Headlines - The flu outbreak is in its final stages

In mainland France, the epidemic began in the Île-de-France region during week 49 of 2017 (December 4–10, 2017), spreading to Bourgogne-Franche-Comté in week 50, and then to all regions in week 51. It is not yet over at the time of publication, but the decline in flu activity indicators, the transition of three regions into the post-epidemic phase, and the end of the epidemic in Provence-Alpes-Côte d’Azur [1] indicate that the end of the epidemic is imminent.

The start of the epidemic was in every respect comparable to that of the previous 2016-2017 season, but the current epidemic has been exceptionally long, as we are currently (week 13, March 26–April 1, 2018) in the 17th week of the epidemic. By comparison, the late epidemic of the 2015-2016 flu season, dominated by B/Victoria, lasted 12 weeks, and that of 2016-2017, dominated by H3N2, lasted 10 weeks. This exceptional duration is explained by the emergence of dominant circulation of B/Yamagata strains starting in February, following an initial phase of circulation of A(H1N1) strains: in outpatient settings during the last quarter of 2017, the predominant strains A(H1N1)pdm09, B/Yamagata, and A(H3N2) accounted for 57%, 15%, and 10%, respectively, of the 329 influenza-positive samples; during the recent February–March period, these same strains accounted for 30%, 65%, and 8%, respectively, of the last 631 positive samples. In Bourgogne-Franche-Comté, analyses from the virology laboratory (see Figure 7) confirm that A and B strains were co-circulating from the start of the epidemic.

Preliminary results from the Sentinelles Network, available in week 12, indicate a vaccine efficacy of 55% [95% CI: 31–71] among people aged 65 and older against all influenza viruses: it is estimated at 74% [95% CI: 46–87] against the A(H1N1)pdm09 virus and 58% [95% CI: 29–75] against B/Yamagata-type viruses. The FLUVAC study, which measures the vaccine’s effectiveness in preventing severe influenza leading to hospitalization in adults, currently indicates a vaccine efficacy of 11% [95% CI: -19–45] in adults against influenza-related hospitalization regardless of the virus: it is estimated at 31% [95% CI: -13–58] against type A viruses and at 6% [95% CI: -55–43] against type B viruses.

An excess of all-cause mortality across all age groups has been observed at the national level, with two distinct peaks in week 01 (January 1–7, 2018) and week 10 (March 5–11). All-cause mortality data have not yet been consolidated beyond week 09-2018, but excess mortality since the start of the epidemic is provisionally estimated at 8%, representing approximately 12,600 deaths during the period from December 4, 2017, to March 4, 2018. Santé publique France estimates that 73% of this excess is attributable to influenza, representing approximately 9,200 deaths across all age groups. Ninety percent of these deaths occurred among people aged 65 and older. At this stage of the epidemic, these initial estimates suggest an excess mortality lower than that observed during the previous season (at the end of the 2016–2017 epidemic, excess mortality from all causes was estimated at 21,200 deaths, 67% of which were attributable to influenza). In Bourgogne-Franche-Comté, the same trend is observed, with a lower peak in January.