Survival in influenza virus-related pneumonia by viral subtype: 2016-2020.

Background Influenza remains a common cause of morbidity and mortality worldwide and viral subtype-related differences in disease outcomes have been documented. Objective To characterize the survival experience of adult inpatients with influenza virus-associated pneumonia by viral subtype during five consecutive flu seasons. Method We performed a retrospective cohort study and data from 4,678 adults were analyzed using the Kaplan-Meier method. A multivariate Cox proportional hazard regression model was fitted. Results The overall in-hospital mortality rate was 25.0 per 1,000 hospital-days. The survival probabilities from pneumonia patients went from 93.4% (95% CI 92.6-94.1%) by day 3 to 43.3% (95% CI 39.2-47.4%) in day 30 from hospital admission. In general, the lowest survival rates were observed in patients with AH1N1 infection. In multiple model, and after adjusting by comorbidities and when compared with A non-subtypified virus, pneumonia patients with AH3N2 or B strains had a significantly decreased risk of a non-favorable disease outcome. The association of other strains was not significant. Conclusions Our findings suggest that the survival of inpatients with influenza virus-associated pneumonia varies according to the pathogenic viral subtype and the lowest survival rates were observed in patients with AH1N1 infection. This effect was independent of the patients' gender, age, and the analyzed underlying health conditions.