Oceania's populations, including Melanesians, are paying a heavy price for dengue fever, which has been circulating actively in the region since the Second World War. In New Caledonia (NC), the incidence of dengue fever is higher among municipalities predominantly populated by Melanesians, suggesting that Melanesians may have an increased susceptibility to symptomatic dengue fever. Differences in antiviral immune responses between populations of different geographical origins are partly the result of population-specific immune regulatory variants. In turn, viruses have imposed considerable selective pressure on human populations. Although crucial to understanding their susceptibility to viral infections, the genetic determinants of the antiviral immune response of Oceanians remain to be characterized.

In this context, the hypothesis is that the genetic origin of Oceanians, and Melanesians in particular, has shaped their antiviral immune response and contributes to their greater susceptibility to certain viral infections. The aim is to characterize the immune response to pathogens affecting the New Caledonian population, and in particular to dengue virus, of Melanesian and European populations, and to identify its genetic determinants. It will be explored whether saliva can be used as a non-invasive sample to study the seroprevalence of dengue in Oceanian populations.

Cross-sectional observational study with prospective sample and data collection Individuals will be identified from among eligible participants in the STEP-BSA21 and COVCAL studies.

Information and consent Questionnaire, saliva collection on Oragene tube, buccal swabbing and 20 mL blood collection on CPT tube

DNA extraction and low-coverage sequencing of participants' complete genomes And Isolation and in vitro stimulation of peripheral blood mononuclear cells (PBMCs) with dengue virus

• single-cell RNA sequencing (scRNAseq)
• multiplex quantification of cytokines and chemokines
• bioinformatics analysis to identify genetic markers associated with a differential transcriptomic and secretory response to dengue virus stimulation.

Inclusion Criteria:

• Adult
• Non-febrile
• Self-declared member of the Melanesian or European community
• Having given consent to participate in the study

Exclusion Criteria:

• People who have taken part in a clinical study in the last 6 months in which they were exposed to a health product as part of the investigation (pharmaceutical product or device or placebo).
• People taking part in an ongoing clinical study
• People declaring themselves to belong to two communities (e.g. people of mixed European and Melanesian descent)
• Pregnant and breast-feeding women (in whom the immune response could be modified)
• People with a long-term medical condition (as defined by the French social security system) that could have an effect on the immune response, excluding dengue risk factors prevalent in New Caledonia such as diabetes, overweight/obesity and hypertension.
• Individuals with an acute infection (viral, bacterial or fungal) within 3 months of inclusion.
• Chronic administration (for more than 14 days) of immunosuppressants or treatments affecting the immune system in the 6 months prior to inclusion. For corticosteroids, this corresponds to a dose equivalent to 20 mg/day of prednisone or equivalent for more than two weeks (inhaled or topical steroids are permitted).
• Administration of a vaccine within 3 months prior to inclusion.
• Administration of blood products or immunoglobulins within 3 months of inclusion.
• People with known allergies to antibiotics, which could have an impact on the in vitro culture of PBMCs in the presence of antibiotics
• Persons not intellectually capable of answering the questionnaire
• Persons under guardianship, curatorship or any other legal incapacity