Abstract
Influenza viruses have a high potential for genetic changes. The objectives of this study were to analyse influenza virus circulation in Bulgaria during the 2019/2020 season, to perform a phylogenetic and molecular analyses of the haemagglutinin (HA) and neuraminidase (NA) sequences of representative influenza strains, and to identify amino acid substitutions compared to the current vaccine strains. Seasonal influenza viruses A(H3N2), A(H1N1)pdm09 and B/Victoria-lineage were detected using a real-time RT-PCR in 323 (23.3%), 149 (10.7%) and 138 (9.9%) out of 1387 patient samples studied, respectively. The HA genes of A(H3N2) viruses analysed belonged to clades 3C.3a (21 strains) and 3C.2a (5 strains): subclades 3C.2a1b + T131K, 3C.2a1b + T135K-B and 3C.2a1b + T135K-A. The clade 3C.3a and subclade 3C.2a1b viruses carried 5 and 14–17 substitutions in HA, as well as 3 and 9 substitutions in NA, respectively, in comparison with the A/Kansas/14/2017 vaccine virus, including some substitutions in the HA antigenic sites A, B, C and E. All 21 A(H1N1)pdm09 viruses sequenced fell into 6B.1A5A subclade. Amino acid sequence analysis revealed the presence of 7–11 substitutions in HA, compared to the A/Brisbane/02/2018 vaccine virus, three of which occurred in antigenic site Sb, along with 6–9 changes at positions in NA. All 10 B/Victoria-lineage viruses sequenced belonged to clade 1A with a triple deletion in HA1 (genetic group 1A(Δ3)B) and carried 7 and 3 substitutions in HA and NA, respectively, with respect to the B/Colorado/06/2017 vaccine virus. The results of this study confirm the rapid evolution of influenza viruses and the need for continuous antigenic and genetic surveillance.
Highlights
Influenza is one of the most common infectious illnesses in humans that affects annually 5–10% of the adult population and 20–30% of children causing a huge number of outpatient visits, hospitalisations and fatal cases, imposing a substantial public health burden [1]
The circulation pattern and genetic characteristics of influenza viruses detected in Bulgaria during the 2019/2020 season were examined
Influenza A(H3N2) viruses prevailed (53.1%), and A(H1N1)pdm09 and B/Victoria-lineage viruses circulated in similar proportions
Summary
Influenza is one of the most common infectious illnesses in humans that affects annually 5–10% of the adult population and 20–30% of children causing a huge number of outpatient visits, hospitalisations and fatal cases, imposing a substantial public health burden [1]. Influenza viruses belong to the Orthomyxoviridae family and have a segmented, negative-sense, single-stranded RNA genome, each segment of which encodes at least one protein [2]. The surface glycoproteins haemagglutinin (HA) and neuraminidase (NA) form discrete spikes and play an important part in a viral life cycle. HA mediates the binding of the virus to the sialic acids as cellular receptors and the entry of the viral ribonucleoprotein into the cell cytoplasm by fusing of viral envelope and cell membrane. NA destroys sialic acid residues on the cell surface and provides the release of newly formed influenza viruses from the infected cells. HA is the primary target for virus-neutralising antibodies and together with NA is a key component of influenza vaccines
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