Graphical abstract

Introduction

After the end of January 2022, the number of confirmed coronavirus disease 2019 (COVID-19) cases in the Republic of Korea began to increase with the emergence of the Omicron variant. The cases peaked in March and then started to decline gradually after April [1]. In response to this outbreak, Korean public health authorities initiated Phase 1 preparations for a return to normal life, which involved easing measures such as social distancing, starting from April 18, 2022 [2]. From May 2022, schools were given the option to switch back to in-person classes from the online learning routine that had been in place to prevent large COVID-19 outbreaks within educational settings. With the resumption of in-person classes, schools also restarted overnight programs, such as field trips [3]. In addition, schools developed COVID-19 prevention and management guidelines in collaboration with public health authorities. These guidelines aimed to establish an effective infection management system, which included monitoring for symptomatic individuals and the use of rapid self-test kits, to support a stable transition back to normal life [4].

Despite these efforts, however, a large outbreak took place among 431 individuals (404 students and 27 staff) who participated in a school field trip at High School A in Jeollabuk-do in June 2022, with 139 confirmed cases. High School A is a girls’ high school with a total enrollment of 737 students and staff (668 students and 69 staff members). The field trip was a 4-day event that took place from July 12 to July 15, 2022, in the Jeju Special Self-Governing Province. The attendees included 215 10th graders, 189 11th graders, and 27 teachers. Typically, the school monitored and tested individuals who showed symptoms to prevent and control the spread of infectious diseases. Confirmed cases were isolated to prevent further transmission within the school. On July 11, the day before the field trip, rapid antigen tests were administered to all participants. The field trip was organized into 4 teams (2 teams per grade), and each class traveled on designated buses. Furthermore, individuals exhibiting symptoms during the field trip were checked and tested daily. Those who tested positive were promptly sent home under staff supervision.

In this study, we aimed to identify risk factors in a large-scale outbreak that occurred during a school field trip when COVID-19 control measures were being eased. We seek to propose strategies and measures to prevent and manage such outbreaks in similar situations in the future.

Materials and Methods

Study Population and Case Definition

The study population comprised 737 students and staff of High School A (668 students, 69 staff), where a large COVID-19 outbreak took place after a school field trip in July 2022.

Cases at High School A were defined for the period from June 28, 2022, to July 21, 2022, using the following criteria: (1) detection of the COVID-19 gene via polymerase chain reaction or isolation of the virus, or (2) a positive result on a rapid antigen test administered by healthcare providers [5]. These criteria were in line with the diagnostic testing guidelines provided in the COVID-19 Response Guidelines. Applying this definition, a total of 190 cases were confirmed [5].

Study Design

A retrospective cohort design was utilized. Data were collected using an epidemiological survey questionnaire distributed to all students and staff. This questionnaire captured personal information, including grade, class, sex, age, participation in a field trip, the date of symptom onset, and the types of initial symptoms. Additional data were obtained from the school health administrator, which provided detailed information about the field trip, such as team divisions, bus boarding logistics, and the itinerary of locations visited at specific times. The Korea Disease Control and Prevention Agency’s COVID-19 Information System was utilized to confirm individual-level COVID-19 diagnosis statuses. This information was instrumental in defining the cases for our study.

The collected data were statistically analyzed and described to investigate the progression of the COVID-19 outbreak at High School A and the risk factors for infection during this large outbreak.

Statistical Analysis

The epidemic curve was constructed based on the time of onset of the first symptom. Among the 190 confirmed cases, 176 individuals were symptomatic (with 14 being asymptomatic); these cases were categorized by whether they were students or teachers, and by their attendance at a school field trip.

In assessing the incidence rate and risk factors, 2 separate analyses were conducted: one encompassing the entire study population and another focusing specifically on school trip attendees. Risk factors were subjected to statistical analysis and the results were expressed as odds ratios (ORs), adjusted for student/teacher status and sex, along with 95% confidence intervals (CIs).

To manage and visualize the data, Microsoft Excel ver. 2013 (Microsoft Corp.) was used, and statistical analyses were performed using the R ver. 3.6.3 (The R Foundation).

Ethics Review

This study was approved by the Institutional Review Board (IRB) of the Korea Disease Control and Prevention Agency (IRB No: KDCA-2023-07-03-PE-01).

Results

Attack Rate

Of the 737 students and staff at High School A, 190 met the definition of a case, resulting in an attack rate of 25.8%. Of the 190 cases, 174 were students, accounting for 91.6% of the cases (attack rate, 26.0%). Among student groups, 10th and 11th graders had the greatest number of cases (63 and 88, respectively), and accounted for 79.5% of the student cases. The highest attack rate was found among 11th graders (41.9%). Of all cases, 139 were attendees of the school field trip; in this group, the attack rate was 32.3%, which was 2-fold higher than that among non-attendees (Table 1).

The symptoms reported among confirmed cases included sore throat (n=100, 52.6%), fever (n=79, 41.6%), cough (n=76, 40.0%), headache (n=33, 17.4%), and phlegm (n=31, 16.3%). There were no cases of severe symptoms necessitating hospitalization (Table 2).

Epidemic Curve

There were 2 distinct waves: the first from June 28 to July 12, 2022, and the second starting after the school field trip.

The first outbreak predominantly affected 10th graders, 12th graders, and staff (since the index case was diagnosed on June 28, 2022), peaking on July 10, 2022, with 14 confirmed cases, after which it subsided.

The second outbreak began with 4 confirmed cases on the day of the school field trip on July 12, 2022. It peaked on July 16, 2022, and eventually began to slowly subside. During the second outbreak, there was an increase in the number of confirmed cases as a result of close contact with a confirmed patient attending the school trip. Most of the confirmed cases during the second outbreak were attendees of the school trip, and the second outbreak was larger than the first due to trip-specific risk factors, such as traveling in the same bus and staying in the same accommodation (Figure 1).

Risk Factors

Among all students and staff at High School A, the OR was 2.39 among school trip attendees (95% CI, 1.66–3.43; p<0.05) compared to non-attendees (Table 3).

Among the 10th and 11th graders who attended the school trip, the number of cases was 1.5 times higher among 11th graders than among 10th graders, with a statistically significant OR of 2.32 (95% CI, 1.53–3.52; p<0.05). However, there were no differences among teams within the same grade level (Table 4).

During the school trip, there were 4 different teams (2 teams in each grade level), and the duration of their contact was 34.2 hours for 10th graders and 32.8 hours for 11th graders. The duration of contact between grade levels was 4.8 hours (Figure 2).

Discussion

In response to the influx and spread of COVID-19 in 2020, educational authorities recommended that schools suspend in-person classes and transition to virtual learning from the first semester of 2020 [6]. However, to address the challenges of maintaining strict disease control and promoting sustainable COVID-19 responses amid the prolonged pandemic, authorities permitted schools to develop their own management systems starting in the second semester of 2022. This included the option to transition back to in-person classes.

The present study analyzed the characteristics and risk factors of a large outbreak that occurred as schools began to ease their control measures and resume school-wide extracurricular programs. Based on the findings, the following conclusions were drawn regarding important considerations when transitioning into phases where control measures are relaxed.

In response to the first outbreak at High School A before the school trip, High School A minimized path-crossing between classes and limited after-school programs. From June 30 to July 6, 2022, the school was conducting midterms, which resulted in only morning classes being held, thereby reducing contact among students. Consequently, from the time the index case was diagnosed until just before the school trip, a total of 35 cases were confirmed. During the subsequent outbreak, from July 12 to July 21, 2022, there were 155 confirmed cases, indicating that 81.6% of the total cases occurred in this second wave. The significant increase in cases during this period is largely attributed to the students spending extended periods together in enclosed spaces, such as buses, and staying in shared accommodations.

Despite confirmed cases of COVID-19 within the school prior to the school trip, the school decided to proceed with the trip to avoid the high costs associated with canceling prearranged accommodations and restaurant bookings. However, a more stringent management of close contacts of COVID-19 patients could have reduced the spread of the virus. Nonetheless, it is a daunting task for a single health care provider to manage all aspects of infection prevention and control in a school of 737 individuals. Therefore, it is essential to determine the optimal provider-to-individual ratio to ensure adequate staffing or resource allocation. This would enable comprehensive testing of all potential participants before a large event, such as a school trip, and the exclusion of those with confirmed infections [7].

During the school trip, an issue that arose was the difficulty in preventing teams from crossing paths, despite plans to minimize such occurrences. Space limitations made complete separation unfeasible. Additionally, the use of shared spaces like restaurants and accommodations heightened the risk of exposure in enclosed, crowded settings, which is particularly problematic for respiratory illnesses. However, the study did not monitor specific exposure scenarios for each team, complicating the task of providing a definitive explanation for the markedly different ORs observed across various grade levels. Despite these challenges, the data showed that teams sharing the same routes for prolonged periods had similar attack rates. To mitigate these risks, schools should consider organizing trips so that different grade levels visit separate regions or scheduling trips during distinct seasons, such as spring and fall [8].

At events where many attendees are expected, it is imperative to note that prevention is the best strategy for management. When planning activities such as school trips off-campus during the ongoing spread of COVID-19, it is important not only to monitor individuals for symptoms but also to screen close contacts to identify potential asymptomatic carriers in the incubation period. Should the school nurse be unable to make these determinations, schools are advised to consult with local epidemiological investigators. Additionally, an effective infection prevention and response system should be established. This system should include instructing attendees to promptly inform the chaperone if they experience symptoms and to rigorously maintain personal hygiene practices. Chaperones, in turn, must regularly check attendees for symptoms and be well-versed in the management manual to effectively handle any cases that arise.

HIGHLIGHTS

• There was a high risk for COVID-19 transmission during extracurricular activities with a large number of participants, such as field trips, even after the Omicron outbreak subsided.

• Even when students are divided into teams and follow distinct routes, designing paths that entirely prevent interactions between teams is challenging. Therefore, students still encounter each other closely during meals and in their accommodations.

• Programs should be designed carefully, and students with symptoms should be identified before and during the program in order to quarantine them promptly.

Article information

Ethics Approval

This study was approved by the Institutional Review Board of the Korea Disease Control and Prevention Agency (IRB No: KDCA-2023-07-03-PE-01).

Conflicts of Interest

The authors have no conflicts of interest to declare.

Funding

None.

Availability of Data

The datasets are not publicly available but are available from the corresponding author upon reasonable request.

Authors’ Contributions

Conceptualization: all authors; Data curation: SJK, EYK; Formal analysis: SJK; Investigation: SJK, EYK; Methodology: JY, SJK; Project administration: JY; Resources: JY; Software: SJK; Supervision: JY; Validation: JY; Visualization: SJK; Writing–original draft: SJK; Writing–review & editing: all authors. All authors read and approved the final manuscript.

Additional Contributions

Korea Disease Control and Prevention Agency provided statistical support and the photographs that constitute Figure 1.

Figure 1.

Number of symptomatic cases by date of symptom onset, school trip attendance, and grade level or staff status.

Figure 2.

Duration of contact at field trip sites between grade levels and teams. Each color denotes a field trip site, and the width of the box denotes the duration of time spent at the corresponding site (minutes).

References

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Figure & Data

References

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