Influenza Epidemics in Zhejiang Province: A Post-COVID-19 Analysis
Influenza viruses readily mutate, requiring continuous, systematic monitoring too track epidemic patterns adn identify emerging mutations. This details is crucial for selecting appropriate vaccine strains and developing effective disease prevention and control strategies [13]. Data from Zhejiang Province in 2020 revealed a significant decrease in both influenza-like illness (ILI) percentage and the influenza virus detection rate compared to pre-pandemic levels.The epidemic remained low throughout the year, lacking typical seasonal patterns, mirroring the national influenza situation at the time [14].
This decline likely resulted from the stringent nonpharmaceutical interventions (NPIs) implemented in China starting January 20, 2020, to combat COVID-19. These public health measures effectively controlled the COVID-19 outbreak and simultaneously curbed influenza transmission [15, 16]. In 2021, as the COVID-19 epidemic stabilized and social activities resumed, public awareness of preventative measures waned, leading to a resurgence of influenza, peaking during winter and spring.
The influenza epidemic in 2022 exhibited three distinct peaks: July, December, and March of the following year. The influenza virus nucleic acid detection rate during this period was comparable to that of 2019, before the COVID-19 pandemic. The July peak may be linked to Zhejiang Province’s geographical location [17]. Situated in the southeast coastal region, the province is frequently enough affected by typhoons and the rainy season during summer, causing temperature drops that favor influenza virus survival and spread.Furthermore, July coincides with the summer vacation period in China, and increased travel and gatherings accelerate virus transmission.
From November 2022, ILI% rapidly increased, peaking in December. however, the influenza virus detection rate remained low, suggesting the rise in ILI% was likely associated with the rapid spread of COVID-19, with most ILI patients infected with SARS-CoV-2 [18]. Research indicates that influenza surveillance during the COVID-19 pandemic should consider the correlation between ILI% and the influenza virus detection rate. Simultaneous testing for both influenza virus and SARS-CoV-2 in ILI and severe acute respiratory infection (SARI) samples is essential for monitoring the pandemic’s impact.
In early 2023, ILI cases in Zhejiang Province increased rapidly, peaking in march, primarily driven by A (H1N1) influenza viruses. This surge might potentially be attributed to the easing of COVID-19-related NPIs in China on January 8, 2023, and the subsequent resumption of social order and travel. Additionally, the immunological debt created by the COVID-19 pandemic [19], reduced immune function caused by SARS-CoV-2 [20, 21], and seasonal factors [22] contributed to the post-COVID-19 influenza outbreak. Similar “triple outbreaks” of influenza,respiratory syncytial virus (RSV),and COVID-19 have been observed in Europe and the United States since the pandemic began [23].
Regarding population distribution, ILI cases in Zhejiang Province were primarily concentrated among children under 5 years old, with fewer cases reported in the elderly. This is likely due to children’s less developed immune systems.