3. Confirming the route of transmission
Regarding the course of disease development in the 28 patients with confirmed COVID-19 as of February 14th, 6 patients developed secondary infection, which they contracted from previously infected individuals, with each group including 2–6 patients (Figure 2).
The first group included the following index patients: Patient #3 was a 54-year-old man who was asymptomatic upon arrival in Korea on January 20th, but had developed a fever, chills, muscle ache, and nasal congestion by 13:00 on January 22nd. Patient #28, who was on the same flight bound from Wuhan to Korea as Patient #3, tested positive on February 10th during self-quarantine. The route of transmission is still under investigation. In addition, since Patient #28 underwent surgery following entry into the country, and was treated with antibiotics and analgesics, the exact date of onset of symptoms could not be calculated. Patient #6, an acquaintance of Patient #3, dined at a restaurant with Patient #3 on the evening of January 22nd for an hour and a half, developed symptoms including the chills on January 26th, and tested positive for COVID-19 on January 30th. Patients #10 and #11, the wife and son of Patient #6 who lived and dined at the restaurant with him, developed symptoms on January 29th and 30th, respectively, and tested positive for COVID-19. Patient #21, an acquaintance of Patient #6 who attended on January 26th, developed symptoms on January 30th and subsequently tested positive for COVID-19.
The index case imported from abroad in the second group involved Patient #5, a 32-year-old man from Wuhan who visited Korea on January 24th and started to develop aches similar to that observed in the common cold on January 26th. Patient #9, who developed a secondary infection, was an acquaintance of Patient #5 who stayed and dined at the same place as Patient #9 until January 29th. Patient #9 developed a headache on January 30th, was managed as a contact case, and tested positive on January 31st.
The third index case imported from abroad involved Patient #12, a 48-year-old man with a history of contact for business reasons, with a Japanese patient who had tested positive for COVID-19 in Japan. This was the first case imported from a country outside China. Following entry to Korea on January 19th, the patient started experiencing muscle aches on January 20th. However, COVID-19 was not suspected because he arrived in Korea from Japan, and thus reporting/testing was delayed. Patient #14 was his wife who stayed with him for more than 10 days before COVID-19 was confirmed. She developed symptoms on January 29th and subsequently tested positive.
The fourth index case involved Patient #15, a 43-year-old man who was being monitored after having close contact with Patient #4 on the plane. He developed a fever and a sore throat on February 1st and subsequently tested positive. Patient #20, a relative living in the same building but on a different floor, experienced a sore throat on February 4th and subsequently tested positive. It is likely that he was infected whilst eating with Patient #15.
The index case imported from abroad in the fifth group involved Patient #16, a 42-year-old woman who developed chills on January 25th after visiting Thailand and tested positive on February 4th. Since Patient #16 developed symptoms after visiting Bangkok and Pattaya, Thailand, between January 15th and January 19th, it is probable that she was either infected at a tourist destination in Thailand or Bangkok airport. The daughter of Patient #16 tested positive on February 5th and was designated as Patient #18. Among the other family members who dined with Patient #18 at her mother’s house on January 25th, the older brother (Patient #22) tested positive for COVID-19 on February 5th. Patients #18 and #22 were initially asymptomatic. Hence, the date of symptom onset could not be estimated. Moreover, Patient #18 had an overlapping history of visiting Thailand and a hospital visit with Patient #16 in Korea thus, the route of transmission was unclear and remained under investigation.
The sixth transmission case originating in Korea involved Patient #25, a 73-year-old woman who tested positive at a screening clinic. Later, her son (Patient #26) and daughter-in-law (Patient #27) tested positive during the process of epidemiological investigation. After patients #26 and #27 entered Korea on January 31st from Zhuhai, Guangdong Province of China, via Macau, the 3 patients remained in the same place of residence in Korea. Considering that Patient #27 developed symptoms on February 4th, earlier than Patients #25 and #26, and Patient #25 did not have any previous history of overseas travel, it is likely that Patients #25 and #26 were infected by Patient #27.
Of the 10 patients with secondary infection and known routes of transmission, 7 were family members including 2 relatives and 3 were acquaintances. All these patients either dined or resided together. Transmission from patients #3 to #6, and from patients #16 to #22 was assumed to have taken place while they dined together at the same place. Patients #6 and #21 did not dine together, but were together for 2 hours at a chapel. Regarding transmission, patients with secondary infection had a history of close contact with the primary patient including physical contact, co-dining, and co-habitation for a considerable amount of time.
In summary, of the 28 confirmed cases, 16 were index cases imported from abroad, 10 were secondary infection cases, and 2 remained under investigation to determine the route of transmission. As of February 14th, the 28 patients had come into close contact with 1,781 people, of whom 1,356 were released from surveillance, and 457 were under self-quarantine, and active surveillance. The suspected sites of transmission for the 10 patients with secondary infection were the home (7 patients, 60.0%), a restaurant, a church, and other locations (1 patient each, 8.3%). The relationship between the primary patient and patients with secondary infection were as follows: family (7 patients, 70.0%, including 2 relatives) and acquaintances (3 patients, 30.0%). This finding suggests that secondary infection occurred among individuals who were in close contact with an infected person for a considerable amount of time whilst they lived and dined together.