Abstract
Background: To optimize seasonal influenza vaccination programs in regions with potentially complicated seasonal patterns, the epidemiological characteristics of seasonal influenza activity in a subtropical city of China were explored. Materials and Methods: Influenza virus data of patients with influenza-like illness (ILI) during 2013–2019 were collected from two sentinel hospitals in a subtropical region of China, Yichang city. The influenza virus positive rate among sampled ILI cases served as a proxy to estimate influenza seasonal characteristics, including periodicity, duration, peaks, and predominant subtypes/lineages. Epidemiological features of different years, seasons and age groups were analyzed, and vaccine mismatches were identified. Results: In total, 8693 ILI cases were included; 1439 (16.6%) were laboratory-confirmed influenza cases. The influenza A positive rate (10.6%) was higher than the influenza B positive rate (5.9%). There were three influenza circulation patterns in Yichang: (1) annual periodicity (in 2013–2014, 2015–2016 and 2018–2019), (2) semiannual periodicity (in 2014–2015), and (3) year-round periodicity (in 2016–2017 and 2017–2018). Summer epidemics existed in two of the six years and were dominated by influenza A/H3N2. Winter and spring epidemics occurred in five of the six years, and A/H1N1, A/H3N2, B/Victoria, and B/Yamagata were codominant. During the study period, the predominant lineages, B/Victoria in 2015-16 and B/Yamagata in 2017–2018, were both mismatched with the influenza B component of the trivalent vaccine. Children 5–14 years old (26.4%) and individuals over 60 years old (16.9%) had the highest influenza positive rates. Conclusions: The seasonal epidemic period and the predominant subtype/lineage of influenza viruses in Yichang city are complex. Influenza vaccination timing and strategies need to be optimized according to the local features of influenza virus activity.
Highlights
Influenza is one of the most serious diseases worldwide, resulting in an estimated 1 billion cases, 3-5 million hospitalizations, and 290,000–650,000 respiratory deaths globally per year [1]
From 2013–2019, 8693 influenza-like illness (ILI) patients were enrolled for influenza virus surveillance in this study
Our findings demonstrate that Yichang may have three different influenza circulation patterns; annual, semiannual and year-round epidemics all occurred in the surveillance years 2013–2019
Summary
Influenza is one of the most serious diseases worldwide, resulting in an estimated 1 billion cases, 3-5 million hospitalizations, and 290,000–650,000 respiratory deaths globally per year [1]. There are three climatic zones in China: Temperate, subtropical, and tropical. Influenza epidemics occur annually and with predictable seasonality; in tropical regions, they can occur year round, with unpredictable peaks [11,12]. To optimize seasonal influenza vaccination programs in regions with potentially complicated seasonal patterns, the epidemiological characteristics of seasonal influenza activity in a subtropical city of China were explored. Materials and Methods: Influenza virus data of patients with influenza-like illness (ILI) during 2013–2019 were collected from two sentinel hospitals in a subtropical region of China, Yichang city. The influenza virus positive rate among sampled ILI cases served as a proxy to estimate influenza seasonal characteristics, including periodicity, duration, peaks, and predominant subtypes/lineages. There were three influenza circulation patterns in Yichang: (1) annual periodicity (in 2013–2014, 2015–2016 and 2018–2019),
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