A pattern shift in SARS-CoV-2 Omicron variant transmission after the city lockdown--observational study based upon daily reported addresses of infected cases
Authors: Yin Lihong , Zhang Guozhou 
Published: September 2022

Background

Varied degrees of lockdown have been imposed for dozens of jurisdictions upon facing the SARS-CoV-2 epidemics during the past two years. Areal lockdown has been demonstrated effective to reduce the morbility and mortality of COVID-19. Even after the strict lockdown the peak of infection will appear around 9-25 days (median 18 days) thereafter. A wave of Omicron variant (BA.2 and BA.2.2) outbreak was seen from March to May 2022, in Shanghai, a megacity in China mainland. Aim To understand the sources of infection cases from outside or inside the isolated locations before and after the strict city lockdown.

Methods

The attributable addresses of SARS-CoV-2 infection were reported daily as well as the infected cases from March 18th, 2022 on through government website, which was publicly accessible. The address data and infected cases were collected until May 29th, 2022. The location (longitude and latitude) of these addresses were retrieved and the pattern of repeatedly reported addresses were analyzed. A tool of simple and meso-scale point-based (location-based) chronological graph was used to visualize and analyze the interactions of these locations.

Results

From March 18thto May 29th2022, 173,350 items representing 35,743 unique addresses and 636,279 infected cases were released. The infection cases peaked 16 days after the city lockdown and were highly clustered in much crowded districts. The proportion of repeatedly reported locations of the previous day increased from around 20% before lockdown to greater than 40% in the plateau and remained at this level for up to one third (20/62) of the lockdown phase. This significantly increased proportion of intra-address infection indicated a pattern shift from inter-addresses to intra-address (D=0.2954, p<0.0001), which might perpetuate to the growth of infection cases. Based upon the day-to-day nearest neighbour transmission assumption the connections between some frequently repeated locations might be complex and heterogeneous.

Interpretation

During the strict areal isolation the intra-address infection may contribute significantly to infected cases of SARS-CoV-2 Omicron variant, the infection might have easily spilled over the boundary of family(with averaged family size of 2.3-3.1 people and family were required stay-at-home compulsively). This significant inter-addresses to intra-address pattern shift necessitated the understanding of intra-location transmission routes and corresponding interventions. Areal isolation and close off with homogeneous assumption inside and outside the isolated areas should be modified and the quantifing of the elevated risk for previously less exposed but much vulnerable sub-population was in pressing need.

Research in context

Evidence before this study It is commonly observable that the infection cases will continue to increase and peaked in several weeks after the first day of imposing areal lockdown. The search syntax of [(SARS-CoV-2 OR COVID-19) AND (transmission OR infection) AND (post-lockdown OR “during lockdown” OR “isolated area*”)] on PubMed hits 1372 records(on 2022-08-10). However, the viral transmission between the isolated locations or within them have not been studied thoroughly, maybe, due to the presumption that it would be stopped eventually by greatly reducing the inter-personal contact.The search syntax of [(SARS-CoV-2 OR COVID-19) AND (transmission OR infection) AND (post-lockdown OR “during lockdown” OR “isolated area*”) AND (strict)] on PubMed hits 57 records. The populations of different ages with heterogeneous risks of exposure to viruses have been revealed.

Added values in this study

The strict city lockdown of Shanghai in facing the Omicron outbreak provided a prototype to understand the sources of infection by using the daily reported address (isolated immediately) involving infected cases. A noticeable pattern shift was revealed in this study with significantly increased proportion of intra-location (small area) self-propagation after the city lockdown.

Implication of all the available evidence

Counter measures should be provided to reduce the risk of transmmision within the small isolated areas; and the elevated risk for the much vulnerable sub-populations within the intra-location (small isolated areas) should be quantified and to trade off between the reduced inter-location risk and increased intra-location risk of infection in policy making.