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 ( R 0 ). 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 ( R 2 > 0.95). The median R 0 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 R 0 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.