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. 2022 Nov 30;14(12):2689.
doi: 10.3390/v14122689.

What's Next for Flu? Out-of-Season Circulation of Influenza Viruses in Southern Italy, August 2022

Daniela Loconsole  1 , Francesca Centrone  1 , Valerio Aprile  2 , Anna Sallustio  3 , Daniele Casulli  3 , Marisa Accogli  1 , Davide Sacco  1 , Riccardo Zagaria  1 , Maria Chironna  1
Affiliations

What's Next for Flu? Out-of-Season Circulation of Influenza Viruses in Southern Italy, August 2022

Daniela Loconsole et al. Viruses. .

Abstract

The COVID-19 pandemic has modified the seasonal pattern of respiratory infections. The objective of the present study is to characterize the out-of-season circulation of influenza viruses and an influenza outbreak that occurred in southern Italy in August 2022. Nasopharyngeal swabs collected from patients with influenza-like illnesses (ILI) were tested for the presence of influenza and other respiratory viruses. Epidemiological investigations on 85 patients involved in an influenza outbreak were performed. Sequencing and phylogenetic analysis of hemagglutinin genes was undertaken on samples positive for influenza A. In August 2022, in the Apulia region (Italy), influenza A infection was diagnosed in 19 patients, 18 infected with A/H3N2 and one with A/H1N1pdm09 virus. Seven influenza-positive patients were hospitalized with ILI. A further 17 symptomatic subjects, associated with an influenza outbreak, were also tested; 11 were positive for influenza A/H3N2 virus. Phylogenetic analysis of 12 of the A/H3N2 sequences showed that they all belonged to subclade 3C.2a1b.2a.2. The A/H1N1pdm09 strain belonged to subclade 6B.1A.5a.2. The out-of-season circulation of the influenza virus during the summer months could be linked to changing dynamics in the post-COVID-19 era, as well as to the impact of climate change. Year-round surveillance of respiratory viruses is needed to monitor this phenomenon and to provide effective prevention strategies.

Keywords: SARS-CoV-2; epidemiology; influenza A virus; out-of-season circulation; outbreak; phylogenetic analysis; respiratory viruses.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Seasonal circulation pattern of influenza viruses in the Apulia region of Italy, from season 2015–2016 to season 2021–2022.
Figure 2
Figure 2
Phylogenetic analysis of the hemagglutinin (HA) gene of 12 A/H3N2 influenza viruses characterized in the Apulia region of Italy in August 2022. Red dots represent strains identified from the outbreak and blue squares represent sporadic strains; green triangles represent the vaccine strains for the 2022–2023 season, and the orange triangle represents the vaccine strain for the 2021–2022 season. The phylogenetic tree was constructed using Mega 11 software, applying the neighbor-joining algorithm. The evolutionary distances were calculated using the Kimura 2-parameter method. Amino acid substitutions defining specific genetic clusters are shown at the nodes. Genetic clades are shown to the right of the tree, and the scale bar represents the nucleotide substitutions per site (GISAID accession numbers: EPI2155546, EPI2155545, EPI2155544, EPI2155543, EPI2155541, EPI2155535, EPI2155534, EPI2155533, EPI2155532, EPI2163106, EPI2179877, and EPI2179878).
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