Epidemiological characteristics and transmission dynamics of dengue in Ningbo City, China before, during, and after the COVID-19 pandemic interventions

BACKGROUND: Ningbo experienced a local dengue outbreak in 2018-2019, followed by the implementation and relaxation of COVID-19 pandemic interventions. Evidence has been limited on dengue epidemiology and the transmissibility across these periods. METHODS: We analyzed routine dengue cases and mosquito surveillance data from 2017 to 2024, using descriptive analyses to assess the epidemiological characteristics, and a mathematical model was developed to estimate the basic reproduction number (R0). RESULTS: A total of 266 dengue cases were reported, with no deaths. Before the pandemic interventions, 51.19% of cases were overseas-imported; no cases were reported during pandemic interventions, whereas 73.47% were domestically imported after pandemic interventions. The median age of cases was 38.50 years (IQR = 31.25-54.00), and 57.52% were unemployed, retired, or in commercial services. The onset-to-diagnosis interval shortened post-interventions (4.46 days vs. 2.67 days). Viral typing increased post-interventions (34.59% vs. 63.27%), with 83.70% of cases identified as dengue virus-1 infections. Dengue cases were reported across all districts, with higher proportions in Haishu (27.07%) and Yinzhou (24.44%). The incidence was 2.82 per 100,000 persons. Incidence did not differ significantly by sex, but was significantly higher among individuals 15-59 years and those aged ≥60years than among those aged ≤14 years (3.18 and 3.01 per 100,000 persons, respectively, vs. 0.45 per 100,000 persons). Incidence was highest in Haishu and Jiangbei (6.90 and 4.93 per 100,000 persons, respectively). A post-intervention incidence rebound was observed, although no statistically significant differences were observed in the ≤ 14 age group, Beilun District, Ninghai County and Xiangshan County. Aedes albopictus was the only vector species. The mean Breteau Index was 16.07 (SD: 8.04) and decreased across phases, and rebounded in Jiangbei, Zhenhai, and Haishu districts. The mean BI was higher in summer compared to other seasons (p < 0.05). Model fit was strong (R2 > 0.95). The median R0 was higher in the outbreak years (3.46, IQR = 2.98-3.95) than years without local transmission (0.00, IQR = 0.00-0.00; p < 0.001). Median R0 was 0.00 (IQR = 0.00-0.71) over 2017-2024 due to the absence of local cases in most years. CONCLUSIONS: Dengue remains non-endemic in Ningbo, and is driven primarily by imported infections rather than sustained local transmission. Despite a reduction in diagnostic delay and vector density, post-PHSM incidences rebounded, and district-level heterogeneity indicates ongoing outbreak potential. Sustained surveillance, rapid diagnosis and district-tailored vector control are critical to prevent re-establishment of local transmission.