Epidemiological characteristics and transmission dynamics of the Omicron outbreak under lockdown policies in Shanghai

In a recent study posted to the medRxiv* preprint server, researchers assessed the epidemiological characteristics of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant outbreak in Shanghai, China.

Study: Epidemiological characteristics and transmission dynamics of the outbreak caused by the SARS-CoV-2 Omicron variant in Shanghai, China: a descriptive study. Image Credit: Graeme Kennedy/Shutterstock
Study: Epidemiological characteristics and transmission dynamics of the outbreak caused by the SARS-CoV-2 Omicron variant in Shanghai, China: a descriptive study. Image Credit: Graeme Kennedy/Shutterstock

Strict containment measures enabled China to reduce the impact of the first wave of coronavirus disease 2019 (COVID-19) infections. However, early March 2022 saw a widespread outbreak of the SARS-CoV-2 Omicron variant spread across Shanghai in China.

About the study

In the present study, researchers described the epidemiological characteristics and transmission dynamics related to the spread of the SARS-CoV-2 Omicron variant.

The team summarized the non-pharmacological interventions (NPIs) that were implemented in Shanghai prior to and during the Omicron outbreak. Before the initial surge in Omicron infections, Shanghai exhibited a baseline level of NPIs against potential outbreaks, such as stringent border control, surveillance based on symptoms, case isolation, contact tracing, occupation-based screening, and travel restrictions.

The team also collected the total number of local as well as imported COVID-19 infections based on the symptomatic status reported in Shanghai from December 2019 recorded by the Shanghai municipal health commission. Lists of SARS-CoV-2 infections were retrieved from publicly available data sources, followed by integration with supplementary information. 

The non-high-risk areas were further divided into two categories according to their spatial proximity to high-risk regions: (1) moderate-risk areas: areas that were adjacent to high-risk regions but were not high risk themselves, and (2) low-risk areas: areas that were neither adjacent to high-risk areas nor were classified as high-risk themselves.

Results

The study results showed that the number of imported COVID-19 infections reached the maximum of 81 cases per day on 24 February 2021, which was over 19 times more than the daily average noted between 2020 and 2021. A total of 1,112 imported cases were observed between 15 February and 15 March 2021. The team found that among the inbound travelers, the test positivity rate was the highest in February 2022, with a maximum of 96.3 infections observed per 1000 travelers.

The sudden increase in the number of infected travelers resulted in a breach of the entry control program. This breach was most probably responsible for the high transmission on Omicron during early March. A total of 14 COVID-19-positive persons were detected on 1 March 2021 among attendees of a dance event. This eventually led to 6,26,000 COVID-19 infections by 31 May 2022 due to the high transmissibility of Omicron and the delayed discovery of the infection outbreak.

Almost 96% of the total infections were detected among quarantined persons. The proportion of infected individuals identified due to community screening decreased to 13.6% in phase 2, which further declined to 2.7% in phase 3. On the other hand, the number of SARS-CoV-2 infections exhibiting symptoms ranged from 15.5% in phase 1 to 9.8% in phase 3.

Approximately 99.5% of the sub-districts throughout Shanghai reported SARS-CoV-2 infections. However, the team observed a highly heterogeneous distribution of infections, with 40% of the city area reporting 80% of all the infections, including the Pudong New area reporting 35.5% of the total infections. Moreover, the total incidence of reported COVID-19 infections was 0.02 infections per 1,000 persons during phase 1, which increased to 2.0 per 1000 in phase 2, and 21.9 per 1000 in phase 3.     

The speed of the spread of COVID-19 consistently slowed down from an average of 544 meters per day reported in the first week of the outbreak from 27 February to 5 March 2021 to 325 meters per day from 25 March to 31 March 2021. 

Between 16 March and 29 March 2021, the total number of COVID-19 infections reported per 1000 individuals was 2.2 in high-risk areas, 1.51 in moderate-risk areas, and 0.53 in low-risk areas. The team noted that implementing intervention strategies before the citywide lockdown was enforced did not sufficiently curb the increase of infections. Altogether, the growth rate of the COVID-19 transmission in phase 2 was 0.26 per day, while the number of infections doubled every 3.21 days.    

Conclusion

Overall, the study findings highlighted the risk of COVID-19 outbreaks in China, while the implementation of strict NPIs along with a citywide lockdown could effectively control the SARS-CoV-2 Omicron outbreak. The researchers believe that studying the impact of each performed intervention could help understand key public health priorities better.

*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.

Journal reference:
Bhavana Kunkalikar

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.

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