The diagnosis of respiratory viral infections is usually based on testing for pathogens suspected of causing the infection using PCR tests that target the pathogens' DNA or RNA. A specific PCR test is required for each pathogen. In most cases, testing is carried out primarily for three specific viruses (SARS-CoV-2, respiratory syncytial virus (RSV), influenza virus) is carried out as a first-line test. However, numerous viruses can cause these infections (adenovirus, metapneumovirus, rhinovirus, parainfluenza virus, bocavirus, etc.). Consequently, performing a specific PCR test for every virus that could cause respiratory symptoms is not feasible, and viral infections are often under-diagnosed because the screening for viruses is not exhaustive.

To address this issue, analysing the host response to determine the microbial aetiology of an infection may represent an innovative alternative for the diagnosis of infections. A common feature of all viruses is their ability to induce type I interferons (IFN-I). Thus, measuring this IFN-I response could help direct the diagnosis of respiratory infections towards a viral origin.

Furthermore, the detection of a virus via PCR in a respiratory sample may indicate an active or recent infection, but may also be detected following an infection that occurred several weeks or months earlier. The viral load in the sample is an important factor in distinguishing between these two scenarios but does not always allow for a definitive conclusion (some patients may have low viral loads whilst still having an active infection). It is therefore sometimes difficult to distinguish, using a PCR test that detects the viral genome, between the virus currently replicating and traces of viral genetic material from dead viruses. It is therefore necessary to have markers that can be associated with an active/replicating infection to aid interpretation in the event of a positive PCR result.

Using nasopharyngeal swabs collected for the diagnosis of SARS-CoV-2 infection, we were able to demonstrate that it is possible to measure the IFN-I response. These proteins, which possess antiviral properties, are secreted by immune cells during infection to limit viral replication.

Thus, the combination of simultaneous testing for the pathogen and the IFN response has made it possible to link the replicative nature of the SARS-CoV-2 virus with IFN-I production.

Our latest research into SARS-CoV-2 has thus led to the identification of a marker of the IFN-I response associated with active viral replication, which we have confirmed using viral culture techniques that detect only live/infectious virus.

However, the IFN-I response may be compromised in older and very old individuals, particularly due to certain comorbidities or treatments, the prevalence of which increases with age.

The aim of this study is to assess the IFN-I response in the context of respiratory infections caused by various viruses, in a population admitted to a geriatric ward and a population from the REFIPA protocol (NCT07239830), a study designed to establish reference values for nasal and blood interferon scores in uninfected elderly subjects.

There is a need to improve the diagnosis of viral infections, particularly in geriatric wards where the burden of viral infections is very significant for patients, carers and the organisation of care. Rapid diagnosis of viral infections would enable the optimisation of isolation measures and hygiene protocols within these wards. Furthermore, it is important to validate new markers that could aid in the interpretation of a positive PCR result for respiratory viruses. Some PCR tests can indeed remain positive for several weeks or even months due to the detection of traces of viral genetic material.

Thus, new markers indicating an active or recent infection could be useful in facilitating interpretation, alongside other clinical and/or virological findings.

The aim is, by combining different results, to improve the diagnosis of viral infections and to avoid over-diagnosing respiratory viral infections due to a positive PCR result that may in fact correspond to a past infection.

Inclusion Criteria:

• Patients admitted to a geriatric ward or participants diagnosed with a PCR-positive infection based on a swab sample taken as part of the REFIPA study (NCT07239830). Nasopharyngeal swab taken for viral detection, where the viral test was carried out as part of standard clinical care (minimum 500 µL required)

Exclusion Criteria:

• The patient's objection to the use of their data in this study

This study focuses on nasopharyngeal swabs taken as part of clinical care to screen for respiratory viruses in patients admitted to geriatric wards, or as part of the REFIPA protocol (NCT07239830) in older, asymptomatic, non-hospitalised participants. For hospitalised patients, first-line microbiological tests (RSV, SARS-CoV-2 and influenza) as well as viral culture are carried out as part of routine diagnostic testing. For participants in the REFIPA protocol (NCT07239830), samples were collected as part of this study.

Thus, all samples are collected as part of standard care in hospitalised patients in geriatric wards presenting with signs of respiratory distress, or as part of the REFIPA protocol in asymptomatic, non-hospitalised elderly participants.

The IFN-I score and the results of second-line tests will not be disclosed to the patient's doctor. They will therefore have no impact on the patient's treatment.