The Asymptomatic Infection of COVID-19 Risen in Imported Population in Shenzhen, China

As the with over The present study aimed to carry out an epidemiological analysis of conrmed and asymptomatic infections in Shenzhen City to provide scientic reference for the prevention and control of COVID-19. The epidemiological information of the 462 conrmed cases and 45 asymptomatic infections


Page 4/19
Background In early December 2019, Wuhan City of Hubei Province reported a novel coronavirus disease (Coronavirus disease 2019,  [1][2][3], caused by the pathogen identi ed as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [1][2][3]. Since the outbreak and spread of COVID-19, patients harboring the virus were the most prominent sources of infection [5], followed by asymptomatic infections (especially those in the latency) [5]. Thus, delayed identi cation and isolation resulted in the continuous accumulation of infection sources in communities, making it di cult to prevent and control the epidemic [6]. Furthermore, patients in the latency and convalescence might continue to spread the virus [6], making them sources of infection. SARS-CoV-2 is transmitted mainly through respiratory droplets and contact as well as by fecal-oral transmission and mother-to-fetus transmission [8], albeit rarely. Aerosol transmission of the virus under speci c conditions is yet to be studied [8]. The novel coronavirus is an emerging pathogen to which humans are not yet immune and are generally susceptible. The novel coronavirus can infect people of all ages, as long as the conditions for transmission are optimal [11]. The close contacts of COVID-19 patients and asymptomatic infections are at a high risk of being infected by novel coronavirus [12].
The clinical performance of COVID-19 varies greatly [13,14]. Mild patients, account for the vast majority (about 80.9%) of the cases [15,16], have no symptoms of pneumonia, but fever, fatigue, and dry cough, with a stuffy nose, running nose, sore throat, and diarrhea are noted in a few cases, these present gentle progression and good prognosis. However, a few developed into severe and critical cases (about 19.1%) with continuous high fever, and then into pneumonia cases with a severe acute respiratory syndrome, renal failure, and hypoxemia that were di cult to treat and cure in the absence of speci c drugs, resulting in high mortality in gerontal patients with underlying diseases (such as cardiovascular disease and diabetes) [15,16].
The high virulence and contagious nature, SARS-CoV-2 made COVID-19 spread rapidly worldwide [17]. As of July 24, 2020, the global reported number of COVID-19 cases had reached more than 15.4 millions with over 640,000 deaths. To date, COVID-19 has been labeled as a disease of "global pandemic". The rapid spread of COVID-19 posed great challenges to the medical, healthcare, and social emergency management of the countries and regions across the world. The rapid spread threatens the life safety and makes a signi cant impact on the economy and society such that no disease in the past 100 years could surpass it, making it a great concern for all state governments and all sectors of society.

Research design
A retrospective study was performed to observe and analyze COVID-19 con rmed cases and asymptomatic infections in Shenzhen City, Southern China.

Data source
The epidemiological investigation reports of all COVID-19 con rmed cases and asymptomatic infections in Shenzhen City from January 1 to June 30, 2020, were collected and investigated, and the demographic information, date of onset, date of report, date of entry, country or region of residence or travel before entry, the region of the report, track of entry, and course of identi cation of all research subjects were collected.

De nitions of cases
For the de nition of COVID-19 con rmed cases and asymptomatic infections, please refer to the Guidelines on Prevention and Control of COVID-19 (6th Revision). Domestic imported cases referred to those with a travel and residential history in Hubei Province (including Wuhan City) or in other provinces in China 14 days before the onset of symptoms, as well as their contacts. Cases without a travel and residential history in cities other than Shenzhen City at 14 days before onset were de ned as domestic local cases. In addition, cases imported from abroad referred to the COVID-19 cases residing in Shenzhen, with a travel and residential history 14 days before onset and subject to no de ned domestic source of infection according to epidemiological investigations. The local cases were associated with those imported from abroad or had a history of contact with such travelers; however, no de ned domestic source of infection was detected by the epidemiological investigations.

Statistical analysis
Excel was used to build a case library of con rmed cases and asymptomatic infections, and SPSS 26.0 software (IBM, US) was used to conduct a statistical analysis. Information of the cases was input and veri ed in both software. Odds ratio (OR) and 95% con dence intervals (CIs) of categorical variables were calculated using two tailed Chi-square or Fisher's exact test. If, in terms of overall comparison, independent sample rates of multiple groups were statistically signi cant, the samples were compared with each other using chi-square test of independence, in which the test level α = 0.05 is corrected with Bonferroni method to obtain α´=0.05/k (k indicates the number of comparisons).

Con rmed cases in Shenzhen City
Shenzhen City reported the rst COVID-19 case on January 19, 2020. The rst COVID-19 case imported from abroad on March 1, 2020 and the last case was on April 30, 2020, with no new cases from May 1 to June 30, 2020 (Fig. 1). From January 19 to April 30, 2020, Shenzhen City reported a total of 462 COVID-19 con rmed cases, including 423 domestic events (91.56%) and 39 imported from abroad (8.44%).
Among domestic cases, the majority were imported from Hubei Province (n = 312, 67.53%), followed by local incidents (n = 69, 14.94%) and those imported from provinces other than Hubei Province (n = 42, 8.87%) ( Table 1, Additional le 1). As shown in Table 1, from January 19 to February 12, 2020 and from February 13 to April 30, 2020, the development of COVID-19 in Shenzhen City changed signi cantly. From January 19 to February 12, 2020, COVID-19 cases in Shenzhen City were mainly imported from Hubei Province (75.70%), followed by local cases and those imported from provinces other than Hubei Province, while none were imported from abroad. However, from February 13 to April 20, 2020, cases imported from abroad accounted for the majority (54.93%) of the reported cases in Shenzhen City, followed by those imported from Hubei Province (22.54%) and those from other provinces (12.68%), while local cases accounted for only 9.68% of the total number of occurrences. This further indicated that the intensive community prevention, control measures, other comprehensive blocking strategies, and measures adopted by Shenzhen achieved signi cant results.

Asymptomatic infections and con rmed cases
Shenzhen City reported a total of 45 asymptomatic infections from January 19 to April 30, 2020, including 31 local ones (68.89%) and 14 imported from abroad (31.11%), while none were imported from Hubei or other provinces (    Similarly, the proportion of con rmed cases in Shenzhen from January to April 2020, declined every month, while that of asymptomatic infections showed a continuously rising trend (Cochran-Armitage test for trend, Z = 11.7896, P < 0.0001). The change in the proportion of asymptomatic infections indicated a statistical signi cance in the four months (χ 2 = 121.8704, P < 0.0001). As there were differences as a whole, 6 (k = n (n-1)/2 = 6, n indicates the number of groups) pair comparisons were conducted, and the P-value was 0.008333 (the corrected test level α'=α/6 = 0.05/6 = 0.008333 by Bonferroni). Thus, it could be deduced that asymptomatic infections in April accounted for a higher proportion than those in March (χ 2 = 11.9911, P = 0.0005, OR = 5.2308, 95% CI: 1.9917-13.7374), those in February (P < 0.0001, OR = 9.4441, 95% CI: 3.8436-23.2053), and those in January (P < 0.0001), while those in March accounted for a higher proportion than those in February (P < 0.0001, OR = 9.4441, 95% CI: 3.8436-23.2053).
Therefore, the ratio of all asymptomatic infections to con rmed cases in Shenzhen was 1:10.27 (45/462), the same of asymptomatic domestic infections to con rmed cases was 1:13.61 (31/423), and that of asymptomatic infections imported from abroad to con rmed cases was 1:2.79 (14/39). The proportion of asymptomatic infections in cases imported from abroad was higher than that of the same in domestic cases (χ 2 = 22.5121, P < 0.0001, OR = 4.8983, 95% CI: 2.4052-9.9756).

Con rmed cases and asymptomatic infections in Shenzhen imported from abroad
Shenzhen City reported the rst con rmed case imported from abroad on March 1, 2020, and 39 con rmed cases and 14 asymptomatic infections imported from abroad on April 30, 2020 ( Table 4). The UK, the USA, and France were the primary source nations of con rmed and asymptomatic cases. The proportion of asymptomatic infections did not differ signi cantly among different countries of import (χ 2 = 7.7202, P = 0.6561).

Discussion
COVID-19 is an emerging infectious disease to which humans are generally susceptible as effective vaccines, speci c therapeutic drugs, and methods of treatment are yet lacking [18,19]. The infection spreads easily from person to person with convenient transmitting media. Therefore, taking strong control measures against the sources of infection, cutting off the routes of transmission, enhancing social distancing, and improving the protection capability of all people are optimal strategies to combat the infection [20]. WHO has termed this approach as "Non-pharmacological Intervention," or the "Blocking Strategy" [21,22].
The present study performed a descriptive and exploratory analysis of COVID-19 con rmed cases and asymptomatic infections in Shenzhen City, and demonstrated that the spread of infection in the city was mainly via through two stages, the early stage (from January 9 to February 12, 2020) in which the cases were mainly those imported from Hubei Province (including Wuhan City) and the late stage (from February 13 to April 30, 2020) in which the cases were mainly those imported from abroad [8], while local cases in Shenzhen were at a low level of incidence and prevalence [8]. As of January 14, 2020, when the rst suspected case (imported from Wuhan City) was reported, con rmed cases imported from Hubei Province accounted for 75.70% of all the con rmed cases in the early stage (296/391), while the same accounted for only 22.54% in the late stage, indicating that the control measures of the Wuhan city (from January 22, 2020 onwards) reduced the risk of an increase in the number of cases in other cities [21], thereby reducing the pressure of prevention and control of infection in other cities [21].
The number of con rmed cases in Shenzhen imported from other provinces in the late stage (12.68%) increased over those in the early stage (8.44%), indicating that targeted prevention and control measures should be improved and enhanced among the population that migrated to Shenzhen from other provinces at the early stage (from February 14, 2020). The ceased local outbreak of COVID-19 in Shenzhen indicated that Shenzhen municipal government implemented intensive joint prevention and control measures across the community to facilitate the legal tracing of the migration and ow tracks of key persons using advanced technology. These measures and the obtained data enable precise prevention and control, which made up for the defective prevention and control in the early stage and improved the faulty tolerance rate of the containment mechanism. In the late stage, the development of COVID-19 in Shenzhen changed fundamentally with the global pandemic as the cases imported from abroad accounted for the majority of the con rmed cases in the city, suggesting the need for timely adjustment of prevention and control measures (for instance, shutting down international ights).
Moreover, precise and scienti c prevention and control are effective strategies for coping with the dynamic development of COVID-19 internationally. The local cases in Shenzhen accounted for 15.86% in the early stage and 9.86% in the late stage, which indicated effective, comprehensive blocking strategies and measures such as community prevention and control that were continuously upgraded according to the dynamic development of COVID-19: timely severance of the routes of transmission, stoppage of the local outbreak, and reducing the prevalence among the population. These effective tools are the key measures that timely curbed and effectively quelled the development of COVID-19 in Shenzhen.
COVID-19 asymptomatic infections refer to those positive in nucleic acid testing with nasopharyngeal swabs but developing no novel coronavirus-related symptoms, such as fever, upper respiratory symptoms (cough and sore throat), fatigue, and gastrointestinal symptoms (for example, diarrhea). They are not referred to as "patients" due to the lack of symptoms [6], but they spread the virus easily while keeping frequent contact with others due to their unawareness of the infection [6,23]. The main reasons for asymptomatic infections might be that the infections are yet to develop clinical manifestations in the early stage of disease treatment or the individuals are strongly immune and develop no clinical manifestations after infection [24]. Such individuals recover after a period or continue to be a carrier. Due to the absence of clinical symptoms, such as cough or sneeze, asymptomatic infections are less prone to discharge their pathogens than con rmed cases, and hence, less prone to spread the virus theoretically.
Asymptomatic infections are di cult to be identi ed, and lack awareness of protection as they do not know about their infection; so their close contacts might result in the same or higher actual infectivity than con rmed cases [25]. Some studies have found that about 11% of asymptomatic infections turned out to be con rmed cases. According to the studies, in key epidemic areas of COVID-19 (for example, Wuhan City), asymptomatic infections amounted to 30,300/100,000 individuals, and in cities where imported cases accounted for the majority, the ratio of asymptomatic infections to con rmed cases was 1:2.17, the infection rate of COVID-19 in close contacts of asymptomatic infections was 4.11%, and the infection rate of close contacts of COVID-19 con rmed cases from asymptomatic infections showed no statistical signi cance. In the current study, the proportion of asymptomatic infections in Shenzhen increased continuously over time, indicating that our epidemic prevention measures did avoid the import of cases and that our monitoring system was strongly sensitive and capable of identifying early cases and infections yet developing symptoms.
The asymptomatic infections are the most insidious and leaky source of infection [26]; thus, the capability to identify these should be improved for such a virulent pathogen that enjoys routes of easy transmission. With less virulence discharge and no clinical symptoms, asymptomatic infections cannot be identi ed spontaneously, thereby rendering it necessary to improve the detection capability (with quantitative uorescence PCR method, trace sequencing method, serum antibody testing method, and multi-method joint diagnosis method) to recognize the asymptomatic infections, which is crucial for controlling the outbreak of COVID-19.
Asymptomatic infections may be identi ed as follows: Among close contacts of COVID-19 cases; during active detection in the investigation of cluster outbreaks; during active detection of the exposed populations when tracing the infection sources of COVID-19 cases; during active detection of COVID-19 cases and persons with travel or residential history in regions of constant transmission.
Another study demonstrated that in Shenzhen City, the proportion of asymptomatic infections from January 19 to April 30, 2020 increased over time (day-by-day), which might be because of the following reasons: 1. We focused mainly on treating symptomatic infections (especially severe and critical ones) in the early stage of the outbreak and paid great attention to the prevention and treatment of asymptomatic infections only when the progress was consolidated in domestic epidemic prevention and control; 2. The continuous improvement of the detection capability and level and testing intensity enhanced the active identi cation of asymptomatic infections; 3. The pathogen might become less virulent and more insidious after infecting people. Asymptomatic infections as a more insidious source of infection accelerated the spread of COVID-19. Therefore, multiple nucleic acid testing stations were set up based on the active identi cation and screening, allowing those should be tested and those willing to be tested both to be tested. Moreover, the measures to avoid imported cases, such as carrying out pre-examination at fever clinics and in inpatients, were taken to identify as many asymptomatic infections as possible and prevent the local outbreak of COVID-19.
The number of asymptomatic infections in Shenzhen in April showed a signi cant increase over that in March, according to our study, although the proportion of the entrants to the city did not increase over time. If international ights were gradually allowed to land in Shenzhen, the proportion of asymptomatic infections in entrants to the city might increase. To this end, it is critical to take strict prevention and control measures, strictly isolate and test the entrants to the city, screen out asymptomatic infections, implement strict medical quarantine and observation measures, and perform active and early treatment, so as to avoid further spread of COVID-19.
Con rmed cases and asymptomatic infections entering Shenzhen from abroad originated mainly from 11 countries, and the ratio of asymptomatic infections to con rmed cases (between 0.00% and 50.00%) had no statistical signi cance. This nding suggested that all individuals imported to Shenzhen from different countries regardless of the severity of COVID-19 in such countries have to be treated indistinguishably and conscientiously, without ignoring the entrants from countries with less severe development of COVID-19. In addition, stringent medical quarantine measures need to be exercised over all entrants to Shenzhen from abroad, which might extend the duration of medical observation of entrants engaging in people-intensive service industries (such as restaurant workers and market and supermarket workers) (quarantine of 21 days, i.e., allow them to work only after 14 days centralized medical observation plus 7 days of home observation) and increase the frequency of nucleic acid testing, in order to identify as many asymptomatic infections as possible.
At present, with the global pandemic of COVID-19 [27], which has no de ned duration, there is always a risk of imported sources of infection and China will always face the risk of imported cases as long as the pandemic is not under effective control, and as insidious sources of transmission, asymptomatic infections may also be one of the major risks in the future development of COVID-19 [28]. Thus, normalized prevention and control measures are required for the prevention and control of COVID-19 at such a time when there are few local cases and a few imported cases and asymptomatic infections [27].
The professional institutions are required to perform effective disease detection and monitoring, build a sensitive, e cient, long-lasting, and well-run monitoring system, identify the epidemic as early as possible and take timely and scienti c emergency disposal measures, and continue effective publicity and education on the prevention and control of COVID-19 while maintaining social distancing and wearing masks in crowded places to avoid the second outbreak of COVID-19 due to slack [29].

Limitation Of This Study
There were several shortcomings in this study needed to be addressed. It was a cross-sectional study and cannot be obtained causal conclusion. At the same time, the cases imported from abroad and with asymptomatic infection was not big, and the results may be unstabitily. Hence, the large-sample study will be needed is future, and the asymptomatic and pre-symptomatic COVID-19 merits additional research.

Conclusions
Asymptomatic infectors accounted for an increasing proportion among cases imported from abroad, indicating that improving the detection capability to identify asymptomatic infections as early as possible, it will be of signi cance for the control outbreak of COVID-19. Furthermore, the detection measures and strategies at airports, seaports, and land ports should be adjusted constantly according to the new changes in COVID-19 development in foreign countries, so as to timely identify the sources of infection and ensure that imported cases do not cause local infections.  The time trend of COVID-19 con rmed cases and asymptomatic infection in Shenzhen City, China