title: Antigenic and genetic characterization of influenza viruses circulating in Bulgaria during the 2015/2016 season.
abstract: Influenza virological surveillance is an essential tool for early detection of novel genetic variants of epidemiologic and clinical significance. The aim of this study was to determine the antigenic and molecular characteristics of influenza viruses circulating in Bulgaria during the 2015/2016 season. The season was characterized by dominant circulation of A(H1N1)pdm09 viruses, accounting for 66% of detected influenza viruses, followed by B/Victoria-lineage viruses (24%) and A(H3N2) viruses (10%). All sequenced influenza A(H1N1)pdm09, A(H3N2) and B/Victoria-lineage viruses belonged to the 6B.1, 3C.2a and 1A genetic groups, respectively. Amino acid analysis of 57 A(H1N1)pdm09 isolates revealed the presence of 16 changes in hemagglutinin (HA) compared to the vaccine virus, five of which occurred in four antigenic sites, together with 16 changes in neuraminidase (NA) and a number of substitutions in proteins MP, NP, NS and PB2. Despite the many amino acid substitutions, A(H1N1)pdm09 viruses remained antigenically closely related to A/California/7/2009 vaccine virus. Bulgarian A(H3N2) strains (subclade 3C.2a) showed changes at 11 HA positions four of which were located in antigenic sites A and B, together with 6 positions in NA, compared to the subclade 3C.3a vaccine virus. They contained unique HA1 substitutions N171K, S312R and HA2 substitutions I77V and G155E compared to Bulgarian 3C.2a viruses of the previous season. All 20 B/Victoria-lineage viruses sequenced harboured two substitutions in the antigenic 120-loop region of HA, and 5 changes in NA, compared to the B/Brisbane/60/2008 vaccine virus. The results of this study reaffirm the continuous genetic variability of circulating seasonal influenza viruses and the need for continued systematic antigenic and molecular surveillance.
authors.affiliation: National Laboratory "Influenza and ARD", National Centre of Infectious and Parasitic Diseases, 44A Stoletov Blvd, 1233 Sofia, Bulgaria. Electronic address: neli_korsun@abv.bg.
authors.forename: Neli
authors.initials: N
authors.lastname: Korsun
authors.affiliation: National Laboratory "Influenza and ARD", National Centre of Infectious and Parasitic Diseases, 44A Stoletov Blvd, 1233 Sofia, Bulgaria.
authors.forename: Svetla
authors.initials: S
authors.lastname: Angelova
authors.affiliation: WHO Collaborating Centre for Reference and Research on Influenza, Crick Worldwide Influenza Centre, The Francis Crick Institute, 1, Midland Road, London NW1 1AT, United Kingdom.
authors.forename: Viki
authors.initials: V
authors.lastname: Gregory
authors.affiliation: WHO Collaborating Centre for Reference and Research on Influenza, Crick Worldwide Influenza Centre, The Francis Crick Institute, 1, Midland Road, London NW1 1AT, United Kingdom.
authors.forename: Rodney
authors.initials: R
authors.lastname: Daniels
authors.affiliation: National Laboratory "Influenza and ARD", National Centre of Infectious and Parasitic Diseases, 44A Stoletov Blvd, 1233 Sofia, Bulgaria.
authors.forename: Irina
authors.initials: I
authors.lastname: Georgieva
authors.affiliation: WHO Collaborating Centre for Reference and Research on Influenza, Crick Worldwide Influenza Centre, The Francis Crick Institute, 1, Midland Road, London NW1 1AT, United Kingdom.
authors.forename: John
authors.initials: J
authors.lastname: McCauley
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000956:Antigens, Viral / Q000235:genetics*; D002031:Bulgaria / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D062665:Epidemiological Monitoring; D005260:Female; D019267:Hemagglutinin Glycoproteins, Influenza Virus / Q000235:genetics*; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000145:classification / Q000235:genetics*; D053122:Influenza A Virus, H3N2 Subtype / Q000145:classification / Q000235:genetics*; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission / Q000821:virology; D008297:Male; D008875:Middle Aged; D009439:Neuraminidase / Q000235:genetics*; D010802:Phylogeny; D012621:Seasons; D017422:Sequence Analysis, DNA
chemical_list: D000956:Antigens, Viral; D019267:Hemagglutinin Glycoproteins, Influenza Virus; D009439:Neuraminidase
keywords: Amino acid substitution; Antigenic and genetic characterization; Influenza virus