Reproduction number of SARS-CoV-2 Omicron variants, China, December 2022–January 2023

Abstract

<p>China had successfully suppressed multiple waves of SARS-CoV-2 epidemics using the ‘dynamic zero-COVID’ strategy before November 2022. Due to the substantially reduced pathogenicity of new SARS-CoV-2 Omicron variants (e.g. BF.7 and BA.5.2) in populations with higher vaccine coverage, and the enormous socioeconomic costs incurred by the dynamic zero-COVID strategy, China began to adjust response strategies from 11 November 2022 (including restricting testing coverage, shortening quarantine periods for inbound travellers and suspending secondary contacts tracing).¹ Starting from 7 December 2022,² the further relaxation of control measures (including the prohibition of regional mass testing and the implementation of home isolation or quarantine) triggered an unprecedentedly large Omicron wave in China, leading to a sharp increase in fatalities. The abrupt change of COVID control measures and the resulting surge of SARS-CoV-2 infection, hospitalization and death in China have raised great international concerns.³</p><p>The time-varying reproduction number (<em>R_t</em>), a measure of instantaneous transmissibility of an epidemic, is defined as the average number of secondary infections caused by a typical primary case at time <em>t</em>, after accounting for the population immunity and the impact of control measures. Reliable estimates of <em>R_t</em> are essential to understand how the transmissibility of COVID-19 changes after the relaxation of the public health and social measures. As the population-wide testing of SARS-CoV-2 was abandoned in mainland China from 8 January 2023, surveillance data about the daily number of new infections were lacking since then. To fill this gap, on 25 January 2023, the Chinese Center for Disease Control and Prevention (CDC) published a unique surveillance data estimating the daily hospitalization number of COVID-19 patients in hospitals from December 2022 to January 2023 in China.⁴ Informed by these data, we evaluated the <em>R_t</em> for the 32 provincial-level administrative divisions in the study period, together with the number of daily new cases, to nowcast the epidemic growth of COVID-19 Omicron variant in China from December 2022 to January 2023.</p>