Asymmetric post-pandemic recovery of influenza A and B since 2020 in Hong Kong: an interrupted time-series (ITS) analysis of weekly surveillance data with subtype and lineage characterization

Influenza circulation declined markedly worldwide after the emergence of COVID-19 in 2020. Although early suppression was widely documented, less information is available on how influenza activity returned during later years, particularly for influenza A and influenza B and their subtype or lineage distributions.

Weekly laboratory surveillance data from Hong Kong covering 2014–2024 were analyzed using interrupted time-series regression. Weekly numbers of tested specimens were included as an offset. Epidemiological week 1 of 2020 was used as the primary interruption point. Influenza A subtype activity, A(H1) and A(H3), and influenza B lineage activity, Victoria and Yamagata, were also examined descriptively. Other respiratory viruses (ORV) were included for general comparison.

Both influenza A and influenza B declined sharply during early 2020, although the decline was greater for influenza B. Influenza A activity increased again during later years, with recurrent seasonal peaks and variation in the relative activity of A(H1) and A(H3). Influenza B remained at comparatively low levels through much of the post-2020 period. Among characterized influenza B viruses, most detections belonged to the B/Victoria lineage, whereas B/Yamagata was not identified in later surveillance data. ORV circulation showed greater variability after 2020 and did not follow the same pattern observed for influenza.

Recovery patterns differed between influenza A and influenza B after 2020 in Hong Kong. Influenza A circulation resumed gradually, whereas influenza B activity remained lower and lineage diversity was reduced. Continued surveillance at the level of virus type, subtype, and lineage may help identify future changes in influenza circulation during the post-pandemic period.