Background: Seasonal influenza infection cases decline significantly in Hong Kong since the COVID-19 pandemic has begun in 2020. This also reduced the opportunities of reinducing influenza-specific antibodies in the community. The levels of antibodies in protecting against recently circulating influenza A viruses in post COVID-19 era are unclear.

Methods: Paired plasma samples which were collected in 2021 and 2022 from 479 healthy adults in Hong Kong were tested. The neutralizing titer in plasma against the influenza A (H1N1) and (H3N2) viruses circulating before or after the COVID-19 outbreak were determined using microneutralization assay.

Results: The H1N1 and H3N2 vaccine strains which have been selected for the 2022/23 season are phylogenetically closed to the recently circulating viruses. In samples collected in 2022, only 14.61% and 0.42% showed neutralization titer MN50 ≥1:20 to the H1N1 A/Wisconsin/588/2019 (H1/Wis19) and H3N2 A/Darwin/6/2021 (H3/Dar21), respectively. Participants who reported receiving flu vaccination annually had higher seropositive rate to H1/Wis19 than those who had never received flu vaccine (28.06% vs 5.30%). SS: Our results indicate adults in Hong Kong are generally lack of neutralizing antibodies against the circulating influenza A viruses especially the H3N2, highlighting the importance of promoting flu vaccination in the post-COVID-19 era.