Influenza activity in the mainland of China was below pre-pandemic COVID-19 levels during the period October 5, 2020 to September 5, 2021 (surveillance Week 41 in 2020 to Week 35 in 2021). The percentage of specimens testing positive for influenza each week ranged from 0.1% to 6.2% in southern China and ranged from 0% to 10.6% in northern China. The positivity rate of specimens collected from ILI cases slightly increased starting at Week 49 in 2020 and had been increasing in 2021 in southern China. The positivity rate in northern China began to increase in Week 10 of 2021, peaking at Week 18 of 2021, and declined since then (Figures 1 and 2).
Influenza positive tests reported by network laboratories in southern China.
Influenza positive tests reported by network laboratories in northern China.
In southern China, the network laboratories tested 308,698 specimens between October 5, 2020 and September 5, 2021; among these specimens, 7,593 (2.5%) tested positive including 79 (1.0%) for influenza A and 7,514 (99.0%) for influenza B. Most of the positive samples (7,299, 96.1%) were collected since 2021. Among the 58 seasonal influenza A positive specimens that were subtyped, 39 (67.2%) were influenza A(H1N1)pdm09 and 19 (32.8%) were influenza A(H3N2). Among the 7,497 influenza B viruses for which lineage was determined, 7,481 (99.8%) belonged to the B/Victoria lineage and 16 (0.2%) belonged to the B/Yamagata lineage (Figure 1).
During the study period, network laboratories in northern China tested 209,219 specimens for influenza; among these, 3,489 (1.7%) were positive for an influenza virus: 19 (0.5%) for influenza A and 3,470 (99.5%) for influenza B. Among the 14 seasonal influenza A viruses subtyped, 7 (50.0%) were influenza A(H1N1)pdm09 and 7 (50.0%) were influenza A(H3N2). Of the 3,465 Influenza B viruses with B lineage results, 3,456 (99.7%) were B/Victoria and 9 (0.3%) were B/Yamagata (Figure 2).
Two A(H1N1)pdm09 viruses were antigenically characterized by HI tests, and one virus was well inhibited by ferret antisera raised against A/Guangdong-Maonan/SWL1536/2019, the egg-propagated reference virus representing the A(H1N1)pdm09 component of the 2020–2021 Northern Hemisphere influenza vaccines (7). One of the viruses was well inhibited by ferret antisera raised against A/Victoria/2570/2019, the egg-propagated reference virus representing the A(H1N1)pdm09 component for the upcoming 2021–2022 Northern Hemisphere influenza vaccines (8). Phylogenetic analysis of hemagglutinin (HA) gene segments determined that 1 belonged to genetic subclade 6B.1A5A with HA amino acid substitutions N129D, T185I, and N260D; the other one belonged to group 6B.1A5A2 with HA amino acid substitutions K130N, N156K, L161I, V250A, and E506D, which evolved from subclade 6B.1A5A.
Antigenic characterization of the 1,819 B/Victoria lineage viruses were conducted using HI tests. A total of 1,141 (62.7%) B/Victoria lineage viruses were not recognized well by the ferret antisera raised against B/Washington/02/2019, the egg-propagated reference virus representing the B/Victoria lineage component of the 2020–2021 and 2021–2022 Northern Hemisphere influenza vaccines (7-8). Among the 797 viruses HA gene segments sequenced and phylogenetically analyzed, all belonged to genetic clade 1A, nearly all belonged to subclade 1A.3, which shared a three amino acid deletion in HA (positions 162–164) and the substitution K136E, and in which, 96.7% belong to group 1A.3a characterized by HA amino acid substitutions N150K, G184E, N197D, and R279K, and was further divided into subgroup 1A.3a1 (520, 65.2%), which had additional HA amino acid substitutions V220M and P241Q, and 1A.3a2 (251, 31.5%), which had HA amino acid substitutions A127T, P144L, and K203R.
No A(H3N2) and B/Yamagata lineage viruses were isolated and available for characterization during the study period.
Of the 2 A(H1N1)pdm09 viruses collected in the mainland of China, neither showed evidence of reduced inhibition by oseltamivir and zanamivir, nor to baloxavir. Of the 1,258 influenza B/Victoria lineage viruses screened for neuraminidase inhibitors susceptibility, all showed normal inhibition by oseltamivir and zanamivir. A total of 797 B/Victoria lineage viruses were screened for susceptibility to the endonuclease inhibitor, baloxavir, by genetic analysis, and none showed evidence of reduced susceptibility.