Nowcasting and Forecasting Seasonal Influenza Epidemics — China, 2022–2023

Abstract

<abstract abstract-type="Abstract"> <sec> <b>Background</b> Seasonal influenza resurged in China in February 2023, causing a large number of hospitalizations. While influenza epidemics occurred across China during the coronavirus disease 2019 (COVID-19) pandemic, the relaxation of COVID-19 containment measures in December 2022 may have contributed to the spread of acute respiratory infections in winter 2022/2023. </sec><sec> <b>Methods</b> Using a mathematical model incorporating influenza activity as measured by influenza-like illness (ILI) data for northern and southern regions of China, we reconstructed the seasonal influenza incidence from October 2015 to September 2019 before the COVID-19 pandemic. Using this trained model, we predicted influenza activities in northern and southern China from March to September 2023. </sec><sec> <b>Results</b> We estimated the effective reproduction number <i>R</i>_<i>e</i> as 1.08 [95% confidence interval (<i>CI</i>): 0.51, 1.65] in northern China and 1.10 (95% <i>CI</i>: 0.55, 1.67) in southern China at the start of the 2022–2023 influenza season. We estimated the infection attack rate of this influenza wave as 18.51% (95% <i>CI</i>: 0.00%, 37.78%) in northern China and 28.30% (95% <i>CI</i>: 14.77%, 41.82%) in southern China. </sec><sec> <b>Conclusions</b> The 2023 spring wave of seasonal influenza in China spread until July 2023 and infected a substantial number of people. </sec>