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J Med Microbiol. 2018 Feb;67(2):228-239. doi: 10.1099/jmm.0.000668. Epub 2018 Jan 3.

Predominance of influenza A(H3N2) viruses during the 2016/2017 season in Bulgaria.

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1​National Reference Laboratory "Influenza and ARD", National Centre of Infectious and Parasitic Diseases, 44A Stoletov Blvd, 1233 Sofia, Bulgaria.
2​Paediatric Clinic, University Hospital Alexandrovska, Medical University, 1"Sv. Georgi Sofiiski" Str., 1431 Sofia, Bulgaria.



Influenza viruses are characterised by high variability, which makes them able to cause annual epidemics. The aim of this study is to determine the antigenic and genetic characteristics of influenza viruses circulating in Bulgaria during the 2016/2017 season.


The detection and typing/subtyping of influenza viruses were performed using real time RT-PCR. Results of antigenic characterisation, phylogenetic and amino acid sequence analyses of representative influenza strains are presented herein.


The 2016/2017 season was characterised by an early start, an exclusive dominance of A(H3N2) viruses accounting for 93 % of total influenza virus detections, and a low circulation of A(H1N1)pdm09 (4.2 %) and type B (2.5 %) viruses. The analysed A(H3N2) viruses belonged to subclades 3C.2a (52 %) and 3C.2a1 (48 %); all studied A(H1N1)pdm09 and B/Victoria-lineage viruses belonged to subclades 6B.1 and 1A, respectively. The amino acid sequence analysis of 56 A(H3N2) isolates revealed the presence of substitutions in 18 positions in haemagglutinin (HA) as compared to the A/Hong Kong/4801/2014 vaccine virus, seven of which occurred in four antigenic sites, together with changes in 23 positions in neuraminidase (NA), and a number of substitutions in internal proteins PB2, PB1, PB1-F2, PA, NP and NS1. Despite the many amino acid substitutions, A(H3N2) viruses remained antigenically similar to the vaccine strain. Substitutions in HA and NA sequences of A(H1N1)pdm09 and B/Victoria-lineage strains were also identified, including in antigenic sites.


The results of this study confirm the genetic variability of circulating influenza viruses, particularly A(H3N2), and the need for continued antigenic and molecular surveillance.


amino acid substitution; antigenic and genetic analysis; influenza virus

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