Estimating transmission dynamics and severity of SARS-CoV-2 Omicron following adjustment of zero-COVID policy in Shenzhen, China : a modeling study

Abstract

Background: Between March 2020 and November 2022, China managed sporadic clusters of COVID-19 outbreaks by adhering to the dynamic “zero-COVID” policy. On 11 November and 7 December 2022, two nationwide relaxations of this policy, referred to as the 20 measures and 10 measures, were introduced. These changes suspended regular mass testing and permitted self-isolation of infected individuals at home. Subsequently, between November 2022 and February 2023, large outbreaks of Omicron variants, including BA.5.2 and BF.7, occurred across various regions of mainland China. I investigated the outbreak in Shenzhen and estimated the transmission dynamics and severity of Omicron following the adjustment of public health and social measures (PHSMs) in late 2022.

Methods: An epidemic model was fitted to data from three sources: (i) the daily number of reported cases from 18 to 30 November 2022 during the enforcement of the zero-COVID policy, (ii) the proportion of online poll participants self-reporting as test-positive in December 2022, and (iii) electronic health record data of COVID-19 hospitalizations at HKU-Shenzhen Hospital between 1 December 2022 and 31 January 2023.

Findings: In the base case scenario, the effective reproduction number (Rt) was estimated to peak at 2.74 (95% credible interval (CrI): 2.30, 3.13) on 11 December 2022, following the announcement of the 20 measures and 10 measures on 11 November and 7 December, respectively. The daily incidence of infections peaked at 1.06 million (95% CrI: 0.74, 1.33) on 18 December, representing 6.03% of the total population in Shenzhen. The estimated cumulative infection attack rate increased from less than 1.5% on 7 December to 97.2% (95% CrI: 96.4%, 98.0%) on 7 January 2023. The daily incidence of COVID-19 hospitalizations at HKU-Shenzhen Hospital peaked on 28-29 December 2022. The estimated mean time interval from symptom onset to hospitalization was 3.04 days (95% CrI: 2.81, 3.30) for patients admitted from the fever clinic and 3.17 days (95% CrI: 2.89, 3.48) for direct hospitalizations. The estimated probability of hospitalization at HKU-Shenzhen Hospital for infections aged over 59 years was significantly higher compared to younger age groups, with the probability for infections referred from the fever clinic being at least 5.27 times (95% CrI: 5.19, 5.49) higher, and for direct hospitalizations, at least 8.50 times (95% CrI: 8.45, 8.55) higher. Sensitivity analysis, accounting for the time interval from infection to symptom onset in online polling, yielded results consistent with the base case scenario.

Interpretation: I integrated real-time mobility data, adjustments of PHSMs, and electronic health record data into the estimation of COVID-19 transmission dynamics and severity. These methods synthesized available data in addition to daily reported cases, and are applicable when PHSMs are gradually phased out and case reporting behaviors change.