The study population was 39 pilgrim travelers comprising 1 priest, 1 travel guide, and 37 tourists. Immediately after 2 confirmed COVID-19 cases in the group, the remaining 37 pilgrim travelers were aggressively tested for SARS-CoV-2 from February 21 to February 22 regardless of symptoms. A COVID-19 case was defined as anyone who tested positive through real-time RT-PCR. All pilgrim travelers except 1 travel guide resided in Gyeongsangbuk-do. Thus, the Gyeongsangbuk-do Provincial Government initiated an outbreak investigation with the cooperation of the Korea Centers for Disease Control and Prevention (KCDC).

The Gyeongsangbuk-do rapid response team was organized to investigate and control the outbreak. Confirmed COVID-19 cases were admitted to 4 designated hospitals for COVID-19 patients in Gyeongsangbuk-do and the National Medical Center in Seoul. Pilgrim travelers who tested negative were home quarantined for 14 days. According to the COVID-19 response guidelines in Korea, contact tracing of COVID-19 cases was conducted from 1 day before symptom onset or 1 day before the case was sampled.13 The criteria were changed to 2 days after the revised guideline on April 2.14

The pilgrim travelers were initially interviewed using a standardized infectious disease report form in Korea for COVID-19 that evaluated demographic characteristics, exposure history, and clinical symptoms. In addition, the pilgrim travelers were interviewed through cellular phones to identify the presence of symptoms, onset of symptoms, and possible source of infection during the pilgrimage trip. The clinical course of COVID-19 cases was obtained by reviewing medical records. Onset of symptoms was determined using information from case interview and supplemented by medical records. The passengers and aircrews list was secured through the KCDC to evaluate the possibility of exposure during the flight from Korea to Israel on February 8. Considering that the early outbreak in Korea was related to the Shincheonji Church,10 the COVID-19 patients and their families were checked if they were on the list of Shincheonji, obtained by the KCDC. In addition, the history of medical facilities and visiting pharmacies, GPS tracking of cellular phones, and card transactions were utilized to confirm if they had visited the Shincheonji Church in Daegu.

The Gyeongsangbuk-do Government Public Institute of Health and Education (GGPIHE) performed real-time RT-PCR on all samples except 1 sample from a travel guide in Seoul, which was performed by the Seoul Research Institute of Public Health and Environment (SRIPHE). These 2 organizations were certified to perform SARS-CoV-2 testing by the KCDC on January 24, 2020.10 Real-time RT-PCR was used to amplify the viral E gene as a screening test and the RdRp region of the orf1b gene as a confirmatory test in accordance with the laboratory guidelines in Korea.15 Cycle threshold (Ct) values below 40 indicated a positive result for SARS-CoV-2. According to the response guidelines in Korea, upper and lower respiratory tract specimens were obtained from nasopharyngeal plus oropharyngeal swabs and the sputum, respectively.13 The first specimens were collected by trained medical personnel in each public health center and immediately transported to the GGPIHE and the SRIPHE. The revised guideline on March 2 recommended to obtain only lower respiratory tract specimens from cases with sputum.16

The first real-time RT-PCR test for SARS-CoV-2 was performed for all 39 pilgrim travelers; those with a negative result were retested 7 days later. All were initially negative for SARS-CoV-2. Ten confirmed cases without symptoms at the first sampling dates initially were selected for follow-up RT-PCR (Case 07, 19, 22, 23, and 25–30). Two cases were revealed to later have symptoms at the first sampling dates (Cases 07 and 19). Follow-up RT-PCR tests were conducted 7 days after the first sampling dates; additional follow-up RT-PCR tests were conducted twice or more per week until hospital discharge. Follow-up specimens were obtained by medical staff in the 4 designated hospitals for COVID-19 patients in Gyeongsangbuk-do and transported to the GGPIHE within 24 hours of collection. In this study, we compared the Ct values that were analyzed by the GGPIHE.

Continuous variables are presented as the median (interquartile range [IQR]) and compared by the Mann–Whitney U test. Categorial variables are presented as the number (%) and compared with the chi-square test or Fishers' exact test. A P value less than 0.05 was considered as statistically significant. Statistical analyses were performed with SPSS (version 22.0; SPSS Inc, Chicago, IL, USA).

Data were collected as part of the public health response and not considered research subject to approval of the Institutional Review Board; informed consent was not required.

The pilgrim travelers boarded an airplane from Korea to Israel on February 8, 2020 (KE957). None of the 219 passengers and 15 aircrew were Chinese. From February 8 to February 15, the pilgrim travelers visited various places, including Netanya, Tiberias, the Dead Sea, Beer Sheba, and Jerusalem. One local guide and 1 local bus driver accompanied the pilgrim travelers. The local guide and bus driver tested negative for SARS-CoV-2 in Israel.1112 The pilgrim travelers left Israel on February 15 and arrived in Korea on February 16 (KE958). Two index COVID-19 cases (Case 08 and 15) in pilgrim travelers were reported on February 21. Another 37 pilgrim travelers were tested for SARS-CoV-2 from February 21 to February 22 regardless of symptoms. Contacts who experienced symptoms within 14 days of home quarantine were also tested for SARS-CoV-2. Then, confirmed COVID-19 cases were admitted to the hospitals.

Out of 39 pilgrim travelers, 30 (77%) tested positive for SARS-CoV-2. The primary case occurred on February 14, and there was a peak of 8 secondary cases on February 17. The onset of secondary cases ranged from February 16 to February 28, within 2 weeks from the onset of the primary case (Fig. 1). None of the travelers wore masks during the pilgrimage because COVID-19 cases had not been reported in Israel.

The date of symptom onset of two index cases was February 17 (Case 08) and February 18 (Case 15). During the interview, the primary case (Case 01) stated that his first symptom was myalgia on February 17, 2020. However, the tour guide informed us that someone else had tried to obtain cold medicine on February 14. One tourist witnessed that the primary case took cold medicine on February 14. Thus, the onset of the primary case was assumed to be on February 14. Among 39 pilgrim travelers and their families, only 1 person who was the primary case's son, was on the list of Shincheonji and tested positive for SARS-CoV-2 on February 26. Although the son stated that he did not visit the Shincheonji Church after 2019, he stayed at the Shincheonji Church in Daegu from January 31 to February 6, 2020, according to GPS records. On February 7, the son came home and met the primary case before departing for Israel.

Thirty persons (77%) were women with a median age of 59.0 years (IQR, 55.0–65.0 years). Chi-square test showed that older individuals had a significant association with SARS-CoV-2 infections (P = 0.028), which were not significant using the Mann–Whitney U test (P = 0.155). The local bus in Israel consisted of 4 seats per row. The travelers placed in aisle seats, where the primary case had been seated, had a higher attack rate than those in the window seats, without statistical significance (Table 1).

Among 30 COVID-19 cases, 8 (27%) experienced no symptoms at the first sampling dates (Cases 22, 23, and 25–30) and 4 (13%) were asymptomatic (Cases 27–30). We could not obtain credible information on the specific symptoms of the primary case. Out of 29 cases, the most common symptoms at the first sampling dates, which was 3 days (median; IQR, 1–4 days) after symptom onset, were febrile/chilling sense (34%) and sore throat (34%). Upon admission, which was around 5 days (median; IQR, 3–6 days) from the onset of symptoms, the most common symptoms were cough (45%) and sputum (45%). The median hospital stay duration was 23 days (IQR, 20–29 days), and no fatal cases were present (Table 2). Out of 25 cases with symptoms, the most frequent first symptom was febrile/chilling sense (44%), followed by sore throat (40%), myalgia (32%), and cough (27%).

Routine blood tests and chest radiograph results showed no significant difference between cases with symptoms and cases without symptoms at admission. Specimens from the 29 confirmed cases were analyzed by the GGPIHE. The first sampling dates were ranged from February 20 to February 22, and majority of the first samples (27/29, 93.1%) were collected on February 21. Available Ct values from specimens of the lower respiratory tract were significantly lower in cases without symptoms (median, 22.8; IQR, 20.1–25.4) than in cases with symptoms at the first sampling dates (median, 27.9; IQR, 23.4–30.4) (Table 3). A lower Ct value indicates higher viral shedding.

Fig. 2A (upper respiratory tract) and 2B (lower respiratory tract) show the available Ct values from 6 symptomatic cases (Cases 07, 19, 22, 23, 25, and 26) and 4 asymptomatic cases (Cases 27–30), relative to the sampling dates. The viral loads gradually decreased over time and were not different between symptomatic and asymptomatic cases. Fig. 2C (upper respiratory tract) and 2D (lower respiratory tract) show the available Ct values for 6 symptomatic cases relative to days from symptom onset; Cases 07 and 19 were postsymptomatic and Cases 22, 23, 25, and 26 were presymptomatic at the first sampling dates. The lowest Ct values from upper respiratory tract specimens (19.1) and lower respiratory tract specimens (17.7) were observed from the first samples of Case 26 collected 7 days before symptom onset.

Out of 328 contacts related to the original COVID-19 cases in the pilgrim travelers, 22 additional cases (attack rate: 7%) were confirmed with SARS-CoV-2 infections (Table 4 and Fig. 3). The son of the primary case was excluded, considering the uncertainty of information on symptom onset. During transmission from secondary to tertiary cases, the attack rate was 7% (19/258) and that from tertiary to quaternary cases was 5% (3/60). No additional quinary cases occurred. The highest attack rate was observed from church contacts (19%, 6/31), followed by cohabiting family contacts (secondary attack rate: 18%, 8/44), caregiver contacts (13%, 2/15), and restaurant contacts (10%, 2/20). There were no confirmed cases out of the 19 clinic and pharmacy contacts. The tertiary case from the aircrew contacts was a stewardess who had boarded the airplane with the pilgrim travelers from Israel to Korea (KE958). The stewardess was in charge of the zone where 29 COVID-19 cases were seated. An additional quaternary case from the aircrew contacts was a stewardess who boarded the airplanes with the tertiary stewardess from Korea to the USA (KE017) and from the USA to Korea (KE012).

One fatal tertiary case occurred, resulting in a total fatality rate of 2% (1/52) in this outbreak. Three tertiary cases were identified to be transmitted by presymptomatic secondary cases. An immobile cohabiting aunt of Case 26 was a tertiary case whose symptoms occurred 2 days before the symptom onset of Case 26 (February 26 for the aunt and February 28 for Case 26, respectively). Case 26 had cared for the aunt at home until hospital admission on February 28. Case 26, a woman in her 60s without any comorbidity, was positive with the highest viral loads from samples collected 7 days before her symptom onset (Fig. 2). Additionally, 2 tertiary cases were transmitted by the presymptomatic caregiver (Case 17), who had visited the home of the tertiary cases until 1 day before the symptom onset of Case 17.