The pre-investigation (between January 31 and February 11)
Of the 2,004 individuals tested, COVID-19 was detected in three patients, including a father and his daughter living in Yantian district, and one patient living in Nanshan district. All patients had been to Hubei and had different initial symptoms. The father and his daughter had dry cough for 2 weeks. The third patient had a single temperature reading above 37.3°C, but presented with a normal temperature and was asymptomatic during the other assessment times as of the date to get NAT on 8 February. The three patients never contacted with any suspicious case during the period in Hubei. The infection rate of home-quarantined individuals was 1.5‰ (95% CI: 0.31‰–4.37‰) based on the results of the pre-investigation. The period of pre-investigation was conducted (between January 31 and February 11) during the first incubation after Wuhan was placed on lockdown. The home-quarantined individuals with epidemic histories (came from Hubei and any other affected regions), but without symptoms remained as the high risk population. The pre-investigation implemented the policy of “early detection, early report, early diagnosis, early quarantine and early treatment” for the high risk population to control and prevent the spread of COVID-19.
The formal investigation – Part 1 (between February 12 and 24)
The formal investigation was divided into two parts by the date of Guangdong province turned down to level-2 public health emergency response on 24 February. Part 1 was between February 12 and 24 (the second incubation after Wuhan closure) and part 2 was between February 25 and March 5 (the third incubation after Wuhan closure). Part 1 expanded the sample size to 44,021 based on the results of the pre-investigation. Figure 3 is the sampling and detection distribution among the 11 districts (including the Shenshan special cooperation zone) in Shenzhen by ARCGIS10.2. There were five positive cases detected from the sample of 44,021 individuals. The 4 of the 5 positive cases including two patients living in Longgang district, two patients living in Futian district. The fifth positive case in Nanshan is asymptomatic and hasn’t been onset so far, so we excluded this case considering the sensitivity of NAT. Thus, the infection rate of home quarantined individuals was 0.09‰ (95% CI: 0.03‰–0.22‰) based on the results of formal investigation part 1. The median age of the respondents was 34 years (range, 0 to 95 years), and 57.1% were male (Figure 4). The proportion of children under 15-years-old was 11.83%, while the proportion of individuals aged over 60-years-old was 3.28%. Among the overall population, 8.4% had at least one coexisting illness (e.g., cardiovascular and cerebrovascular disease, hypertension, chronic obstructive pulmonary disease diabetes, liver disease, blood disease, or malignant tumors).
A total of 40% individuals had an epidemic history (came from Hubei and any other affected regions), and the remaining 60% were their family members. Among the epidemic histories, 93.21% had been to Hubei province, including 21.68% that visited Huanggang, 16.85% that visited Jingzhou, and 11.41% that visited Xianning. The dates of return to Shenzhen were mainly concentrated the period between January 23 and February 26 (Figure 5). There were two peaks in figure 5, the first of which concentrated on January 25, which indicated that many people left Hubei for Shenzhen after the Wuhan closure on January 23, and those individuals are the high risk group and were required to take immediate quarantine observation. The second peak concentrated on February 24, which Guangdong turn down to level-2 public health emergency response. This showed that prevention and control strategies were effective to control the epidemic in the early stage. Meanwhile, Shenzhen started to work on February 9. Many people returned to Shenzhen to begin working after COVID-19 was effectively controlled.
Considering the epidemic histories, 97.32% of individuals stayed in Hubei less than 100 days, and the mean duration was 0.37 days. A total of 2.18% lived in Hubei (more than 1 year), and 6.31% stayed in Hubei less than 24 hours as they were passing through or in transit. There were two peaks in Figure 6, the first one concentrated on 4 days, and the second one concentrated on 34 days. The most common mode of transportation was driving a private vehicle (86.58%). The advantages of private vehicles include reducing the number of close contacts and the risk of transmission. The second most common mode of transportation included riding high-speed trains/bullet trains/other types of trains (10.32%). The third one was plane (1.99%), and the fourth one was taking long-distance buses (0.21%). Only 0.6% of individuals had been exposed to individuals with fever and respiratory symptoms, 0.37% were exposed to confirmed cases, mild cases or asymptomatic cases, and 0.61% were exposed to suspected cases, confirmed cases or fever patients in their family, friends and colleagues.
In order to protect vulnerable populations, prevent and control the epidemic, the government advocated to everyone to take home quarantine, wear masks outdoors, wash hands frequently, leave homes less frequently, cancel parties, dinners, and other visits, and cover the mouth and nose with paper or towels when sneezing or coughing. The government also strengthened public health surveillance and hygiene knowledge to improve public health awareness and behavior. The results of our research indicated that the rate of health self-protection awareness among home-quarantined individuals with epidemic histories (came from Hubei and any other affected regions) was more than 80%, and females have a higher rate than males (Table 1).
The formal investigation - Part 2 (between February 25 and March 5)
No positive cases were detected in the 12,991 individuals tested during part 2 (the third incubation). So the infection rate of home quarantined individuals was 0. Additionally, the number of new confirmed cases per day had been 0 for eight consecutive days from February 22 to 29 in Shenzhen. Thus, the prevention and control strategies as well as the national cooperation mechanisms were effective at controlling COVID-19 during the early stage.
Seven confirmed cases
The total sample size for this investigation (including the pre-investigation, and the formal investigation) was 59,016 individuals. Of which seven cases were confirmed positive. Therefore, the infection rate of home-quarantined individuals was 0.12‰ (95% CI: 0.05‰-0.24‰).The seven patients age from 13 to 66 and had been to Hubei(Figure 7), which included a father (Case 1) and his daughter (Case 2). There were four patients with different initial symptoms, including Case 3 who had a temperature higher than 37.3°C once, but exhibited a normal temperature and was asymptomatic during the other assessments as of the date to get NAT on 8 February (Table 2). Case 1 and Case 2 had dry coughs for 2 weeks. Case 7 had a runny nose for 4 days. Six patients did not exhibit fever, and three patients were asymptomatic. These seven patients received medical treatment in the Third People’s Hospital of Shenzhen (designated hospitals) immediately following positive NAT results. They were mostly detected between February 8 and 18, which was during the second incubation period after Wuhan closure. So the home-quarantined individuals with epidemic histories (came from Hubei and any other affected regions), but without symptoms were still at high risk of infection and transmission. Shenzhen started to work on February 9 which means businesses began to reopen, and more people returned to Shenzhen during this time. We still needs to be vigilant to prevent and control COVID-19.