Influenza virus affects the respiratory tract by directly infecting via a virus or by damaging the immune system response. Virus transmission happens via contact with a susceptible individuals aerosols or respiratory fomites from an infected individual. Humans possess the respiratory epithelium as the sole site where the hemagglutinin (HA) molecule is effectively cleaved, creating infectious virus particles. In this illness, the lungs may be unable to perform their primary function of gas exchange because of multiple mechanisms, including airway obstruction, alveolar structure loss, lung epithelial integrity loss from direct epithelial cell killing, and degradation of the critical extracellular matrix.

Nearly 30–40% of hospitalized patients with laboratory-confirmed influenza present acute pneumonia diagnosis. These patients with pneumonia are more likely to be < 5 years old, > 65 years old, Caucasian, and nursing home residents; have chronic lung or heart disease and smoking history; and with an immunocompromised state.

Influenza has a high predisposition to bacterial sepsis and ARDS. Influenza A is the adults′ predominant viral etiology of acute respiratory distress syndrome (ARDS). Risk factors alone associated with ARDS are age between 36 to 55 years, pregnancy, and obesity. In contrast, protective factors are female sex, influenza vaccination, and infections with Influenza A (H3N2) or Influenza B viruses.

Influenza primarily causes severe pneumonia but can also present with or followed by a secondary bacterial infection, most commonly by Staphylococcus aureus and Streptococcus pneumoniae. Viral infections occurring simultaneously with bacterial pneumonia are now known to occur with a frequency of 30–50% in both adult and pediatric populations. The H3N2 subtype causes unprecedentedly high intensive care unit (ICU) admission levels.

Thus, in the ICU, mainly during the winter season, it is crucial to suspect influenza not only in patients with typical symptoms and epidemiology, but also in patients with severe pneumonia, ARDS, sepsis with or without bacterial co-infection, along with patients showing encephalitis, myocarditis, and rhabdomyolysis.

Kalil AC, Thomas PG. Influenza virus-related critical illness: pathophysiology and epidemiology. Crit Care.2019;23. https://doi.org/10.1186/s13054-019-2539-x