# Social Mixing Patterns and Chikungunya Reemergence Risk in French Polynesia

## TL;DR

A study in French Polynesia finds high immunity to chikungunya, but models suggest the virus could reemerge by 2028 if reintroduced.

## Contribution

The study integrates seroprevalence data and social mixing patterns to project chikungunya reemergence risk under different assumptions.

## Key findings

- Chikungunya seroprevalence was 62.8% in schoolchildren and 67.6% in adults.
- Modeling suggests the effective reproduction number (Reff) could exceed 1 by 2028 under assortative mixing.
- Age-specific contact matrices improve outbreak risk assessments.

## Abstract

What is the risk of chikungunya reemergence in French Polynesia a decade after the first outbreak was recorded?

In this cross-sectional study with seroprevalence data from 457 schoolchildren and 1906 adults, chikungunya seroprevalence among schoolchildren was 62.8% and weighted seroprevalence in adults was 67.6%. Modeling inferred a pre-outbreak basic reproduction number of 1.78 in 2014 to 2015, estimated an effective reproduction number (Reff) of 0.95 for 2025, and projected that the Reff would exceed 1 by 2028 assuming assortive social mixing.

The findings of this study suggest that despite high overall immunity, chikungunya transmission could be sustained following reintroduction; additionally considering age-specific contact matrices in risk assessment would help to refine estimated timing of reemergence.

This cross-sectional study examines the seroprevalence of chikungunya virus in French Polynesia and estimates the effective reproduction number for 2025 and later.

Chikungunya virus has reemerged in some French overseas territories and mainland France, raising concern for its reintroduction in French Polynesia.

To assess current population immunity to chikungunya virus and estimate future outbreak risk.

This cross-sectional study included data from 2 serosurveys and used a modeling approach that integrated seroprevalence, contact matrices, and demographic characteristics. The first serosurvey was conducted on the island of Tahiti (June 2018) among schoolchildren aged 6 to 16 years. The second serosurvey (November 2019 to December 2021) included adults aged 18 to 69 years across French Polynesia, a French overseas territory in the Southeast Pacific that comprises 75 inhabited islands grouped into 5 subdivisions with 279 000 people according to the 2022 census. Data analysis was conducted from April to June 2025.

Main outcomes were seroprevalence rates and factors associated with seropositivity; age-stratified infection proportions for the 2014 to 2015 outbreak, pre-outbreak the basic reproduction number (R0), the effective reproduction number (Reff) for 2025, and Reff projections were estimated.

Serological data were available for 2363 participants (457 schoolchildren; median [IQR] age, 11 [8-13] years; 244 [53.3%] female, and 1906 adults; 38 [28-52] years; 1003 [52.6%] female). Seroprevalence was 62.8% (95% CI, 58.0%-67.0%) in schoolchildren, and the weighted seroprevalence was 67.6% (95% CI, 64.8%-70.3%) in adults. In adults, lower odds of seropositivity were associated with male sex (adjusted odds ratio [aOR], 0.71; 95% CI, 0.55-0.92), age groups 30 to 44 years and 45 to 69 years compared with 18 to 29 years (ages 30-44 years: aOR, 0.46; 95% CI, 0.32-0.66; ages 45-69 years: aOR, 0.33; 95% CI, 0.22-0.48), living in the Austral subdivision (aOR, 0.38; 95% CI, 0.27-0.55), and higher education (ie, university or more education: aOR, 0.51; 95% CI, 0.32-0.81). Living in larger households and being unpartnered were associated with higher odds of seropositivity (larger households: aOR, 1.69; 95% CI, 1.12-2.56; being unpartnered: aOR, 1.95; 95% CI, 1.03-3.69). Modeled infection rates for 2014 to 2015 ranged from 52.6% (95% credible interval [CrI], 43.6%-60.0%) in children aged 0 to 4 years to 80.8% (95% CrI, 73.3%-86.2%) in those aged 10 to 14 years. Inferred pre-outbreak R0 was 1.78 (95% CrI, 1.73-1.82), and Reff for 2025 was estimated at 0.95 (95% CrI, 0.85-1.04). Assuming assortative mixing, projections suggest Reff would exceed 1 (lower 95% CrI >1) by 2028, but assuming random mixing, Reff would exceed 1 in 2051, more than 20 years later than the other estimate.

In this cross-sectional study, high chikungunya seroprevalence was observed, with heterogeneity between subdivisions, age groups, and other sociodemographic characteristics. Modeling analysis estimated Reff near 1 in 2025, suggesting sustained transmission could occur if chikungunya is reintroduced. These findings suggest that incorporating contact matrices could improve outbreak risk assessments, while assuming random mixing might underestimate reemergence timing.