The effectiveness of active surveillance measures for COVID‐19 cases in Pudong New Area Shanghai, China, 2020

Abstract The aim of this study was to thoroughly document the effects of multiple intervention and control methods to mitigate the ongoing coronavirus disease 2019 (COVID‐19) outbreak in Pudong New Area, Shanghai. After identification of the first confirmed case of COVID‐19 in Pudong on January 21, 2020, the local Center for Disease Control and Prevention (CDC) launched a case investigation involving isolation, close‐contact (CC) tracing and quarantine of persons with a potential exposure risk to prevent and control transmission. Epidemiological features of cases detected by three different strategies were compared to assess the impact of these active surveillance measures. As of February 16, 2020, a total of 108 confirmed COVID‐19 cases had been identified in Pudong, Shanghai. Forty‐five (41.67%) cases were identified through active surveillance measures, with 22 (20.37%) identified by CC tracing and 23 (21.30%) by quarantine of potential exposure populations (PEPs). The average interval from illness onset to the first medical visit was 1 day. Cases identified by CC tracing and PEPs were quarantined for 0.5 and 1 day before illness onset, respectively. The time intervals from illness onset to the first medical visit and isolation among actively screened cases were 2 days (p = .02) and 3 days (p = .00) shorter, respectively, than those among self‐admission cases. Our study highlights the importance of active surveillance for potential COVID‐19 cases, as demonstrated by shortened time intervals from illness onset to both the first medical visit and isolation. These measures contributed to the effective control of the COVID‐19 outbreak in Pudong, Shanghai.

already arrived in the Pudong New Area. Moreover, any new arrivals from Hubei Province were screened at every city entrance, including airports, train stations, and so on. All the information of the identified PEPs at city entrances was immediately sent by a big data network to the community in which they lived. PEPs were quarantined under compulsory medical observation at home or centralized facilities for 14 days, and during that time, they were assessed for fever or respiratory symptoms by medical staff twice daily. Persons in quarantine were transferred by ambulance directly to fever clinics when they developed any related symptoms during the quarantine period, while others were released after 14 days without fever and any respiratory symptoms.

| COVID-19 case detection and management
Patients transferred by ambulance from centralized facilities or their homes and people with clinical manifestations of COVID-19 who sought medical care on their own were sent to fever clinics immediately when they visited a hospital. According to the COVID-19 case definition in Shanghai, 13 suspected cases were diagnosed and sampled in fever clinics.
Within 2 h, the information of suspected cases was reported online, and specimens were sent to the Pudong Center for Disease Control and Prevention (CDC) for testing by real-time reverse transcriptionpolymerase chain reaction (RT-PCR). Moreover, patients were isolated in the hospital until COVID-19 confirmation and then transferred to a designated hospital for isolation and treatment. The other suspected cases were excluded by two consecutive negative laboratory test results of samples taken at intervals of more than 24 h.

| Identification and quarantine of CCs
An epidemiological investigation of all suspected cases was conducted by the Pudong CDC within 24 h to identify CCs and collect basic demographics, signs, symptoms, and exposure histories. CCs were identified through contact tracing and defined as those who lived in the same apartment, shared a meal, traveled, or socially interacted, and had close (within 1 m) and prolonged (generally ≥15 min) contact with any suspected COVID-19 patients without effective protection from 2 days before the patient's illness onset to the time of patient isolation. Detailed information of CCs was sent to their communities for further confirmation and management. Similar to PEPs, the CCs were quarantined, but only at centralized facilities for 14 days. The release was conditional on a negative RT-PCR result for the related suspected case or the absence of fever and any respiratory symptoms for 14 days if the related suspected case was confirmed.

| Data analysis
All confirmed COVID-19 cases reported between January 21 and  CC management, PEP management, and screening of self-admitted patients in fever clinics. Important intervention dates were plotted to compare intervals from illness onset to quarantine initiation, the first medical visit, isolation, and confirmation.
All statistical analyses were performed using R 3.5.1 (R Core Team, R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria). The χ 2 test or Fisher's exact test was used for categorical variables, and the Wilcoxon rank sum test or Kruskal-Wallis test was used for continuous variables, as appropriate.
p < .05 were considered statistically significant.

| RESULTS
Between January 21 and February 16, 2020, the Pudong CDC confirmed 108 COVID-19 cases (  14 and peaked approximately 1 week later. The epidemic was effectively controlled within 1 month, and no additional cases were identified at the end of February (Figure 2A).
On average, quarantine was initiated 1 day before illness onset among those with a potential exposure risk and 0.5 days before illness onset among CCs. A total of 52.17% (12/23) of the PEPs and 50.00% (11/22) of the CCs were quarantined before illness onset ( Figure 2B).
The median intervals from illness onset to the first visit, isolation, and confirmation were 1, 2 and 4 days, respectively. Compared to those of self-admission cases, the intervals from illness onset to the first medical visit, isolation, and confirmation of cases identified from CCs and PEPs were all significantly shorter (p = .02, .00, and .00, respectively; Table 1 and Figure 3).

| DISCUSSION
Nearly half of the confirmed COVID-19 cases in the Pudong New Area were detected through active surveillance measures, according to our study. Large proportions of the CCs and potentially exposed F I G U R E 1 Flowchart of active surveillance for COVID-19 cases in Pudong New Area, Shanghai, January-February 2020 individuals were identified and quarantined before the development of symptoms. On average, quarantine was initiated 1 day before the onset of illness among potentially exposed individuals and 0.5 days before the onset of illness among CCs. The time interval from illness onset to isolation for patients identified by active surveillance was significantly shorter than that for self-admitted patients.
At the beginning of the intervention, all of our surveillance measures of monitoring the population with higher infected risk, such as close contacts and travelers from high-risk areas, aimed to identify and isolate them as soon as they became patients. We did not actually consider avoiding presymptomatic transmission at that time. However, with the implementation of the intervention measures, increasing evidence from related research globally had indicated the occurrence of asymptomatic and presymptomatic transmission of COVID-19, 15,16 which increase the necessity of monitoring population with high infectious risk before they became a patient. Our results also showed that a lot of cases had been quarantined before they were detected with SARS-CoV-2.
As the prodromal symptoms are mild and do not begin abruptly, early signs and symptoms of COVID-19 are difficult to recognize. 17,18 Given the individual differences in the symptoms of this disease as well as subjective and constantly changing symptoms, most patients might not seek prompt medical care when they develop any symptom, especially at the early stage of a pandemic. A lack of timely detection of cases directly leads to an increased risk of community transmission; however, these problems can be averted by active quarantine and compulsory medical observation of the specific population. At the early stage of the COVID-19 outbreak, the proportion of persons who visited a fever clinic within 2 days of illness onset was 65.74% in Pudong New Area, Shanghai, compared with 61% in Gansu Province of China, 19 27% in Wuhan in China, 20 and 23.1% in South Korea. 21 Patients with mild symptoms promptly sought timely medical care, and detection and isolation were performed within a short period, which significantly reduced CC transmission, the risk of community transmission, and the likelihood of a patient developing severe illness. Overall, 74.07% of the patients in our study were isolated within 5 days after illness onset, which is similar to the rate of 68% in Gansu 19 and higher than the rate of 11% in Wuhan at the early stages of the disease. 7 In Singapore, the mean interval from symptom onset to hospital isolation or quarantine was 5.6 days. 22 Routine health management ensured timely case identification and confirmation, effectively protecting additional community members from infection. As a benefit from the positive control T A B L E 1 Characteristics of the confirmed COVID-19 patients in Pudong New Area, Shanghai, January-February 2020 This investigation has two limitations. First, we did not calculate the actual number of cases averted by each of our strategies due to a lack of information from the limited cases. However, previous studies using epidemiological models have demonstrated that early identification of potential cases plays a substantial role in determining whether an outbreak is controllable. 10,25,26 Second, the intervention we conducted in Pudong might need strong policy support from the government and need