Epidemic dynamics of the SARS-CoV-2 Omicron variant in China during the transition from 'zero-COVID' to little-to-no intervention efforts

By Pooja Toshniwal PahariaReviewed by Danielle Ellis, B.Sc.Feb 17 2023

In a recent study posted to the medRxiv* preprint server, researchers quantified the magnitude and rate of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant outbreak in China following the abrupt exit from the ‘zero-coronavirus disease (COVID)’ guidelines.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

In December 2022, China transitioned from a stringent ‘zero-COVID’ policy (comprising frequent SARS-CoV-2 testing, extensive city-wide lockdowns, travel limitations, and mandatory quarantine of infected individuals and their contacts in central facilities) to sudden abandonment of nearly all interventions. The abrupt relaxation raised public health concerns over the unchecked transmission of Omicron among a large proportion of individuals with low levels of pre-existing immune protection.

A few weeks following exit from zero-COVID, increased incidence of COVID-2019 (COVID-19)-associated hospitalizations were reported. Further, at the end of December 2022, 40.0% to 50.0% of flight passengers traveling from China to Italy were diagnosed with COVID-19. The findings indicate widespread SARS-CoV-2 presence in China by the end of December 2022, contrasting with official statistics. However, data on the Omicron epidemic dynamics in China are limited.

About the study

In the present study, researchers modeled Omicron infection dynamics in China between November and December 2022, during the transitional period from zero COVID to negligible interventions.

The COVID-19 scenario during the Omicron wave in China was modeled by integrating the official statistics before 11 November and data from the national web-based survey conducted by the Chinese Ministry of Human Resources and Social Security on 26 December. Official data were retrieved from the Sina Pandemic Map dataset, comprising data from NHC and the Johns Hopkins COVID-19 dashboard.

The survey questionnaire obtained data on the status of SARS-CoV-2 infection, prior COVID-19 history, and the duration of their ongoing COVID-19-associated symptoms from 47,897 Chinese residents. The model estimates were validated using the Chinese Center for Disease Control and Prevention (CDC) and the Sichuan province Center for Disease Control and Prevention Health datasets.

The Sichuan web-based survey of Sichuan province residents, conducted on 24 December and 25 December, comprised 158,506 individuals to obtain data on the status of SARS-CoV-2 infection based on antigen testing or polymerase chain reaction (PCR) and symptom duration. Data on the SARS-CoV-2-positive test date and symptom onset date were also obtained.

The rates of growth in COVID-19 cases during three different periods were estimated by fitting the model with regression modeling data and the official COVID-19 statistics. The three periods were (i) the zero COVID period between late October and 11 November; (ii) the 20 Measures period between 12 November and 7 December; and (iii) the 10 Measures period from 8 December onward.

The susceptible-exposed-infected-recovered (SEIR) model was expanded to encompass daily COVID-19 cases from widespread official testing and symptom-based survey responses. Individuals were modeled under the following categories: susceptible individuals (S), exposed individuals (E-1 and -2), infectious and pre-symptomatic individuals (IP), infectious and symptomatic individuals (IS), symptomatic but not infectious individuals (RS-1, -2, and -3), recovered and asymptomatic individuals (RS), individuals asymptomatic throughout SARS-CoV-2 infection (IA-1 and -2) and recovered individuals who never experienced symptoms (RA).

Results

Under zero COVID and 20 Measures policies, the Omicron outbreak in China grew exponentially at rates of 0.150 per day and 0.170 per day, respectively. After exiting from the zero-COVID policy, the rate of Omicron transmission was 0.40 daily, with a 1.60-day doubling duration, during the initial and middle period of December, before peak incidence around 23 December.

Omicron spread more rapidly than the ancestral SARS-CoV-2 Wuhan strain (previously reported as 0.30 daily, with a 2.40-day doubling duration and a reproduction number of 5.7). The estimates indicated that nearly 97% (1.30 billion individuals) were SARS-CoV-2-positive in December. In particular, 88% of the population of China, i.e., greater than 1.0 billion individuals, were infected with SARS-CoV-2 between 15 December and 31 December 2022.

The findings indicated that interventions should have been relaxed gradually, rather than abruptly, in conjunction with pharmaceutical measures to reduce the COVID-19 burden, lower COVID-19-associated hospitalizations and deaths, ensure adequate care to all patients, and prevent a COVID-19 overshoot.

The model findings were consistent with those of the Chinese CDC and Sichuan survey datasets. Thus, the estimated growth rate (0.4 per day) and the reproduction number (3.1) estimates might approximate Omicron BA.5 and BF.7 transmissibility among individuals residing in densely populated regions with low levels of pre-existing immunity Quantifying Omicron dynamics in the pre-and post-zero-COVID periods indicated that the interventions lowered Omicron transmission by 56.0%.

Conclusion

Overall, the study findings highlighted the Omicron infection dynamics in China before and after the abrupt exit from the zero COVID policy. The magnitude and timing of future waves would depend on antibody waning and the transmissibility and immune-evasiveness of novel SARS-CoV-2 variants. Modeling analyses to evaluate the effects of interventions and continued SARS-CoV-2 surveillance efforts are critical in reducing COVID-19-associated morbidity and mortality.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.