CNUH is a major acute teaching hospital with over 1,000 beds and a 24 h accident and emergency service, serving over 3.5 million people in the Chonnam province and Gwangju metropolitan city. CNUH is composed of seven buildings (buildings 1, 2, 3, 5, 6, 7 and 8) (Supplementary Fig. 1A). Building 1 was constructed in 1979 and building 8 was constructed in 2001. Buildings 1 and 8 each have twelve floors. Building 7 is next to building 8, and the two buildings are connected on floors 1, 2 and 7. Buildings 1 and 7 are linked on floor 2 by a sky bridge. The general ward of building 1 comprises two wings (A and B), a communal toilet, and a communal bathroom. In addition, there is one single-patient room, five two-patient rooms, and five six-patient rooms in each wing (Supplementary Fig. 1B). There are toilet facilities in each single or two-patient room in the ward.

All visitors and patients filled out a questionnaire regarding travel and contact history and underwent thermal scanning for fever before entry into the hospital premises was permitted. All patients with scheduled operations were only admitted following presentation of a negative SARS-CoV-2 reverse transcription-polymerase chain reaction (RT-PCR) test within 48 hours of admission.4 Each outpatient was only allowed 1−2 accompanying person(s) when attending the specialist outpatient clinic. Each inpatient was restricted to only one attending caregiver throughout the period of hospitalization, except near the time of death; however, caregivers could be changed without restrictions. Patients with fever (> 37.5°C) or respiratory symptoms were transferred to the triage room in the emergency area or outpatient clinic for SARS-CoV-2 RT-PCR testing. Medical staff in the triage room wore personal protective equipment (PPE) including gowns, gloves, N95 respirator and face shields or goggles. Everyone in the hospital was encouraged to wear masks and follow hand hygiene practices. Our administrative office recommended to avoid large events and gatherings, but it was not mandatory.

The first confirmed COVID-19 case was a 30-year-old second-grade neurosurgery resident. On 11/13/2020, the patient visited the triage room presenting with cough and myalgia. The patient had no history of travel or close contact with confirmed COVID-19 cases. Following confirmation of COVID-19 infection on the evening of 13/11/2020, our infection control team announced the closure of the emergency room (ER) of the hospital; the ER and neurosurgical intensive care unit (NCU) were the main workplaces of the neurosurgery resident diagnosed with COVID-19. We then conducted contact tracing and tested HCWs, patients and caregivers in all areas served by neurosurgeons (ER, NCU, general neurosurgery wards [6A and 6B]) for COVID-19.

RT-PCR was performed using combined nasopharyngeal and oropharyngeal swabs in the CNUH laboratory, a diagnostic facility for COVID-19 authorized by the Korean Center for Disease Control and Prevention (KCDC). RNA was extracted using an automated nucleic acid extraction system (AdvanSure™ E3 System; LG Chem, Korea). Extracted RNA was amplified using a commercial kit (PowerChek™ 2019-nCoV Real-time PCR Kit; Kogenebiotech, Korea) and the envelope (E) and RNA-dependent RNA polymerase (RdRP) genes were detected (CFX96™ Real-time PCR detection system; Bio-Rad, USA). A positive RT-PCR result was defined as both E and RdRP genes had Ct values of 35 or less, whereas a Ct values over 40 was considered negative; a Ct values of 35−40 was considered indeterminate. In total, 439 respiratory samples from 344 HCWs, 47 patients and 48 caregivers were obtained between 11/13/2020 and 11/14/2020.

Close contacts of suspected/confirmed COVID-19 cases, as defined according to the World Health Organization guidelines,5 were tested for COVID-19. Close contacts with negative test results were obliged to self-quarantine for 14 days following their most recent contact with a confirmed COVID-19 patient, then retested at the end of the quarantine period. In cases of contact but not close contact, self-quarantine was not enforced, but active and passive monitoring were implemented. During active monitoring, the local public health authority assumed responsibility to establish regular communication with exposed people to assess for symptoms. Passive monitoring required exposed people to self-monitor by performing regular temperature checks.

This study was approved by the Institutional Review Board of Chonnam National University Hospital (IRB No. CNUH-2021-186). Informed consent was waived because of the retrospective nature of the study.

Following confirmation of COVID-19 in the second-grade neurosurgery resident, we traced contacts made with the COVID-19 case from 11/09/2020 to 11/13/2020; we found a symptomatic third-grade neurosurgery resident and discovered that 13 neurosurgeons had attended a get-together (‘hoesik’ in Korean) for 6 hours with the first COVID-19−confirmed HCW on 11/12/2020. Fig. 1A shows the estimated transmission routes of 22 cases of COVID-19 in the neurosurgery outbreak. Through contact tracing and testing, we diagnosed COVID-19 in two other neurosurgery residents (Fig. 1A, Case numbers 2 and 3) and a nurse (Fig. 1A, Case number 4) of ward 6A of building 1 on 11/14/2020. We quarantined all neurosurgery doctors and nurses, healthcare assistants, and cleaners of NCU and neurosurgery wards (6A, 6B) from 11/14/2020 to 11/27/2020. The ER remained closed throughout this period because we were unable to respond to emergency neurosurgery calls due to the possibility of COVID-19 in HCWs. We recommended patients to be discharged from the hospital and quarantine at home for 14 days, and the remaining inpatients and caregivers were quarantined for 14 days within the hospital. We performed RT-PCR follow-up tests every 3 days for the remaining NCU patients, and semi-weekly RT-PCR follow-up tests for the remaining patients/caregivers in the general wards during the 14 days following last exposure. The patients and caregivers who were discharged from the neurosurgery areas between 11/09/2020 and 11/13/2020 were invited by call to undergo RT-PCR testing. From our surveillance, we found that three patients/caregivers discharged from ward 6A and two patients/caregivers discharged from ward 6B were infected with SARS-CoV-2. Of the follow-up RT-PCR tests, one patient in the NCU (Fig. 1A, Case number 10) was diagnosed with COVID-19 on 11/17/2020. Unexpectedly, a nurse based in the intervention room (Case number 11) who had performed several neurovascular interventions was diagnosed with symptomatic COVID-19 on 11/18/2020. During the quarantine period of the neurosurgery-based HCWs, three symptomatic and seven asymptomatic HCWs were diagnosed with COVID-19.

Epidemiological evaluation of Gwangju city revealed that the neurosurgery outbreak originated from the community outbreak spot of the Gwangju city.

COVID-19 was confirmed in a total of four HCWs (three neurosurgery residents and one nurse, Case numbers 1−4 of Fig. 1A) by 11/14/2020. Our rapid response team was gathered in the morning of 11/15/2020, and their roles were activated according to our hospital's public health crisis situations manual (Table 1). Following this, we expanded RT-PCR testing to all hospital employees, including outsourced agencies and all hospitalized patients and their caregivers, to find asymptomatic COVID-19 patients. The first complete enumeration test (CET) was conducted from 11/13/2020 to 11/17/2020, during which a total of 730 patients, 498 caregivers, and 3,346 hospital staffs, employees and business company employees had been subjected to COVID-19 testing by RT-PCR. Table 2 shows the number of SARS-CoV-2 RT-PCR tests performed according to the professions of HCWs.

During the first CET, we found five additional asymptomatic COVID-19 cases, including a patient (Fig. 1B, Case number 5) on Floor 11 of building 1; two bankers who worked in Kwangju bank (Case numbers 6 and 7 of Fig. 1C) on floor 1; a radiologist (Case number 8, husband of Case number 6) on floor 2 of Building 1; and a caregiver on floor 9 of building 1 (Case number 9 of Fig. 1D). The overall COVID-19 positivity rate of the first CET (100 × positive tests/total number of tests conducted, 9/4,496) was 0.2%. We were later informed by a community health center on 11/16/2020 about one asymptomatic COVID-19 case (caregiver of a patient from floor 7A who was discharged on 11/14/2020 after 3 days' admission), and one symptomatic COVID-19 case (patient from floor 8 discharged on 11/13/2020 after 4 days' admission). Fig. 2 shows a schematic diagram of COVID-19 occurrence sites in CNUH. The majority of COVID-19 cases were from within building 1, and the cases in building 8 were epidemiologically related to the COVID-19 cases of building 1.

After performing four CET cycles in CNUH, and testing people outside of CNUH following calls or emergency message alerts, we discovered a total of 40 COVID-19 cases with suspicious links to CNUH, including the 22 cases detected during the neurosurgery outbreak. The second large cluster (cluster 2) of COVID-19 cases was linked to a patient on floor 11 (Fig. 1B, Case number 5). The initial SARS-CoV-2 RT-PCR test for Case number 5 was performed in the ER on 11/05/2020 and the result was negative; however, the same patient tested positive on 11/14/2020. Case number 5 had a 2D-echocardiograph on 11/12/2020. There were ten COVID-19 cases in cluster 2. Cluster 3 included a banker and a radiologist who were a husband-and-wife couple; the parents and children of the wife (banker) had tested positive for SARS-CoV-2 following RT-PCR testing. Case number 9 from floor 9 cared for her mother for 3 months, but the mother tested negative for SARS-CoV-2. In four cases of sporadic COVID-19 (Fig. 1D), there was no evidence of secondary transmission in the hospital.

During the quarantine period, the hospital quality improvement team ensured mask wearing compliance by all visitors and HCWs. Following the CNUH quarantine, our infection control policy was amended. Universal screening of all inpatients and resident caregivers was mandatory upon admission. A negative SARS-CoV-2 RT-PCR test within 48 h of admission was required. Whenever a caregiver changes, a negative SARS-CoV-2 RT-PCR test result was required.

Outpatient clinics re-opened on 11/18/2020 and the ER re-opened on 11/27/2020. On 12/01/2020, building 1 re-opened after 17 days' quarantine. Total patients admitted were 922 on 11/12/2020 before the first confirmed COVID-19 cases. The lowest number of admitted patients was 248 on 11/28/2020, and the total number of patients admitted on 12/07/2020 after the quarantine was 540. The official total of COVID-19 cases associated with the CNUH, including n order transmission, was 67.

Universal screening of all inpatients and resident caregivers is now mandatory upon admission and on day 4 of hospitalization, and HCWs are tested monthly. For HCW and patients with potential exposure to patients with COVID-19, additional RT-PCR testing every 3 days for active monitoring and PCR every 5 days for passive monitoring are mandatory. Ventilation is an important factor in preventing the virus that causes COVID-19 from spreading. We identified areas that are not well ventilated and set split air conditioning units, fan coils or any system that runs with a recirculation mode not to recirculate. In cases where the recirculation of air is unavoidable, we increased outdoor air exchange by opening windows.

We recommended keeping windows in patient rooms open and opening emergency ramp doors once per duty, but always keeping the patient doors and the curtains between beds in the multi-occupancy room closed. An audit team including infection control team, quality improvement team, and nursing staff measured the performance rate of the guidelines (hand washing, mask, ventilation, etc.) regularly.