Background: The link between influenza virus (IV) viral load (VL) in respiratory samples and disease severity is not clearly established. This study was designed to assess IV-VL in respiratory samples from flu patients admitted to intensive care unit (ICU).

Methods: Patients admitted to ICU for IV infection, as documented by RT-PCR, with respiratory failure were included in the study during 5 flu-seasons (2014-2018). Routine ICU management parameters were recorded. Real-time amplification Ct-values for IV and cell GAPDH gene were measured in each respiratory sample collected at ICU admission.

Results: Among 105 included patients, 59 (56.1%) presented an acute respiratory distress syndrome (ARDS). The overall mortality was 21%. IV-load assessed by amplification Ct-values and virus-over-cell ratio (expressed as log10) in each respiratory sample ranged from 20 to 40 and -5.2-3.7, respectively, and did not differ according to the type of sample and IV-A or -B type. Cell richness was higher in samples from ARDS patients compared to non-ARDS (p = 0.0003) but no difference was noted for IV Ct-values. In ARDS-patients, IV Ct-values (p = 0.020) and the virus-per-cell ratio (p = 0.038) were significantly higher in sample from patients who eventually died compared to those who survived. These 2 parameters remain independently associated with mortality with an odd-ratio of 1.21 and 2.19, respectively (p < 0.05).

Conclusions: While IV-VL does not seem to predict disease evolution in ICU flu-patients, normalized measurement of IV-VL in respiratory samples could be useful in ARDS patients to identify patients at higher risk of mortality.