Changing Epidemiology of Influenza Infections Among Children in the Post-Pandemic Period: A Case Study in Xi'an,...

The epidemiology of influenza was disrupted during the COVID-19 pandemic. Following the relaxation of non-pharmaceutical interventions, influenza viruses have re-emerged and caused epidemics with shifts in age distribution and seasonality. This study aimed to characterise the post-pandemic epidemiology of influenza infections among children in Xi'an, China.

A retrospective analysis of laboratory-confirmed paediatric influenza cases spanning three periods [pre-pandemic (1 January 2010-22 January 2020), intra-pandemic (23 January 2020-8 January 2023), and post-pandemic (9 January 2023-31 August 2025)] was conducted. Age-specific incidences were determined by subtypes (lineage) and compared across periods. Seasonal parameters were estimated using a generalised linear model with harmonic terms. Associations between influenza infection and risk of co-detection with other respiratory pathogens were assessed using logistic regression models.

Influenza peak activity in the post-pandemic period was 10-fold higher than in the intra-pandemic period. The mean age of infected children increased by 1.4 years (95% CI: 1.2-1.7), shifting towards school-aged children (6-17 years). The seasonal pattern re-established with an earlier peak (13.9 weeks earlier than the pre-pandemic period, 95% CI: 10.4-15.2) and increased amplitude (10-fold and 4-fold higher than the intra- and pre-pandemic periods, respectively). It was observed that A(H1N1)pdm09 positivity was elevated in preschool and school-aged children, whereas B/Victoria infections showed renewed susceptibility among infants [0-5 months vs. 6-35 months vs. 3-5 years vs. 6-17 years: 11.0% (95% CI: 5.1-19.8) vs. 2.8% (1.9-4.0) vs. 4.0% (3.2-5.0) vs. 5.2% (4.5-6.0); p = 0.00014]. Influenza infection was associated with higher risk of bacterial co-detection with Streptococcus pneumoniae (aOR = 1.52, 95% CI: 1.22-1.91) and Haemophilus influenzae (aOR = 1.46, 95% CI: 1.19-1.80), but lower risk of co-detection with SARS-CoV-2 (aOR = 0.52, 95% CI: 0.27-0.99), RSV (aOR = 0.29, 95% CI: 0.11-0.79), and parainfluenza viruses (aOR = 0.16, 95% CI: 0.04-0.65).

The post-pandemic landscape of paediatric influenza in Xi'an has undergone substantial reconfiguration, characterised by intensified activity, altered seasonality, and a marked shift in age distribution. The increased bacterial co-detection points out the potential for more severe respiratory co-infections. These findings highlight the importance of optimising vaccination timing and prompting school-aged-children-targeted immunisation programmes in the post-pandemic era.