Epidemiology and genomic characterisation of travel-associated and locally-acquired influenza, Marseille, France

Background The purpose of the study was to challenge the hypothesis of an introduction of influenza viruses by international travellers and subsequent local circulation in Marseille, France. Methods We analysed the epidemiological data of PCR-confirmed cases over an eight-year period and compared the genomic data of local and imported influenza viruses during a six-month period. Results Between June 2013 and December 2020, 12,434 patients in the Assistance Publique–Hospitaux de Marseille were diagnosed with an influenza virus infection at the laboratory of the Institut Hospitalo-Universitaire Méditerranéee Infection of Marseille. Half of the patients were below the age of 20. Most of the imported cases were diagnosed outside of epidemic periods. Fourteen genomes of the influenza A virus, including six in international travellers returning from Europe or from the Arabian Peninsula and eight from patients who had not travelled were analysed. Sequences of influenza A/H1N1 virus genomes detected in subjects who had travelled to Saudi Arabia were in the same clade and differed from sequences detected later in a traveller returning from Italy, and in non-travellers who were infected in Marseille. This suggests that influenza viruses imported from Saudi Arabia did not subsequently circulate in Marseille. Conclusion Future studies with higher numbers of genomes are needed to confirm this result.