The impact of COVID–19 lockdown on dengue transmission in Sri Lanka; A natural experiment for understanding the influence of human mobility

Background Dengue is one of the major public health problems in Sri Lanka. Its outbreak pattern depends on a multitude of drivers, including human mobility. Here we evaluate the impact of COVID–19 related mobility restriction (lockdown) on the risk of dengue in Sri Lanka. Methodology Two-stage hierarchical models were fitted using an interrupted time-series design based on the notified dengue cases, January 2015 to July 2020. In the first stage model, the district level impact was estimated using quasi-Poisson regression models while accounting for temporal trends. Estimates were pooled at zonal and national levels in the second stage model using meta-analysis. The influence of the extended period of school closure on dengue in children in the western province was compared to adults. Findings Statistically significant and homogeneous reduction of dengue risk was observed at all levels during the lockdown. Overall an 88% reduction in risk (RR 0.12; 95% CI from 0.08 to 0.17) was observed at the national level. The highest impact was observed among children aged less than 19 years showing a 92% reduction (RR 0.8; 95% CI from 0.03 to 0.25). We observed higher impact in the dry zone having 91% reduction (RR 0.09; 95% CI from 0.05 to 0.15) compared to wet zone showing 83% reduction (RR 0.17; 95% CI from 0.09 to 0.30). There was no indication that the overall health-seeking behaviour for dengue had a substantial influence on these estimates. Significance This study offers a broad understanding of the change in risk of dengue during the COVID–19 pandemic and associated mobility restrictions in Sri Lanka. The analysis using the mobility restrictions as a natural experiment suggests mobility patterns to be a very important driver of dengue transmission. Author summary Every year about 100 million symptomatic dengue cases occur with the highest-burden reported in Southeast Asia followed by Latin America. The burden is increasing and considered driven by population densities, human mobility, vector proliferation, and climate variability. Evaluation of the impact of human mobility on dengue transmission is methodologically challenging, but an important aspect in public health research. The ongoing COVID-19 pandemic has resulted in dramatic reductions in human mobility due to social distancing measures. These population-level interventions in most countries have changed the exposure patterns to Aedes vectors and the spread of the dengue virus. The mobility restrictions due to the COVID-19 response in Sri Lanka provided a natural experiment to understand the importance of human mobility on dengue in an endemic setting. Using interrupted time series design, we evaluated the impact of mobility restriction on dengue transmission in all 25 administrative districts, climate zones, and age groups in Sri Lanka. The results of this empirical study provided evidence of an overall statistically significant reduction of dengue during the period of mobility restriction in the country, which appears not to be related to changes in health seeking behaviour. Our study provides a methodological framework to quantify the impact of lockdown interventions on dengue transmission and its heterogeneous nature at different geographical, climatic, and socio-demographic levels within and between countries.