The NoseFlu-Kids project aims at characterizing the immune response in the upper respiratory tract in children aged 2 to 5 with a laboratory-confirmed influenza infection. The immune response during the acute phase of the infection and after recovery will be compared to that of control children with no infection or vaccinated with the inactivated flu vaccine by the nasal route (recruited as part of a mirror study in Oxford).

The primary objective of this observational study is to quantify the inflammatory response in the nasal cavity and to correlate it with viral load and with clinical parameters. The study also aims to compare the inflammatory response measured in the nose to that measured in the blood.

Participants will have two study visits including a blood draw, several nasal samplings (nasal lining fluid and nasal cells) and a saliva sampling, one within 72 hours of their hospital admission and another one month later. Nasal lining fluid and saliva will be obtained every two or three days until symptoms disappear. During those visits, questions regarding symptoms will be asked.

Inclusion criteria:

• Aged 2-5 years
• Presenting clinical signs of an influenza-like illness
• Having a concomitant positive laboratory test for influenza.

Exclusion criteria:

• Immunosuppression
• Intubation or under invasive mechanical ventilation
• Co-infection with RSV or SARS-CoV-2

Participants will be recruited among patients hospitalized or visiting the emergencies at the Geneva University Hospitals (HUG).