Geographical and Epidemiological Characteristics of Case Series With COVID-19 Outbreaks Caused by Asymptomatic Carriers From June 2020 to January 2021 in China: A Narrative Research

With the COVID-19 epidemic quickly under control in China within the early months of 2020, importing the SARS-CoV-2 virus to the country now poses great challenges in epidemic control and prevention. Asymptomatic carriers play a critical role in the transmission of the virus and transmission on a large-scale poses enormous concern. We obtained data from new cluster outbreak regions with COVID-19 caused by asymptomatic carriers from June 2020 to January 2021, and reported the epidemiological characteristics, clinical data and the possible routes of viral transmission and infection. These results indicate the importance of regularly screening high-risk populations critical for epidemic control and provide the basis for suppressing the spread of the SARS-CoV-2 virus. transmission of SARS-CoV-2 23, 24 and the superspreading events that play a signicant role in spreading the virus which constitutes the foundation for international public health development strategies. Our results indicate the importance of regular screening of high-risk populations, aggressive contact tracing and quarantine of close contacts of people with asymptomatic infection for epidemic control and the basis for suppressing the spread of the SARS-CoV-2 virus.


Introduction
Since December 2019, a new type of coronavirus pneumonia (coronavirus disease 2019,  was identi ed and developed into a global epidemic. This was later declared a pandemic by the World Health Organization (WHO) on March 11, 2020. By the end of January 2021, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the new coronavirus behind the disease COVID-19, had infected over 100 million people around the world and caused about more than 2 million deaths. Facing the biggest global COVID-19 pandemic of this century, led governments and the scienti c community in various countries to work hard to uncover the public health strategy for effective scienti c solutions [1][2][3][4] .
It is well-known that the typical three transmission routes of infectious respiratory viruses, include the droplet-borne route transmitted by appropriate droplets 5 , suitable fomite route through contacts with contaminated object surface 6,7 , and airborne route through aerosols that can remain suspended in air for longer periods 8,9 . The transmission of SARS-CoV-2 through fomite and droplet-borne routes was considered as the main pathway, but more and more recent studies have revealed the possibility of airborne transmission [10][11][12][13][14][15][16][17][18] , particularly in crowded and inadequately ventilated indoor spaces [19][20][21] . Fennelly et al 22 measured particle size distribution of infectious aerosols and observed that pathogens were more commonly found in small particles (<5μm).
Some studies have identi ed airborne transmission as a likely major pathway for asymptomatic transmission of SARS-CoV-2 23,24 and the superspreading events that play a signi cant role in spreading the virus 25 .
With the epidemic COVID-19 quickly under control in China within the early months of 2020, the Chinese public strategy and related health research have greatly advanced our understanding of sporadic COVID-19 outbreaks and strengthened the preparedness and combat against COVID-19 disease all over the world. We collected and analyzed data on geographical and epidemiological characteristics of case series with COVID-19 outbreaks caused by asymptomatic carriers from June 2020 to January 2021 in China.

Methods
At the beginning of June 1, 2020, we prospectively gathered COVID-19 epidemic data from the Chinese Center for Disease Control and Prevention every day. Once we obtained the new report of con rmed or asymptomatic cases, we tracked this epidemic and collected its epidemiological characteristics from announcements by the local Health Commission, and presented a narrative research for geographical and epidemiological characteristics of case series with COVID-19 outbreaks caused by asymptomatic carriers.
We searched epidemiologic data published on the website of WHO, the China Center for Disease Control and Prevention, National Health Commission, the Health Commission of Beijing and Tianjin city, Jilin, Shandong, and Heilongjiang Province from June 2020 to January 2021. Using the keywords "asymptomatic", "COVID-19", "SARS-CoV-2", "2019-nCoV", and we periodically searched the published medical literature using the PubMed service maintained by the U.S. National Library of Medicine of NIH. Con rmed COVID-19 cases are de ned as persons who tested positive for SARS-CoV-2 and had clinical symptoms. Asymptomatic carriers refer to persons without clinical symptoms who tested positive for SARS-CoV-2.

Results
Geographical and epidemiological characteristics of case series with COVID-19 outbreaks The geographical and occupational distribution of case series in local sporadic outbreak regions with COVID-19 is illustrated in Figure 1. Data of epidemiological characteristics in new sporadic outbreak regions is presented in Table 1. Data from 5 cities showed that asymptomatic SARS-CoV-2 carriers induced local sporadic outbreaks (Table 1)  All persons associated with Cases 3 and 4, including those who were in the same Ward Area and had close contact with SARS-CoV-2-positive patients, were tested for SARS-CoV-2 nucleic acids. Contact tracing results indicated that 9 cases were infected with SARSCoV-2, including 4 con rmed cases (Case 3-6) and 5 asymptomatic cases (Case 7-11) on October 12.
In addition, the Chinese Center for Disease Control and Prevention sequenced the entire genomes of 11 samples from this cluster.
Viral genomes were identical in 11 cases, indicating that SARS-CoV-2 came from the same point of origin.
By October 16, Qingdao city had reported 13 con rmed cases since the asymptomatic cases were rst detected on September 24. Building. Buildings. The results showed that these viruses belonged to the European family branch 1 of the L genotype, indicating that SARS-CoV-2 among these three buildings came from the same point of origin.
Patient B1, B2 and A0 were in the same train carriage on January 5. Both B1 and B2 tested positive for SARS-CoV-2 on January 11, and remained asymptomatic, indicating that B1 and B2 were previously infected with SARS-CoV-2 and were asymptomatic carriers.
Based on epidemiological surveillance, A0 tested positive for SARS-CoV-2 nucleic acid on January 12 and symptoms of COVID-19 were noted on January 16, suggesting that A0 was the rst con rmed case in this cluster, raising Jilin's health commission concerns.
Jilin, therefore, launched a mass COVID-19 testing programme in some areas including Market-training Hall 1 and 2 on January 11.
From 11 to 18 January, 34 new COVID-19 cases and 80 asymptomatic carriers had been identi ed in the Jilin Province. Among these cases, 102 were related to case-patient A0. By February 9, Jilin Province had reported 3 deaths among 573 con rmed cases since the asymptomatic cases were rst detected in the Changchun city on 11 January.  Timeline of exposure and connections between SARS-CoV-2 cases among persons in Qingdao, China. Cases 1 and 2 were quarantined in hospital when they tested positive for SARS-CoV-2 nucleic acid on September 24, 2020, and were determined to have asymptomatic infection. By October 16, 2020, Qingdao city had reported 13 con rmed cases since the asymptomatic cases were rst detected on September 24, 2020.  Timeline of exposure and connections between SARS-CoV-2 cases among persons in Beijing, China. On December 10, 2020, A0 (Case 1), an Indonesian national, returned to Shunyi District, Beijing after 14 days of isolation in Fujian Province and had a negative nucleic acid test. He was diagnosed with an asymptomatic infection on December 28. Case 1 was the source of a clustered epidemic and Case 3 is a key case in the transmission chain. Note: The data is based on public reports and might be incomplete.  Timeline of exposure and connections between SARS-CoV-2 cases among persons in Jilin Province, China. A0 came to Changchun city from Heilongjiang Province on January 5, and A0, B1 and B2 were in the same train carriage. Both B1 and B2 had positive SARS-CoV-2 nucleic acid tests on January 11 and remained asymptomatic, indicating that B1 and B2 were previously infected with SARS-CoV-2 and were likely asymptomatic carriers. Based on epidemiological surveillance, A0 tested positive for SARS-CoV-2 nucleic acid on January 12 and noted symptoms of COVID-19 on January 16, suggesting that A0 was the rst con rmed case in this cluster.