Epidemiological characteristics of imported influenza viruses at Shanghai entry–exit ports from 2017 to 2024

TL;DR

This study analyzed imported influenza cases at Shanghai ports from 2017 to 2024, finding stable transmission but shifts in origin and age groups, suggesting a need for dynamic surveillance.

Contribution

The study identifies post-pandemic shifts in imported influenza patterns, emphasizing the need for risk-based surveillance strategies.

Findings

Imported influenza showed stable positivity rates before and after the pandemic, with a rebound in case numbers.

Geographic origins shifted, with increased cases from the Western Pacific Region and fewer from the Americas.

Influenza A remained dominant, though influenza B showed a partial rebound in 2024.

Abstract

To characterize long-term epidemiological patterns of imported influenza at Shanghai entry–exit ports from 2017 to 2024 and to compare changes before and after the COVID-19 pandemic, thereby providing a basis for optimizing cross-border surveillance strategies.

Surveillance data were collected from febrile inbound travelers identified by infrared temperature screening (≥37.3 °C) at major Shanghai ports during 2017–2019 and 2023–2024. Throat swabs were tested using real-time RT-qPCR for influenza A and B viruses. Positivity rates, seasonality, demographic characteristics, geographic origins, and viral type distributions were compared between pre- and post-COVID-19 periods using appropriate statistical tests.

Among 33,118 febrile inbound travelers screened, 6,163 influenza cases were confirmed, yielding an overall positivity rate of 18.61%. No significant difference was observed between the pre- and post-COVID-19 periods (18.94% vs. 18.52%, p = 0.34), despite a marked increase in case numbers following travel resumption. Imported influenza exhibited a persistent bimodal seasonal pattern, with winter–spring peaks largely associated with Northern Hemisphere sources and summer peaks primarily driven by Southern Hemisphere imports. The geographic origin of cases shifted significantly, with the Western Pacific Region accounting for 66.2% of cases in 2024, while contributions from the Americas declined (p < 0.05). Adults aged 25–59 years increasingly predominated, and the proportion of travelers aged ≥60 years rose over time. Influenza A viruses remained dominant throughout the study period, accounting for 93.3% of cases in 2023, while influenza B showed a partial rebound in 2024.

Imported influenza at Shanghai entry–exit ports rebounded rapidly after COVID-19, with stable transmission intensity but notable shifts in geographic origin, age composition, and viral distribution. The persistence of bimodal seasonality and increasing concentration of cases from the Western Pacific Region highlight the need to transition from fixed seasonal screening toward dynamic, risk-based port surveillance with emphasis on high-traffic routes and adult travelers.

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