influenza virus subtype emerged (A/California/07/04). Indeed, the highest peak of morbidity in the decade occurred in February 2005 (12.6 per 1,000 inhabitants). In 1999-2008, H1N1 subtype strains circulated and co-circulated with strains belonging to the H3N2 subtype and B type. Regarding B viruses in 2001-02, viruses belonged to the B/Victoria/02/07 lineage re-emerged, and in subsequent years co-circulated with viruses belonging to the B/Yamagata/lineage. The estimated costs of seasonal epidemics from 1999-2008 in Italy ranged from €15 to €20 billion, and the costs of the H1N1v pandemic ranged from €1.3 to €2.3 billion. This Italian study yields interesting conclusions: the results of influenza surveillance in several developed countries vary markedly; influenza imposes a considerable social, healthcare and economic burden; most cases that occurred during the pandemic involved subjects under 14 years of age and, although the clinical course of H1N1v influenza was usually mild, the related economic burden was heavy.