Influenza virus type/subtype and different infection profiles by age group during 2017/2018 season

Background: Influenza has a major impact in hospitalization during each influenza season. We analysed the influenza type/subtype distribution by age group and medical care wards (ambulatory, hospital, intensive care unit). Material and Methods: During 2017/2018 season, 14 hospitals from Portugal mainland and Atlantic Island (Azores and Madeira) reported to the National Influenza Centre 13747 cases of respiratory infection, all tested for influenza type and/or subtype. Epidemiological data: age, sample collection, hospital dwelling service and patient outcome were reported. Results: From the 13747 reported cases, 3717(27%) were influenza positive of which 2033 (55%) were influenza B, 722 (19%) A unsubtyped, 505 (14%) AH3, 442 (12%) AH1pdm09 and 15(0,1%) mixed infections. Influenza A was detected in 71% (204/208) of toddlers(<5 years) although in the remaining age groups influenza B was detected in more than 50% of the confirmed flu cases. Influenza B was the predominant virus in hospitalized and ICU influenza cases between 5-14 years (69% and 75%, respectively) and played a major role in elderly (65+ years) hospitalized and ICU cases(57% and 67%, respectively). AH1pdm09 virus was detected in 30% of the influenza confirmed ICU patients, 2.1 times more than in hospitalized cases in other wards and 3.3 times more than influenza AH1pdm09 cases in ambulatory care. Influenza mixed infection were detected sporadically,mainly in hospitalized and ICU patients. From 2080 known outcomes, 40(1.9%) patients deceased, influenza was confirmed in 11(28%) of these cases. Conclusions: Cocirculation of different influenza virus type/subtype may indicate different infection profiles by age groups and should guide influenza preventive/treatment measures.