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Biacabe S.

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Zeller H.

Raoelina Y.

Talarmin A.

Richard V.

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Ezzedine K.

Giry C.

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Lernout T.

D'Ortenzio E.

Pettinelli F.

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Salez N.

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Leparc-Goffart I.

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de Lamballerie X. Serological evidence of contrasted exposure to arboviral infections between islands of the union of Comoros (Indian Ocean).

The islands from the SWIO have witnessed several dengue epidemics over the past years. However, data are scattered and when available they mainly referred to post- or inter-epidemic sero-epidemiological studies [], but seem to express a complex or contrasted epidemiological situation in this multiple insular area. The last reported outbreak that largely affected the SWIO region goes back to 1976–1977 on the Seychelles and subsequently to 1977–1978 on Reunion Island, involving DENV-2 []. This outbreak had also very likely affected the Comoros at the same period []. A dengue epidemic due to DENV-1 also hit the Comoros and Mayotte in 1993 [], and later on Madagascar in 2006 []. Since 2005, there have been sporadic cases in Reunion Island, mainly imported from neighboring islands including Madagascar, Mayotte (DENV-2, DENV-3) or occasionally from the Caribbean islands (DENV-1) and Thailand (DENV-1, DENV-3). In 2006, the health authorities alerted the inhabitants of Reunion Island about a local and autochthonous circulation of DENV-1, and to a lesser extent, of DENV-2 and DENV-3 []. A co-circulation at low-level transmission of DENV-1 and DENV-2 was also observed in 2017. Usually, such low circulation ends when the austral winter arrives, coinciding with the decrease of the vector populations of mosquitoes. Surprisingly, dengue circulation continued throughout this season. On March 2018, Reunion Island was officially declared under dengue epidemic crisis by the health authorities. Positive biologic diagnosis of DENV-2 infection was made by specific RT-qPCR at the National Reference Centre of Arboviruses of Reunion Island. Up to 5,700 cases were reported from November 2017 to July 2018 []. Besides, since 2015 the Seychelles have also experienced dengue epidemics mostly due to DENV-1 and DENV-2 []. By analyzing the two samples available from consenting adult volunteers, we provided evidence that the DENV-2 circulating on Reunion Island belonged to Cosmopolitan lineage. Access to more samples is required for further epidemiological study. Nevertheless, we undoubtedly established that both the Reunion Island and the Seychelles ongoing epidemics of DENV-2 involved strains that clustered together, and originated probably from India or China. Despite the low number of sequences available for the SWIO, the putative introduction dates of DENV-2 are respectively for the Seychelles and Reunion Island, end of 2015 [t2.89 y.b.p., 95% HPD = 1-1 y.b.p.] and end of 2016 [t1.06 y.b.p., 95% HPD = 0.03–2.5 y.b.p.], consistently with the recording of the first reported cases for these two geographic locations. Yet, the beginning of this cross-circulation between the SWIO islands was estimated around the end of 2013 [t= 4.21 y.b.p., 95% HPD = 2.09–6.62 y.b.p.], showing that the DENV-2 probably circulated silently without being detected almost two years before ( Fig. 2 , red square numbered 2 in the panel). Interestingly, the SWIO lineage including the Indian origin strain has a common ancestor whose testimate indicated a circulation date for the whole SWIO area in 2008 ( Fig. 2 , red square numbered 3 in the panel), that is to say more than 9 years ago. This suggests an endemic circulation at low-level transmission in SWIO neighboring islands. This observation also confirms and reinforces previous sero-epidemiological studies on Mayotte and on the Comoros, which similarly showed that there must have been DENV fluxes between some islands, which occurred quite unnoticed by the relevant authorities [].