BackgroundSince 2011, the Global Influenza Hospital Surveillance Network (GIHSN) has used active surveillance to prospectively collect epidemiological and virological data on patients hospitalized with influenza virus infection. Here, we describe influenza virus strain circulation in the GIHSN participant countries during 2017-2018 season and examine factors associated with complicated hospitalization among patients admitted with laboratory-confirmed influenza illness.MethodsThe study enrolled patients who were hospitalized in a GIHSN hospital in the previous 48h with acute respiratory symptoms and who had symptoms consistent with influenza within the 7days before admission. Enrolled patients were tested by reverse transcription-polymerase chain reaction to confirm influenza virus infection. "Complicated hospitalization" was defined as a need for mechanical ventilation, admission to an intensive care unit, or in-hospital death. In each of four age strata (<15, 15-<50, 50-<65, and65years), factors associated with complicated hospitalization in influenza-positive patients were identified by mixed effects logistic regression and those associated with length of hospital stay using a linear mixed-effects regression model.ResultsThe study included 12,803 hospitalized patients at 14 coordinating sites in 13 countries, of which 4306 (34%) tested positive for influenza. Influenza viruses B/Yamagata, A/H3N2, and A/H1N1pdm09 strains dominated and cocirculated, although the dominant strains varied between sites. Complicated hospitalization occurred in 10.6% of influenza-positive patients. Factors associated with complicated hospitalization in influenza-positive patients included chronic obstructive pulmonary disease (15-<50years and65years), diabetes (15-<50years), male sex (50-<65years), hospitalization during the last 12months (50-<65years), and current smoking (65years). Chronic obstructive pulmonary disease (50-<65years), other chronic conditions (15-<50years), influenza A (50-<65years), and hospitalization during the last 12months (<15years) were associated with a longer hospital stay. The proportion of patients with complicated influenza did not differ between influenza A and B.ConclusionsComplicated hospitalizations occurred in over 10% of patients hospitalized with influenza virus infection. Factors commonly associated with complicated or longer hospitalization differed by age group but commonly included chronic obstructive pulmonary disease, diabetes, and hospitalization during the last 12months.