Given the history of Ae. albopictus colonization, the occurrence of local transmission of viruses carried by this vector is now expected every year in southern France. However, in 2022, metropolitan France experienced an exceptional situation regarding dengue virus transmission, both in terms of the number of transmission events and the number of cases. Since the surveillance system was established in 2006 and prior to the 2022 season, 19 episodes of local transmission had been identified in mainland France, accounting for a total of 48 cases. In the 2022 season alone, this number of cases was exceeded, with a total of 65 locally acquired cases spread across 9 transmission clusters. Furthermore, an increase in the intensity of transmission episodes was observed in 2022, notably including the largest dengue outbreak ever documented in Europe. This outbreak, comprising 34 cases belonging to a single transmission chain, affected two municipalities in the Alpes-Maritimes department. Another notable event this season is the occurrence of local transmission episodes in departments that had previously been spared. This is the case this season in the western part of the Occitanie region: clusters of local cases have been identified in the Pyrénées-Orientales, the Hautes-Pyrénées, Tarn-et-Garonne, and Haute-Garonne, whereas in previous years, only Gard and Hérault—further east in the region and colonized by the tiger mosquito for a longer period—had experienced such cases. A first episode of local transmission was also reported in Southern Corsica.

This year seems all the more exceptional given that France is the only European country to have reported indigenous cases of dengue fever.

There are many factors that influence arbovirus transmission; these are primarily shaped by the interactions between vector populations, virus strains, hosts, and the broader environment.

First, the establishment of a local transmission cycle requires the introduction of the virus into the mainland through individuals who became infected while traveling in areas where the virus is circulating. International travel, in the current context of globalization, is therefore a factor to consider in the occurrence of local transmission episodes. However, the risk is not directly correlated with the number of imported cases: from May to November 2022, 255 imported cases of dengue were identified, which is about three times fewer than in 2019 (657 cases) and 2020 (834 cases)*. A key determinant is the compatibility of the vector-pathogen pair: certain viral strains are better adapted to the local vector, and the introduction of a single imported case can trigger a local transmission chain.

Finally, environmental conditions have a major impact on the geographic distribution of vectors, their density and activity, as well as on the multiplication and transmission of pathogens. Temperatures, rainfall, and land use thus influence the effectiveness of the vector system, that is, its ability to ensure efficient circulation of the virus. Spring and summer 2022 were particularly hot, which may have favored vector activity and the efficiency of dengue virus transmission.

Further research is needed to better characterize the determinants (climatic, socioeconomic, environmental) of local transmission episodes and their spread.

*164 imported dengue cases in mainland France in 2021.

The surveillance system established in mainland France is highly sensitive and unique in Europe; it enables the rapid implementation of mosquito control measures around confirmed cases (see box). However, it requires significant human and material resources, particularly when import pressure is high or when it faces numerous episodes of local transmission, as was the case in 2022.

This year undoubtedly represents a foretaste of what lies ahead, against a backdrop of the vector’s continuously expanding range and, consequently, an increased risk across the country. As of January 1, 2022, 67 metropolitan departments were considered colonized by the Aedes albopictus vector (34 departments in 2016). As resources allocated to the surveillance and management of these public health events are limited, discussions must be held to adapt the system in order to anticipate the increase in episodes of local transmission. This will likely rely on prioritizing investigations, whether entomological or epidemiological.

Healthcare professionals, just like travelers, play a vital role in preventing arboviruses in mainland France. Social mobilization and the involvement of local authorities remain the main drivers in the fight against vector populations to reduce their densities and mitigate associated health risks.

To ensure the sustainability of the surveillance system, it is necessary to maintain the involvement of key stakeholders by strengthening the network of biological analysis laboratories, raising patient awareness of the need to consult a healthcare professional if symptoms arise, and educating healthcare professionals on the diagnosis of these arboviral diseases.