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Coronavirus disease 2019 (COVID-19) is an infectious disease caused by a novel coronavirus (also known as severe acute respiratory syndrome coronavirus 2; SARS-CoV-2) that was declared a pandemic by the World Health Organization (WHO) in March 2020. COVID-19 is widespread globally but has been well controlled in China with only scattered, importation-related local outbreaks following containment in April 2020.
On January 3, 2021, Nangong City (part of Xingtai City, Hebei Province with 484,000 residents in 2017) reported its first symptomatic case of COVID-19. China CDC, Hebei CDC, and Xingtai CDC jointly carried out a field epidemiological investigation and traced the outbreak. On January 6, Nangong City started its first round of population-wide nucleic acid screening. On January 9, Nangong City was locked down by conducting control and prevention measures that included staying at home and closing work units for seven days. As of January 27, 2021, 76 cases had been reported in Nangong City; among these, 8 were asymptomatic. No additional cases have been reported to date, and there were no COVID-19-related deaths in the outbreak.
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The index COVID-19 case in Nangong City is a 34-year-old male office worker (Patient A) when he noticed a fever and took his temperature, finding it to be 38.1 ℃ at 14∶00 on January 1, 2021. At 15∶30, he drove to the fever clinic of Nangong People’s Hospital for evaluation and treatment and was screened for COVID-19 by nucleic acid testing. Patient A was informed on January 2 that his nucleic acid test was positive. On January 3, Patient A was diagnosed as a confirmed case of symptomatic COVID-19 and treated in isolation.
The field epidemiological investigation and case tracing found that Patient A went to the respiratory department of Hebei Children’s Hospital in Shijiazhuang City on December 25, 2020 and had close contact with a symptomatic case of COVID-19 in a waiting area. The contact, Patient B, female, from Gaocheng District, Shijiazhuang City, had asthma and was febrile on December 25 (highest temperature 38.5 ℃). Video monitoring showed Patient A and B were in a room 4.5 meters apart from each other for 16 minutes. During this period, they did not talk or touch the same objects. Patient B removed her face mask twice to eat, and Patient A pulled down his face mask to make phone calls. Whole genome sequence traceability analysis from Hebei CDC found that the viral nucleic acid sample from Patient A, compared with the Wuhan reference strain (NC_045512), had mutation characteristics of the gene locus of the European family of the L genotype and was on the B.1.1.123 branch. The genome sequence has 21 nucleotide mutations that are same as virus samples from Gaocheng District cases. The genetic sequences of Patient A’s and Patient B’s viruses were highly homologous, presumably representing a common transmission source.
Considering dates of illness onset identified by the field epidemiological investigation and the sampling date of the first positive nucleic acid test, the first COVID-19 infection in Nangong City occurred on January 1. Since then, the number of cases reported each day increased rapidly and peaked on January 13 with 12 cases reported. Following the peak, the number of cases reported each day decreased until the last case was reported on January 23. The cumulative number of cases increased fastest from January 9 to January 13 (Figure 1). Among all 76 cases, 46 (60.5%) were among males; 63 (82.9%) were among young to middle-aged (15–59 years old) individuals; the ages ranged from 2 to 83 years with a median of 34.5 years; 34 (44.7%) were farmers, 11 (14.5%) were office staff, and 10 (13.2%) were health care workers; 59 (77.6%) came from Fenggang Street, and 17 (22.4%) came from seven other streets or townships (there are 15 streets and townships in Nangong City) (Figure 2, Table 1). Symptoms among the 68 symptomatic cases were generally moderate or mild.
Figure 1.Distribution of date of illness onset based on field epidemiological investigation and cumulative incidence of COVID-19 cases from January 1 to January 23, 2021 in Nangong City, Hebei Province, China.
Figure 2.Geographic distribution of the 76 COVID-19 cases reported in Nangong City, Hebei Province, China, January 3 to January 27, 2021.
Characteristics Symptomatic cases Asymptomatic cases Total N Percentage (%) N Percentage (%) N Percentage (%) Total 68 89.5 8 10.5 76 100.0 Gender Female 29 42.6 1 12.5 30 39.5 Male 39 57.4 7 87.5 46 60.5 Age group (years) <15 5 7.4 1 12.5 6 7.9 15–29 17 25.0 2 25.0 19 25.0 30–44 20 29.4 3 37.5 23 30.3 45–59 19 27.9 2 25.0 21 27.6 ≥60 7 10.3 0 0 7 9.2 Occupation Farmer 31 45.6 3 37.5 34 44.7 Office staff 11 16.2 0 0 11 14.5 Health care worker 8 11.8 2 25.0 10 13.2 Workman 5 7.4 1 12.5 6 7.9 Teacher 5 7.4 0 0 5 6.6 Student 4 5.9 1 12.5 5 6.6 Child 2 2.9 1 12.5 3 3.9 Business services 2 2.9 0 0 2 2.6 Location Fenggang Street 52 76.5 7 87.5 59 77.6 Other streets 16 23.5 1 12.5 17 22.4 Table 1. Epidemiological characteristics of the 76 COVID-19 cases reported in Nangong City, Hebei Province, China, January 3 to January 27, 2021.
Two cases, including the index case, were identified by fever clinics; 16 were found by community-wide nucleic acid screening; 5 were detected by daily routine tests for medical staff conducted by hospitals; and 53 were discovered by regular, confirmatory nucleic acid screening of close contacts or sub-close contacts including 48 (63.2%) identified during centralized isolation or quarantine.
Transmission chains were characterized by mixed existence of intra-unit transmission, family clusters, nosocomial infections, and community transmission associated with the index case Patient A. There was 1 intra-unit transmission with 23 (30.3%) people infected, 2 family clusters (identified by ≥5 people infected) with 17 (22.4%) people infected, 3 re-transmissions in medical institutions with 8 (10.5%) people infected, and 1 community cluster (identified by ≥5 people infected) with 6 (7.9%) people infected. There was the possibility of cross-infection in isolation places, but there is no definitive supporting evidence. All COVID-19 cases in Nangong City were related to the index case, Patient A; eight generations of infection were observed in the transmission chains.
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