Analysis of viral pneumonia and risk factors associated with severity of influenza virus infection in hospitalized patients from 2012 to 2016

Background Influenza viruses cause pneumonia in approximately one-third of cases, and pneumonia is an important cause of death.

The aim was to identify risk factors associated with severity and those that could predict the development of pneumonia.

Methods This retrospective, observational study included all adult patients with confirmed influenza virus infection admitted to Son Espases University Hospital during four influenza seasons in Spain (October to May) from to 2012–2016.

Results Overall, 666 patients with laboratory-confirmed influenza were included, 93 (14%) of which were severe; 73 (10.9%) were admitted to Intensive Care Unit (ICU), 39 (5.8%) died, and 185 (27.7%) developed pneumonia.

Compared to less severe cases, patients with severe disease: were less vaccinated (40% vs. 28%, p  = 0.021); presented with more confusion (26.9% vs. 6.8%), were more hypoxemic (Horowitz index (PaO_2/FiO_2) 261 vs. 280), had higher C-reactive protein (CRP) (12.3 vs. 4.0), had more coinfections (26.8% vs. 6.3%) and had more pleural effusion (14% vs. 2.6%) (last six all p  < 0.001).

Risk factors significantly associated with severity were pneumonia [OR (95% CI) = 4.14 (2.4–7.16)], history of heart disease (1.84, 1.03–3.28), and confusion at admission (4.99, 2.55–9.74).

Influenza vaccination was protective (0.53, 0.28–0.98).

Compared to those without pneumonia, the pneumonia group had higher CRP (11.3 vs. 4.0, p  < 0.001), lower oxygen saturation (92% vs. 94%, p  < 0.001), were more hypoxic (PaO_2/FiO_2 266 vs. 281, p  < 0.001), and incurred more mechanical ventilation, septic shock, admission to the ICU, and deaths (all four p  < 0.001).

Higher CRP and lower oxygen saturation were independent variables for predicting the development of pneumonia.

Conclusions Pneumonia, history of heart disease, confusion and no influenza vaccination were independent variables to present complications in patients admitted with influenza infection.