Key result
Bulgarian 2019-2020 influenza A H3N2 strains show multiple amino acid substitutions compared to vaccine viruses.
Why the study?
Influenza viruses have a high potential for genetic changes, necessitating analysis of circulating strains and identification of amino acid substitutions compared to current vaccine strains.
Observational (n=1,387)
Yes
The study confirms the rapid evolution of influenza viruses in Bulgaria during the 2019-2020 season, highlighting the need for continuous antigenic and genetic surveillance.
Signals potential regional vaccine mismatch; leaves open implications for strain selection and requires broader validation.
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.
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Korsun et al. (2021) conducted an observational in Acute respiratory infection (Influenza) (n=1,387). Influenza virus infection was evaluated on Detection and genetic characterization of seasonal influenza viruses. During the 2019-2020 season in Bulgaria, influenza A(H3N2) viruses predominated, and genetic analysis revealed multiple amino acid substitutions in circulating strains compared to vaccine viruses.
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