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.

流感病毒具有很大的遗传改变潜力。本研究的目的是分析 2019/2020 季节期间保加利亚的流感病毒传播，对代表性流感病毒株的血凝素 ( HA ) 和神经氨酸酶 ( NA ) 序列进行系统发育和分子分析，并鉴定氨基酸替换与目前的疫苗株相比。使用实时 RT-PCR 在 1387 例患者中检测到季节性流感病毒 A(H3N2)、A(H1N1)pdm09 和 B/Victoria 谱系，其中 323 例 (23.3%)、149 例 (10.7%) 和 138 例 (9.9%)分别研究患者样本。分析的 A(H3N2) 病毒的HA基因属于进化枝 3C.3a（21 株）和 3C.2a（5 株）：亚进化枝 3C.2a1b + T131K、3C.2a1b + T135K-B 和 3C.2a1b + T135K-一个。与 A/Kansas/14/2017 疫苗病毒相比，进化枝 3C.3a 和亚进化枝 3C.2a1b 病毒在HA 中分别携带 5 个和 14-17 个替换，在NA中分别携带 3 个和 9 个替换，包括一些替换测序的所有21个 A(H1N1)pdm09 病毒均属于 6B.1A5A 亚支。氨基酸序列分析显示，与 A/布里斯班/02/2018 疫苗病毒相比， HA中存在 7-11 个替换，其中 3 个替换发生在抗原位点 Sb 中， NA中的位置有 6-9 个变化。与 B/Colorado/06 相比，所有 10 个 B/Victoria 系病毒均属于 1A 分支， HA1中存在三重缺失（遗传组 1A(Δ3)B），并且在HA和NA中分别携带 7 和 3 个取代/2017疫苗病毒。 这项研究的结果证实了流感病毒的快速进化以及持续抗原和基因监测的必要性。