Estimating the epidemic reproduction number from temporally aggregated incidence data: A statistical modelling approach and software tool

by Rebecca K. Nash, Samir Bhatt, Anne Cori, Pierre Nouvellet

The time-varying reproduction number (R_t) is an important measure of epidemic transmissibility that directly informs policy decisions and the optimisation of control measures. EpiEstim is a widely used opensource software tool that uses case incidence and the serial interval (SI, time between symptoms in a case and their infector) to estimate R_t in real-time. The incidence and the SI distribution must be provided at the same temporal resolution, which can limit the applicability of EpiEstim and other similar methods, e.g. for contexts where the time window of incidence reporting is longer than the mean SI. In the EpiEstim R package, we implement an expectation-maximisation algorithm to reconstruct daily incidence from temporally aggregated data, from which R_t can then be estimated. We assess the validity of our method using an extensive simulation study and apply it to COVID-19 and influenza data. For all datasets, the influence of intra-weekly variability in reported data was mitigated by using aggregated weekly data. R_t estimated on weekly sliding windows using incidence reconstructed from weekly data was strongly correlated with estimates from the original daily data. The simulation study revealed that R_t was well estimated in all scenarios and regardless of the temporal aggregation of the data. In the presence of weekend effects, R_t estimates from reconstructed data were more successful at recovering the true value of R_t than those obtained from reported daily data. These results show that this novel method allows R_t to be successfully recovered from aggregated data using a simple approach with very few data requirements. Additionally, by removing administrative noise when daily incidence data are reconstructed, the accuracy of R_t estimates can be improved.