Average rates of excess mortality associated with influenza epidemics and proportion of deaths attributable to influenza by disease outcome, age group, and dominant viral subtype, Portugal 1980–2004.

*age-standardized rates; % IS: proportion of winter death attributable to influenza; calculated as the ratio of excess deaths to death occurring from October to May , by age group, mortality outcome, and season;**Mann-Whitney test for comparison of excess mortality during A(H3) and A(H1) or B seasons;***data not presented due to small death counts.Rates are per 100,000 population.