Context: Seasonal influenza affects 2.5 to 3 million people each year in France, resulting in 1500 to 2000 severe cases seen in intensive care units. The severity of influenza is related to, among other things, its respiratory or neurological complications, observed especially in children. Early determination of the severity of influenza is a critical step to avoid in appropriate treatment and care for patients and to improve their survival. Viral, human but also environmental factors have been described as having an important role in determining this severity. Several studies suggest that the nasopharyngeal microbiome may be involved in the incidence and severity of respiratory viral infections. During influenza infection, the respiratory microbiota is significantly altered. In animal models, particularly murine models, the microbiota regulates the immune response to influenza virus infection. In a retrospective preliminary study, the investigators showed that the composition of the nasopharyngeal bacterial microbiota is different between children who develop a severe or moderate influenza. This difference was observed on respiratory specimens at admission to pediatric emergencies within two days of onset of symptoms.

Hypotheses :

• The respiratory microbiome is a determining factor in the clinical course of influenza infection (benign vs. severe with respiratory or neurological complication)
• The respiratory microbiome can be used as a prognostic biomarker of the clinical course of influenza Originality: There are currently no clinical and / or virological markers to predict the clinical course of influenza infection. This study will define biomarkers of the respiratory microbiome to discriminate patients who will develop a severe influenza from those who will develop a moderate influenza. These prognostic biomarkers could be used to rapidly refer patients at risk to intensive care units, thus improving patient management and care. Moreover, at the fundamental level, this study will specify the role of the microbiome in the severity of influenza infection.

Inclusion Criteria:

• Age < 5 years
• Seen in an emergency department with influenza confirmed by a positive test (polumerase chain reaction, rapide antigen detection test).

Exclusion Criteria:

• Presence of associated significant comorbidity (chronic respiratory, cardiac, neurological or metabolic pathology, prematurity, known immunodeficiency)
• Documented non-respiratory bacterial infection
• No consent
• Patient not affiliated to a national health cover