The optimal strategy for seasonal influenza vaccination to prevent high-intensity level of influenza epidemics in Zhejiang, China: an integrated transmission-dynamic and health-economic modeling analysis

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BACKGROUND: Although Seasonal Influenza Vaccination (SIV) is a crucial preventive measure, achieving sufficient coverage to completely control influenza epidemics poses a significant challenge. This study aims to evaluate optimal strategies for SIV to prevent high-intensity level of influenza epidemics in Zhejiang Province, China. High-intensity outbreaks were defined as weekly incidence rates above 72.2 per 100,000. METHODS: This study estimated the incidence of influenza from 2018 to 2023 in Zhejiang Province, China, using influenza weekly surveillance data. We developed a Susceptible-Vaccinated-Infectious-Recovered-Susceptible (SVIRS) model to simulate influenza transmission and used a decision tree to assess seven vaccination strategies aimed at preventing high-intensity influenza outbreaks. These strategies differed in vaccine coverage across the three age groups: 0-14 years, 15-59 years, and 60 + years, despite having the same overall vaccination coverage. RESULTS: Between 2018 and 2020, influenza incidence in Zhejiang Province followed typical seasonal patterns. However, during the COVID-19 pandemic, these patterns became irregular, culminating in a high-intensity influenza season in 2022-2023. Model simulations indicated that increasing population-wide vaccination coverage to 36.17% could effectively prevent outbreaks from escalating to high-intensity levels. Among various allocation strategies for additional vaccines, targeting individuals aged 0-14 years and those aged 60 years and older was found to be the most effective. This approach reduced the peak weekly incidence rate from the observed 182.83 to a predicted 42.28 per 100,000, saved 23.45 CNY per capita, and yielded a health gain of 0.0102 quality-adjusted life days (QALDs) per person. CONCLUSION: Achieving a population-wide vaccination coverage beyond 36.17% is projected to prevent high-intensity level of influenza epidemics. Targeting the 0-14 years and 60 + years age groups proves to be the most cost-effective strategy under constrained coverage conditions. These findings highlight the importance of age-prioritized vaccination policies to optimize resource allocation and mitigate the impact of influenza outbreaks, particularly in regions with limited healthcare capacity.

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