Evaluating the association between COVID-19 transmission and mobility in Omicron outbreaks in China

Background Prior research has suggested a positive correlation between human mobility and COVID-19 transmission at national or provincial levels, assuming constant correlations during outbreaks. However, the correlation strength at finer scales and potential changes in relationships during outbreaks have been scarcely investigated. Methods We gathered case and mobility data (within-city movement, inter-city inflow, and inter-city outflow) at the city level from Omicron outbreaks in mainland China between February and November 2022. For each outbreak, we calculated the time-varying effective reproduction number ( R _t ). Subsequently, we estimated the cross-correlation and rolling correlation between R _t and the mobility index, comparing them and identifying potential factors affecting these correlations. Results We identified 57 outbreaks during Omicron wave 1 (February to June) and 171 outbreaks during Omicron wave 2 (July to December). Cross-correlation estimates varied between waves, with values ranging from 0.64 to 0.71 in wave 1 and 0.45 to 0.46 in wave 2. Oscillation models best fit the rolling correlation for almost all outbreaks, and there were significant differences between extreme values of rolling correlation and cross-correlation. Additionally, we estimated a positive relationship between the GRI and rolling correlation during the pre-peak stage, turning negative during the post-peak stage. Conclusions Our findings suggest a positive relationship between Omicron transmission and mobility at the city level. However, significant fluctuations in their relationship, as demonstrated by rolling correlation, indicate that assuming a constant correlation between transmission and mobility may lead to inaccurate predictions or decisions when using mobility as a proxy for transmission intensity.