Influenza A(H1N1)pdm09-related pneumonia and other complications.

Influenza A(H1N1)pdm09 virus infection was associated with significant morbidity, mainly among children and young adults. The majority of patients had self-limited mild-to-moderate uncomplicated disease. However, some patients developed severe illness and some died. In addition to respiratory complications, several complications due to direct and indirect effects on other body systems were associated with influenza A(H1N1)pdm09 virus infection. The main complications reported in hospitalized adults with influenza A(H1N1)pdm09 were pneumonia (primary influenza pneumonia and concomitant/secondary bacterial pneumonia), exacerbations of chronic pulmonary diseases (mainly chronic obstructive pulmonary disease and asthma), the need for intensive unit care admission (including mechanical ventilation, acute respiratory distress syndrome and septic shock), nosocomial infections and acute cardiac events. In experimentally infected animals, the level of pulmonary replication of the influenza A(H1N1)pdm09 virus was higher than that of seasonal influenza viruses. Pathological studies in autopsy specimens indicated that the influenza A(H1N1)pdm09 virus mainly targeted the lower respiratory tract, resulting in diffuse alveolar damage (edema, hyaline membranes, inflammation, and fibrosis), manifested clinically by severe acute respiratory distress syndrome with refractory hypoxemia. Influenza A(H1N1)pdm09-related pneumonia and other complications were associated with increased morbidity and mortality among hospitalized patients.