As of December 22, 2020, no local cases of coronavirus disease 2019 (COVID-19) had been reported from Shunyi District of Beijing for 319 days. On December 23, the Shunyi District CDC received a report of a confirmed COVID-19 case, and another case was reported 2 days later. By January 31, 2021, 42 cases had been reported in Shunyi — a confirmed, local COVID-19 outbreak.
Shunyi CDC immediately launched an epidemiological investigation with laboratory testing to identify the source of infection, determine routes of transmission, assess the scale of the outbreak, and provide recommendations for stopping the outbreak and preventing recurrence. The investigation showed that all confirmed COVID-19 cases were associated with an asymptomatic carrier who was an international traveler from Indonesia. The investigation serves as a reminder that the government should pay attention to asymptomatic infections in our COVID-19 prevention and control strategies, including international entrant screening policies and practices.
At 05∶08 on December 23, 2020, the index case of this local outbreak (Patient A) was reported to Shunyi CDC as a confirmed COVID-19 infection. Shunyi CDC conducted in-person interviews with a detailed questionnaire that used China CDC definitions for suspected and confirmed COVID-19 cases (1-2). A series of response measures were implemented but contact tracing did not immediately lead to source identification.
At 05∶30 on December 25, 2 days later, a second case (Patient B) was reported to Shunyi CDC. Patient B had nausea, sore throat, and dizziness at 18∶00 on December 16, but did not seek medical attention at that time. An investigation found that Patient B had a roommate who was a 28-year old Indonesian man who had returned from Indonesia on November 26. The person from Indonesia had entered China at Fuzhou Airport; en route, he sat in seat 60L; a nearby passenger in seat 60J was later confirmed as a COVID-19 case. Because he was a close contact of a confirmed case, the person from Indonesia was required to be in medical quarantine from November 26 to December 10. During quarantine, five consecutive COVID-19 nucleic acid tests and two consecutive virus-specific antibodies tests were performed; all tests were negative.
After completing the 14-day quarantine, the individual from Indonesia travelled from Fuzhou to Beijing, arriving at 00∶05 on December 11. He rented an apartment that day in a community in Shunyi District through a housing agency; there were four others in the apartment. One of the other four was Patient B.
From December 12 to 20, the person from Indonesia was required to conduct quarantine at the apartment, but he did not strictly abide by the quarantine regulations. During this period, he went to malls, supermarkets, and restaurants. At 15∶17 on December 13, he went to Hualian Mall and returned to the rental unit at 16∶51. Two people with whom he had close contact in Hualian Shopping Center became Patient C (a salesperson in the mall) and Patient D. Further investigations revealed that Patient A had close contact with Patient C while at Hualian Shopping Center on December 17.
From December 21 to 24, the person from Indonesia took the subway to work at a company in Wangjing, Chaoyang District around 09∶00 every day and took the subway back to the apartment after getting off work around 19∶00, arriving home around 20∶00. He stated that he was wearing a disposable mask during shopping and work. Between December 10 and 24, no throat swabs or serum specimens from this person were obtained.
At 09∶28 on December 25, the person from Indonesia was determined to be a close contact of Patient B and was transferred to a hotel designated for medical quarantine at 15∶14. Throat swabs and serum specimens were collected and tested for viral nucleic acids and virus-specific antibodies respectively. Sera tested positive for IgM and negative for IgG specific to the COVID-19 virus.
At 14∶30 on December 27, the person from Indonesia was transferred to Beijing Ditan Hospital for further diagnosis and treatment. On December 28, his throat swab and stool specimens were positive for COVID-19, with cycle threshold (Ct) values of 39.0/37.0 (ORF1ab/N), and 24-28/24-28. At 22∶00, he was diagnosed as an asymptomatic COVID-19 carrier (Patient E).
Between December 10, 2020 and January 31, 2021, 42 COVID-19 cases were identified and linked to this outbreak. Starting on December 25, the number of cases significantly increased, reaching a peak on December 27 and 28 (Figure 1).
Figure 1. Epidemiological curve of a local outbreak of COVID-19 in Shunyi District, Beijing, 2020–2021.
The 42 cases involved 5 generations of infection - 1 case in the first generation (Patient E), 3 in the second generation, 13 in the third generation, 22 in the fourth generation, and 3 in the fifth generation. The second-generation cases were infected when living together or during close contact in a shopping mall with Patient E. Among the second-generation cases, one resided with Patient E and the other two were close contacts of Patient E in the Hualian Mall on December 13. The third-generation cases were infected when living together or traveling in cars, or when in close contact with second-generation cases in the mall. The fourth and fifth generation cases were infected when eating or working with cases.
According to our epidemiological investigation, 31 of the 42 outbreak cases had clear last exposure times. Based on data from these 31 cases, we calculated the median incubation period to be 5 days, with a range of 2 to 23 days.
In total, 41 cases were reported from Shunyi District, and 1 was reported from another district in Beijing. The median age was 32 years (8 months – 62 years); the male∶female ratio was 1.6∶1; 18 (42.9%) were workers, and 8 (19%) were unemployed. There were no severe or critically ill cases. Overall, 24 cases (57.1%) were moderate illness, 10 (23.3%) were mild illness, and 8 (18.2%) were asymptomatic. Except Patient E, the remaining 7 cases of asymptomatic infection were confirmed by the test results of throat swabs or anal swabs for COVID-19 (Table 1).
Patient Throat swab Anal swab Serum specimen ORF 1a/b (A lab/B lab) N (A lab/B lab) ORF 1a/b (A lab/B lab) N (A lab/B lab) IgM IgG E －/－ －/－ －/－ －/－ ＋ － F ＋/＋ ＋/＋ －/－ －/－ － － G ＋/n.a. ＋/n.a. －/n.a. －/n.a. － － H ＋/＋ ＋/＋ ＋/＋ ＋/＋ － － I ＋/＋ ＋/＋ －/－ －/－ － － J －/－ －/－ ＋/＋ ＋/＋ － － K ＋/＋ ＋/＋ ＋/＋ ＋/＋ － － L ＋/－ －/＋ －/－ ＋/－ － － Abbreviation: ORF 1a/b=Open reading frame 1a/b；n.a.=not applicable.
Table 1. The first sampling and testing results of 8 cases of asymptomatic infection of a local outbreak of COVID-19 in Shunyi District, Beijing, 2020–2021.
The investigation for Patient E obtained 203 environmental specimens during December 26 and 27, including 21 samples from the apartment, among which 9 tested positive for COVID-19 virus, and 183 samples from the workplace, among which 5 were positive and 1 was indeterminate.
Full-length genome sequences were determined for 14 specimens from people, including Patient E, and 5 environmental samples (2 were from the apartment and the workplace of Patient E). Sequence alignment and phylogenetic analyses showed that all 19 strains were highly homologous, with similarities over 99.993% and having only 1–2 sites with nucleotide variations. The strain that caused this local outbreak of COVID-19 in Shunyi District belonged to the L-Lineage European Branch 2.3 (B.1.160.3) and has high homology with COVID-19 strains found in Southeast Asia during the same period.
Ultimately, Shunyi CDC identified asymptomatic Patient E as the source of infection for this outbreak in Shunyi District.