Chikungunya virus (CHIKV) seroprevalence in the local population of the Cook Islands and Vanuatu and associated environmental and social factors A first CHIKV seroprevalence study in two South Pacific communities

BackgroundArthropod-borne diseases pose a significant and increasing risk to global health. Given its rapid dissemination to all inhabited continents, initiating large-scale outbreaks with severe human infections and economic loss, the Chikungunya virus (CHIKV) is one of the most important arboviruses worldwide. Despite its significance, the real global impact of CHIKV remains underestimated as outbreak data are often incomplete and solely based on syndromic surveillance. During 2011-2016, the South Pacific Region was severely affected by several CHIKV-epidemics, still the area is underrepresented in arboviral research. Methods465 outpatient serum samples collected between 08/2016 and 04/2017 on three islands of the island states Vanuatu (Espiritu Santo) and the Cook Islands (Rarotonga, Aitutaki) were tested for anti-CHIKV specific antibodies using Enzyme-linked immunosorbent Assays. ResultsA total of 30% (Cook Islands) and 8% (Vanuatu) of specimens were found positive for anti-CHIKV specific antibodies with major variations in national and intranational immunity levels. Seroprevalence throughout all age groups was relatively constant. Comparing the different study settings, four potential outbreak-protective factors were identified: presence of Ae. albopictus (in absence of ECSA E1-A226V-mutation CHIKV), as well as low levels of local population densities, residents travel activity and tourism. ConclusionThis is the first seroprevalence study focussing on an arboviral disease in the Cook Islands and Vanuatu. It highlights the impact of the 2014/2015 CHIKV epidemic on the Cook Islands population and shows that a notable part of the Vanuatu test population was exposed to CHIKV although no outbreaks were reported. Our findings supplement the knowledge concerning CHIKV epidemics in the South Pacific Region and contribute to a better understanding of virus dissemination, including outbreak modifying factors. They may support preventive and rapid response measures in affected areas, travel-related risk assessment and infection identification in returning travellers. Author SummaryMosquito-borne viral infections pose an increasing risk to global health. As they mainly circulate in tropical regions comprised of low-resource countries and affect already vulnerable populations, these diseases have an immense impact on the socio-economic sector. Worldwide, the Chikungunya virus (CHIKV) represents one of the most important arboviruses. Yet, despite its significance, reliable data to understand many populations real disease burden is still missing. One example is the South Pacific Region (SPR), which has been severely affected by numerous CHIKV outbreaks since 2011. Epidemiological data is indispensable for the implementation of strategies for optimal allocation of limited resources, efficient early intervention, vector control as well as to understand the virus geographical distribution and its contribution to global morbidity. We therefore conducted a CHIKV seroprevalence study in the local populations of two Pacific Island states - the Cook Islands and Vanuatu. Our results show that during the 2014/2015 CHIKV epidemic on the Cook Islands about 30% of the local population has been affected. Although no outbreak was ever officially reported in Vanuatu, a notable part of the respective test population was proven to have been exposed to CHIKV as well. By comparing our two study settings we further identified several environmental and social factors with potential outbreak modifying effect.