Preplanned Studies: Characteristics and Containment of 74 Imported COVID-19 Outbreaks: Experiences, Lessons, and Implications — China, 2020–2021

After the successful containment of the initial coronavirus disease 2019 (COVID-19) outbreak in Wuhan, China adopted a dynamic-zero policy in March 2020 aimed at eradicating all imported outbreaks. In this report, we provided a comprehensive documentation and analyses of all the imported outbreaks before 2022. Data on daily COVID-19 infections were retrieved from the website of the National Health Commission of China. Results of epidemiological investigations of the outbreaks were retrieved from 1,504 publications by local governments or mainstream social media. Seventy-four outbreaks were identified consisting of 10,082 symptomatic cases and all were successfully contained. Characteristics of the outbreaks were summarized including source of the first case(s), time, place, scale, and duration. These data were then analyzed to identify potential problems and plan for future emerging infectious diseases alike. China’s experience in successfully containing 74 consecutive outbreaks provides important evidence that COVID-19 or newly emerging infectious diseases alike can be contained at their early stage to prevent the occurrence of pandemics, or at least gain experiences and win time for the development of vaccines and drugs.

Data on daily number of imported cases, domestic cases, symptomatic domestic cases, and close contacts from March 19, 2020 to December 31, 2021 were retrieved from the official reports of Daily Briefing on Novel Coronavirus Cases. Our analyses of outbreaks only included symptomatic domestic cases from all the outbreaks; cases found in quarantined inbound cross-border travelers were excluded. For each symptomatic case, epidemiological investigations were traced via official reports from local governments. A total of 1,504 reports were retrieved and scrutinized. The definitions for outbreaks are presented in Supplementary Figure S1, available in https://weekly.chinacdc.cn/.

The national daily numbers of cases, which included data on location and magnitude of these outbreaks, were described chronologically and geographically. Characteristics of the outbreaks were compared among 3 study periods divided according to the announcement dates of the 7th and 8th editions of the Protocol on Prevention and Control of COVID-19 and the source of the first case (1-2). All statistical analyses were performed by using R software version 3.6.2. A full description of methods is in Supplementary Materials, available in https://weekly.chinacdc.cn/.

Overall, the study identified 74 outbreaks with a total of 10,082 symptomatic cases between March 19, 2020 and December 31, 2021 (Figure 1). The median number of cases in an outbreak was 10, ranging from 1 to 1,506. Out of the 74 outbreaks, 43 (58.1%) were detected via proactive surveillance, and 57 (77.0%) were contained at their origin within the provincial-level administrative divisions (PLADs) (Table 1). The outbreaks on average lasted for 10 days, ranging from 1 to 62 days. The ratio of daily number of close contacts over daily number of cases, an approximate indicator of people quarantined per case, was 60, ranging from 4 to 2,830. Detailed characteristics for each outbreak were presented in Supplementary Table S1 available in https://weekly.chinacdc.cn/.

Figure 1. 

Occurrence and size of outbreaks by region and time in the mainland of China between March 19, 2020 and December 31, 2021.

Note: The number next to each outbreak refers to the unique ID number assigned for each outbreak in Supplementary Table S1. Cases with the same numbers belong to the same outbreak.

Abbreviation: PLADs=provincial-level administrative divisions.

Due to heavy international air traffic, it was anticipated that the earliest imported outbreaks occurred in Guangdong, Beijing, Shanghai, and PLADs near them. These outbreaks were generally small and quickly contained. However, as outbreaks continued to spread, an increasing trend emerged including a higher frequency of outbreaks and higher number of PLADs involved (Figure 1). By December 31, 2021, all PLADs in the mainland of China had been involved in at least one outbreak except Xizang (Tibet). No seasonal trends were observed.

Geographically, PLADs including Beijing, Shanghai, Guangdong, and their neighboring regions had the highest frequency of outbreak attacks (Supplementary Table S2). Five PLADs with the highest number of outbreaks included Beijing (11), Shanghai (10), Liaoning (10), Guangdong (9), and Heilongjiang (9). The 5 regions with the largest number of cases accumulated were Shaanxi (1,494 cases), Heilongjiang (1,145 cases), Hebei (1,103 cases), Xinjiang (906 cases) and Jiangsu (826 cases), totally accounting for 54.3% (5,474/10,082) of all cases.

Chronologically, the daily number of imported cases that were diagnosed during quarantine and did not cause community outbreaks considerably fluctuated during the study period. Although there was no clear increasing trend with time, there was a slight elevation after June 2021 (Supplementary Figure S2). The daily number of symptomatic domestic cases and close contacts showed a similar pattern.

In addition, the average size and duration of outbreaks and the average number of PLADs involved in each outbreak are shown in Table 1. Although the number of outbreaks and that of cases accumulated were chronologically increasing during the 3 periods of study, the last period had the highest proportion of outbreaks detected via active surveillance (77.1%), the smallest number of patients per outbreak (6 cases), shortest duration of outbreaks (7 days), the largest proportion of outbreaks involving only one PLAD (77.1%), and an average number of 68 close contacts quarantined per patient. These findings showed that the number of outbreaks and thus prevention and control intensity increased over time, but the effect and efficiency of response actions also increased. As a result, the situation remained largely controllable.

Regarding the source of the first case(s) or the origin of an outbreak, 35 (47.3%) of the 74 outbreaks occurred at areas labeled as the first frontier, i.e., land borders, airports, and ports (Table 2). These outbreaks contributed 5,654 (56.1%) cases to the total number of cases from the 74 outbreaks (Supplementary Figure S3).

Surprisingly, 23 (31.1%) outbreaks occurred at the second frontier, including those at designated care hospitals, quarantine places, and among inbound travelers whose incubation time might be longer than the quarantine time or who got infected during quarantine (Table 2). However, these outbreaks were relatively small in size, quickly contained, and accounted for only 8.0% of the total number of symptomatic cases from the 74 outbreaks.

Lastly, 16 (21.6%) outbreaks were identified in communities such as shopping malls and food markets. Some were possibly caused via cold-chain logistics, while the rest had no clearly identifiable source of infection (Table 2). These outbreaks were most difficult to control when detected as well as difficult to detect once occurred, as only 31.3% (5 out of 16) were detected by proactive surveillance. As a result, they were most likely (43.8%) to involve 2 or more PLADs, resulting in more cases and longer durations per outbreak.