Autochthonous transmission patterns of dengue virus serotype 2 in Italy: evidence from outbreaks in 2024.

BACKGROUNDIn 2024, multiple transmission foci of dengue virus serotype 2 (DENV-2) were detected across four Italian regions, resulting in the largest number of autochthonous cases ever recorded in mainland Europe.AIMWe aimed to characterise DENV-2 transmission patterns in Italy in 2024.METHODSWe analysed 296 locally acquired cases with symptom onset between 31 July and 31 October. Using detailed spatiotemporal data, we reconstructed transmission chains between cases with well-documented exposure sites, applying a Bayesian framework. We estimated the generation time and net reproduction number (Rt) for the main transmission foci, quantified the proportion of transmission occurred across varying distances and assessed the influence of different factors, including temperature and control interventions, on secondary transmission.RESULTSThree major foci were identified, with peak Rt estimates ranging from 1.35 to 3.33. The mean generation time was 18.0 days (95% credible interval (CrI): 15.5-20.0 days). Household transmission accounted for 15.4% (95% CrI: 12.6-17.4%) of infection events. Among cases with an identified source of infection, < 1% of transmissions occurred beyond 400 m. Transmissibility declined significantly after outbreak detection, with the average number of secondary cases per infection dropping from 1.4 to 0.4. Vector control interventions were associated with a 41.0% reduction (95% CI: 6.6-63.3%) in transmission; transmission also increased by 19.8% (95% CI: 11.3-29.0%) for each 1°C rise in temperature.CONCLUSIONAutochthonous dengue outbreaks in Italy in 2024 were primarily driven by short-distance transmission. Our findings support that early case detection and rapid vector control are instrumental in reducing transmission.