Pandemics require an assessment of its severity. The objective was to determine the characteristics of pandemic influenza in Catalonia. We conducted a surveillance study on the incidence of ILI and severe cases between June 2009 and May 2010. A network of 55 doctors reported influenza-like illness and collected nasopharyngeal swabs. The severity of the pandemic was assessed through severe cases confirmed.We calculated the lethality and incidence rates and mortality. The differences are studied

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... ith the relative risk (RR) with confidence intervals (CI) of 95%. The incidence of ILI was much higher in the 5 to 14 and from 0 to 4 years old (1227.8 and 1048.4, respectively, in the week of maximum incidence). Of positive samples, 52.4% (477/916) were positive for influenza virus and the majority, 456 (95.6%) were the pandemic influenza virus (H1N1) 2009.The incidence of severe cases was 10.3 per 10(5) and was higher in children (14,5) and adults under 65 years (10,2) (p < 0.0001). Overall mortality was 0.7 per 10(5) and was also higher in children and adults under 65 but the differences were not statistically significant. The fatality rate was estimated at 0.03% and was much higher in the 15 to 44 years (0,03%) (RR = 3.1, 95% CI 1.2 to 8.3), 45 to 64 years (0,11%) (RR = 11.9 95% CI 4.3 to 32.7) and over 64 years (0,73%) (RR = 79.6, 95% CI 25.3 to 250.9) compared to group 5 to 14 years (0,01%). The incidence of severe hospitalized cases was similar to or lower than that observed in the southern hemisphere. The lower risk of hospitalization in people over 64 years suggests the relative protection of this age group against the pandemic virus.