The diagnosis of respiratory viral infections is mainly based on PCR tests targeting the DNA or RNA of suspected viruses, including SARS-CoV-2, RSV and influenza viruses. However, this method is limited because it only tests for a small number of viruses, while many other pathogens can cause similar symptoms. As a result, respiratory viral infections are often underdiagnosed because not all possible viruses are systematically tested for.

Another limitation of PCR tests is that they can detect residual viral genetic material long after the infection has ended, making it difficult to distinguish between an active infection and a past one. Viral load can help interpret the result, but it is not always reliable. It is therefore essential to have complementary markers that can indicate whether the detected virus is still in the active replication phase.

An innovative approach is to measure the host's immune response, in particular the production of type I interferons (IFN-I), which are markers of active viral infection. Studies have shown that joint analysis of the IFN-I/III response and PCR tests improves the detection of viral respiratory infections by better discriminating between active infections. This method shows promise for refining diagnosis, particularly in cases where the viral load is low or ambiguous.

These advances are particularly important for older patients, in whom viral infections have a severe impact. Ageing leads to a decline in immune function (immunosenescence), including a reduction in IFN-I production. This alteration could further complicate the interpretation of immune biomarkers in older people, highlighting the need to establish reference values specific to this population.

In this context, the RESPIGERIA study (compliance with MR004 No. 24-5127) was launched to evaluate the IFN response in geriatric hospitalised patients with respiratory viral infections. However, there is still a lack of reference data on the IFN response in uninfected older individuals. Establishing a baseline IFN score in this population is essential in order to adapt diagnostic tools to age-related specificities.

Inclusion Criteria:

• Subject aged 80 or over
• Patient admitted to the Charpennes hospital day unit or outpatient clinic for follow-up OR accompanying patient who receive care at the Charpennes geriatric hospital OR subject recruited through the local press
• Weight greater than or equal to 45 kg
• Subject living in the Lyon metropolitan area

Exclusion Criteria:

• Subjects with symptoms of active infection (symptom questionnaire or temperature > 37.5°C).
• Subjects with an autoimmune disease linked to a dysregulated interferon response.
• Subjects participating in another interventional study involving an exclusion period that is still ongoing or that may interfere with the present protocol.
• Subjects deprived of their liberty by judicial or administrative decision
• Subjects receiving psychiatric care
• Adults subject to legal protection measures (guardianship, curatorship)
• Subjects not affiliated with a social security scheme or beneficiaries of a similar scheme