Post-Pandemic Influenza Resurgence in Guangzhou, China: Impact of COVID-19 Interventions and Immune Alterations.

The COVID-19 pandemic and its associated nonpharmaceutical interventions (NPIs) dramatically altered influenza epidemiology. This study aimed to characterize the post-pandemic influenza resurgence in Guangzhou, China, and explore the interplay between NPIs, SARS-CoV-2 infections, and influenza activity. We conducted a retrospective analysis of 73 652 patients with flu-like symptoms at a major tertiary hospital in Guangzhou between February 2016 and June 2024. Cases were stratified by age and period (pre-pandemic, pandemic, post-pandemic). We compared influenza positivity rates via rapid antigen tests and analyzed hematological parameters (WBC, neutrophil, lymphocyte counts) in influenza-positive patients. Influenza positivity rates plummeted during the pandemic (1.99%) compared to pre-pandemic levels (10.20%). However, a dramatic surge occurred in the first post-pandemic year, reaching 24.96%. Influenza A rebounded earlier and more intensely than influenza B. Adults (≥ 18 years) exhibited a significantly greater percent increase in positivity rates compared to children and showed more pronounced decreases in WBC and neutrophil counts post-pandemic. Lymphocyte counts initially increased but later declined. The post-pandemic influenza surge in Guangzhou was both prolonged and intense, likely driven by a dual mechanism: an "immunity gap" created by prolonged NPIs and potential immune alterations following widespread SARS-CoV-2 infection. Sustained vaccination and proactive public health measures are crucial for managing future respiratory virus threats.