Background: Fiji, an archipelago in the South Pacific comprising 332 islands distributed among 4 health administrative divisions (Central, Western, Eastern, Northern), is particularly vulnerable to the (re-)emergence of arboviruses and respiratory viruses due to its sub-tropical climate, the presence of several mosquito vector species, and connections with many countries in the Pacific, Asia and North America. Over the past decades, the epidemiological landscape of arboviruses has shifted from the sequential circulation of each of the four dengue virus (DENV) serotypes to the emergence of Zika virus (ZIKV) and chikungunya virus (CHIKV), concomitantly to the concurrent circulation of multiple DENV serotypes. The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 2020 significantly challenged Fiji's healthcare system, with the Delta variant alone accounting for approximately 700 deaths, while other respiratory viruses, such as influenza A and B, cause seasonal outbreaks. Despite these threats, comprehensive and up-to-date seroprevalence data remain scarce, limiting the capacity to inform and adapt public health policies.

Methods: The cohort study of Arbovirus and other Emerging Virus Infections in Fiji (AEVI-Fiji cohort study) aims to estimate the prevalence of several arboviruses and respiratory viruses, track the evolution of individual immunity, and analyse transmission dynamics of these viruses within the Fijian population. This longitudinal study will span 38 months and will include about 900 willing participants aged six years and older, recruited from at least 210 households randomly selected across the Central Division. Four visits will be conducted 12 months apart in each household. During each visit, participants will complete a questionnaire capturing their demographic characteristics and history of infections with major arboviruses and respiratory viruses and will provide a blood sample for serological analysis. During the whole study period, participants with a suspected acute infection by an arbovirus or respiratory virus will be screened.

Discussion: For the first time in Fiji, the AEVI-Fiji cohort study will generate longitudinal data to explore the determinants of both arbovirus and respiratory virus infections. The findings are expected to guide targeted public health strategies and enhance preparedness for future infectious disease threats in Fiji and the broader Oceania region.

Inclusion Criteria:

• Individuals aged 6 years and older who reside in the Central Division of Fiji
• Have lived in the selected household for at least six months at the time of enrollment

Exclusion Criteria:

• Pregnant women;
• Women in labor, or breastfeeding mothers;
• Individuals deprived of liberty by judicial or administrative decision;
• Individuals under psychiatric care or admitted to a health or social care facility for purposes other than participation in the study;
• Adults under legal protection or unable to provide informed consent;
• Homeless individuals;
• Individuals with severe disabilities preventing mobility;
• Individuals unable to understand or complete the study questionnaire

The study will specifically target residents of the Central Division of Fiji, which accounts for approximately 42.7% of Fiji's total population. Most inhabitants are located within the Nausori-Lami corridor, a region characterized by a mix of urban, peri-urban, and rural areas. The Central Division is also notable for its ethnic diversity, comprising Indigenous iTaukei, Indo-Fijians, and other minority groups, making it representative of Fiji's broader demographic composition.