ENSO Impact on Global Chikungunya Virus Transmission, 2008-2024: A Multi-Country Distributed-Lag Time-Series Analysis.

Chikungunya is undergoing global expansion, but how El Niño-Southern Oscillation (ENSO) influences its transmission remains unclear. We aim to assess the impact of ENSO phases on chikungunya incidence via temperature and precipitation teleconnections and to project future risk burden under climate change. We compiled annual national and subnational chikungunya case data (2008-2024), quantified ENSO-climate teleconnections using the E-index (eastern Pacific El Niño) and C-index (central Pacific La Niña), and applied distributed-lag time-series models to assess the teleconnection-mediated impact of ENSO on chikungunya incidence. We further projected future risk burden under climate change scenarios. Modelling shows that El Niño increased chikungunya risk after a 2-year lag (cumulative relative risk [CRR] = 1.40, 95% confidence interval [CI]: 1.02-1.85), while La Niña suppressed it (CRR = 0.09, 95% CI: 0.07-0.13). Temperature teleconnections were the dominant modifier of spatial heterogeneity in effects. Under all scenarios, El Niño-driven warming led to positive median excess cases, the highest under SSP2-4.5, whereas La Niña-driven changes projected smaller, highly uncertain reductions. ENSO influences chikungunya transmission through asymmetric and spatially heterogeneous teleconnection pathways. Integrating ENSO forecasts into surveillance efforts has the potential to enhance preparedness in climate-sensitive regions.