The 2019-2020 winter season was characterized by the occurrence after the flu epidemic of the COVID-19 pandemic. The Influenza epidemic in Belgium lasted 6 weeks and was a flu season of moderate intensity characterized by the co-circulation of A(H1N1)pdm09 and A(H3N2), with the predominance of A(H1N1). The epidemic threshold was crossed at week 4-2020 (January 13 to January 19, 2020 with an incidence of 245 consultations /100.000 inhabitants and the peak was reached in week 5 with 550 consultations/100.000 inhabitants. After week 5- 2020, the incidence of ILI consultations decreased but remained above the threshold for several weeks likely due to the COVID-19 epidemic with a new ILI peak at week 13 exceeding the influenza peak seen in week 5 (Fig. 1). The emergence of COVID-19, spreading through respiratory transmission, required the implementation of physical distancing measures likely contributed to an abrupt decline of the influenza season.
The majority of the H1N1 viruses fell in the 6B.1A5A subgroup represented by the reference strain A/Norway/3433/2018.
About half of the sequenced A(H3N2) viruses belonged to the clade 3C.2a1 and the remaining belonged to the clade 3C.3a close the vaccine strain for the northern hemisphere A/Kansas/14/2017.
Most of the seqenced influenza B-Victoria viruses were triple-deletion variants similar to B/Washington/02/2019.
Respiratory samples were also analysed for other respiratory viruses. In the ILI population, 70 % of the patients were positive for at least one respiratory virus (including Influenza and co-infections). In the SARI population, 52% of the patients were positive for at least one respiratory viruses (including influenza, SARS-COV-2, other respiratory viruses or different combination of co-infection). From week 10 , the first SARS-CoV-2 patient were diagnosed.
These patients were mostly adults and children above 14 years old.
Severity was moderate in comparison to the previous season and comparable to previous seasons.
None of the analyzed strains presented mutations known to be associated to resistance to antivirals neuraminidase inhibitors (Oseltamivir et Zanamivir).
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