Epidemiologic characteristics
Examination of demographic characteristics of the 28 confirmed patients in Korea showed that 15 (53.6%) and 13 (46.4%) were male and female, respectively. Six of the patients were Chinese nationals, with three identified as visitors from China, while the other three were residents in Korea. The remaining 22 patients (78.6%) were Korean nationals. The median age of all patients was 42 years (21-73) (all adults, with 6, 6, 6, 8, 1 and 1 patient in their 20s, 30s, 40s, 50s, 60s and 70s, respectively) (
Table 1). In total, 16 patients constituted index cases, while 9 and 3 were first-generation and second-generation patients, respectively (
Figure 1).
Geographical regions in which 11 (68.8%), 1, 1, 1, and 2 index cases are speculated to have been infected include Wuhan, Guangdong, Japan, Thailand, and Singapore, respectively. Of the twelve first-generation and second-generation patients, 8 (66.7%) were family members of the index cases, while the remaining 4 patients were acquaintances who had been in close contact with index patients. All of these patients were identified during a control process for close contacts after confirmation index cases (
Figure 1).
The epidemic curve of the outbreak according to date of symptom-onset ranges from January 10, 2020 to February 8, 2020, with January 26, 2020 showing the highest number of cases (n=3). However, dates of symptom-onset show a wide distribution, overall. The epidemic curve plotted according to the date of diagnosis, ranges from January 20, 2020 to February 10, 2020 and shows that the highest number of cases were on February 5, 2020 (
Figure 2). On the other hand, patient #2 first developed symptoms on January 10, 2020 while in China, and was diagnosed on January 24, 2020, after his arrival in Korea on January 22, 2020. Thus, considering only those patients who developed onset of symptoms in Korea, the initial symptom-onset of the infection occurred on January 18, 2020, in patient #1. With regard to affected geographical regions, symptom-onset occurred in Incheon, Bucheon, Pyeongtaek, Gwangju, and Goyang in order (
Figure 3).
Major areas where the diagnosed patients were exposed before being isolated, were Seoul and Gyeonggi-do with 8 and 13 cases (4, Goyang; 3, Siheung; 2, Bucheon; 1, Guri; 2, Suwon; 1, Pyeongtaek), respectively. The rest included Incheon, Gunsan, Gwangju, and Naju with 1, 1, 2, and 1 case, respectively (
Figure 4). After identifying a tourist from Wuhan who visited Jeju-do (on January 21-25, 2020) and was confirmed of the infection on January 30, 2020 in China, 11 contacts of the patient were quarantined for 14 days, but were all released on February 8, 2020, after none were found to be infected.
The time-interval for entry into Korea for the 16 index cases ranged from January 19-31, 2020, excluding that for the 2 Koreans, who were transported from Hubei in a chartered aircraft, by the Korean government, along with other Korean citizens who had been residing there (
Figure 5).
Incubation period is the time-interval between time of infection and onset of symptoms. However, as the exact time of exposure could not be ascertained in index patients and the time of symptom-onset could not be determined in those who were asymptomatic, these cases were excluded from the calculation. The estimates of incubation period included 0-15 days based on seven of the first-generation patients (#6, #20, #9, #14, #25, #26, and #28), and 1-4 days based on three of the second-generation patients (#10, #11, and #21), with mean and median values of 3.9 days (range 0-15) and 3.0 days, respectively (
Table 1).
The mean and median serial interval was estimated to be 6.6 days (range 3-15) and 4.0 days, respectively, based on data of both first-generation and second-generation patients (
Table 1).
The mean duration between symptom-onset and quarantine/isolation was 4.3 days (0-15). It was especially longer (10-12 days) in index patients who were infected in Japan, Thailand, and Singapore, which were not considered as possible risk-areas (
Table 1).
Reproduction number (R) was estimated based on data of 26 (14, index; 9, 1st generation, and 3, 2nd generation) patients among total 28 confirmed cases, excluding the 2 Korean patients who had been immediately quarantined after being transported from Wuhan and therefore did not come into contact with the Korean population. As of now, the R is estimated to be 0.48 in Korea (Poisson 95% confidence interval, 0.25 to 0.84) (
Table 1).
As of February 8, 2020, patient #6 had transmitted the infection to the highest number (n=3; 2 family members and 1 acquaintance). As super spreading event is defined when a patient transmits the infection to 5 or more people, this case has not yet been observed in Korea (
Figure 5).
Of the 28 infected patients diagnosed in Korea, 3 were asymptomatic. Of them, patients #18 and #22 were first-generation patients infected by patient #16 and these patients were asymptomatic even though they were tested positive with the virus at the beginning of quarantine.
Of 28 patients, 4 have been discharged as of February 10, 2020. Patients #2, #1, #11, and #4 were discharged on day-13 (February 5), 19 (February 6), 11(February 10), and 15 (February 9) of hospitalization, respectively (
Figure 5). There are currently no clinically serious patients among those undergoing treatment. After more patients recover and are discharged in the future, further detailed epidemiologic information (such as mean duration of hospitalization, duration of disappearance of symptoms and viral shedding since hospitalization) would be suggested.