Graphical abstract

Introduction

Coronavirus disease 2019 (COVID-19) is an acute respiratory infectious disease caused by the severe acute respiratory syndrome coronavirus 2 [1]. Since its initial outbreak in Wuhan, China, in December 2019, COVID-19 has led to over 770 million confirmed cases worldwide, with more than 6.9 million deaths reported as of August 27, 2023, corresponding to a fatality rate of 0.9% [2]. In the Republic of Korea, from the first confirmed case on January 20, 2020, to August 30, 2023, there have been a total of 34,572,554 confirmed cases and 35,605 deaths, yielding a significantly lower fatality rate of 0.10% [3].

The World Health Organization declared a public health emergency of international concern on January 31, 2020 [4], and later classified the situation as a pandemic on March 12, 2020 [5]. In the Republic of Korea, following a 1-month delay after the first confirmed case on January 20, 2020, authorities elevated the crisis alert to the highest level, “serious,” on February 23 [6]. Initially, COVID-19 was categorized as a class 1 statutory infectious disease under the category of emerging infectious diseases. Thanks to effective measures such as a high vaccination rate and the availability of oral medications, the disease was reclassified to class 2 on April 25, 2022, and subsequently downgraded further to class 4 on August 31, 2023.

Severity classification is critical for the development and management of effective medical response systems. It informs decisions regarding the allocation of hospital beds, options for home treatment, distribution of healthcare resources, and the formulation of treatment plans. During the early stages of the COVID-19 outbreak in the Republic of Korea, the government enforced stringent control measures. These included mandatory isolation, hospitalization, and daily health monitoring for all infected individuals, symptomatic or not, to prevent the virus from spreading. As the strategy shifted towards adapting to life with COVID-19 (“with COVID”), the focus on managing severe and critical patients intensified. Prompt identification and treatment of these high-risk cases are crucial to avoid a significant rise in mortality rates as normal activities resume. Proactive measures are particularly important, considering the potential for rapid increases in cases, as observed in other countries.

In the Republic of Korea, continuous assessments of severity throughout the pandemic have guided the allocation of hospital beds, the estimation of healthcare resources, and the production of weekly reports on severity and fatality rates. This data is a vital indicator for public health strategies, aiding in the reduction of COVID-19’s impact on community health, healthcare systems, lifestyles, and economic conditions [7].

Therefore, we analyzed the characteristics, severity, and fatality rates of patients with severe/critical COVID-19 in the Republic of Korea, from the first confirmed case in January 2020 to the conclusion of mandatory surveillance on August 30, 2023. This analysis aims to inform future pandemic control policies, resource allocation, and healthcare preparedness.

Materials and Methods

Study Population

This study examined the magnitude of outbreaks and the demographic characteristics of patients who developed severe/critical illness among confirmed COVID-19 cases, as reported by the Korea Disease Control and Prevention Agency (KDCA) COVID-19 Information Management System. The analysis covered the period from January 20, 2020, the date of the first confirmed case in the Republic of Korea, to August 30, 2023. A confirmed COVID-19 case was defined as an individual with a confirmed infection identified either through COVID-19 gene detection or virus isolation, regardless of clinical symptoms. This definition also included individuals who exhibited COVID-19 symptoms and had a confirmed infection determined by rapid antigen testing (implemented by experts since March 14, 2022) or emergency screening tests [8].

We classified the severity of COVID-19 into 8 stages: stages 1 to 2, mild/less severe illness (stage 1, no interference with daily life; stage 2, interference with daily life, but no need for oxygen therapy); stages 3 to 4, moderate illness requiring oxygen therapy (stage 3, oxygen therapy with nasal prongs; stage 4, oxygen mask); stages 5 to 7, severe/critical illness involving advanced oxygen support or organ failure (stage 5, non-invasive mechanical ventilation/high-flow oxygen therapy; stage 6, invasive mechanical ventilation; stage 7, multi-organ damage, extracorporeal membrane oxygenation, and continuous renal replacement therapy); and stage 8, death [8].

We primarily analyzed data from stages 5 to 7 (severe/critical) and stage 8 (death) to calculate the severity and fatality rates. However, we recognize the potential for underreporting of severe/critical cases, particularly regarding deaths at long-term care facilities. To account for this possible underestimation, we included death data in our adjustments [7].

The clinical status of confirmed cases was monitored through the KDCA COVID-19 Information Management System and the Integrated Reporting Portal for Health and Medical Resources. When necessary, phone calls and emails were utilized. Data concerning COVID-19 fatalities were gathered using the KDCA system in accordance with the Infectious Disease Prevention and Management Act in the Republic of Korea. Reported death cases were subject to expert review, and those attributed to other diseases or external causes were excluded to maintain data accuracy [9].

All collected data were anonymized to protect patient privacy. Personal information was removed, and unique identification numbers were assigned to each dataset. The anonymized data were then stored on encrypted computers with restricted external access.

Data Analysis

To analyze the characteristics of severe/critical patients, we conducted a frequency analysis based on demographics such as sex, age, and reported region, with the data categorized by year. We assessed community severity by classifying cases into 2 categories: severe/critical and death. We then calculated severity and fatality rates by year and age group. The severity rate indicates the percentage of confirmed cases that were classified as severe, critical, or deceased during the mandatory surveillance period. This rate was determined by calculating the proportion of severe, critical patients, and deaths among the confirmed cases and then multiplying by 100% to generate a percentage (%). Similarly, the fatality rate was calculated as the proportion (%) of deaths among confirmed cases during the same period. To compute age-standardized severity and fatality rates, we used mid-year population data, which reflects the population distribution for the respective years [10]. Age groups were divided into 10-year intervals to enable a comparison of severity rates across different ages. Basic statistical analyses were conducted using Microsoft Excel ver. 2018 (Microsoft Corp.).

Ethics Approval

The study protocol was approved by the Institutional Review Board (IRB) of the KDCA (IRB-2022-10-03-PE-A). Obtaining informed consent was exempted by the IRB as there was no personal information in the study.

Results

General Characteristics

During the mandated COVID-19 surveillance period from January 20, 2020, to August 30, 2023, a total of 34,572,554 confirmed cases were recorded. Among these, 38,112 (0.11%) progressed to a severe/critical condition. Notably, the highest number of confirmed cases was observed in 2022, accounting for 52.8% (20,136) of all severe/critical cases identified during the monitoring period (Table 1).

Daily monitoring of severely/critically ill patients showed a peak in new severe/critical cases on March 29, 2022, with 244 reported cases. However, the number of hospitalized severe/critical cases reached a peak of 1,315 on March 31, 2022, with a daily average of 285. This is more than 10 times the daily average of new severe/critical cases (Table 1; Figure 1).

Men constituted the majority of severe/critical cases, totaling 22,005 cases or 57.7%. The average age of patients in severe/critical conditions over the period was 71.3 years, with a standard deviation (SD) of 15.7 years. This average age showed slight variations annually: it was 70.6 years (SD, 12.2 years) in 2020, dropped to 65.7 years (SD, 15.3 years) in 2021, and increased to 73.2 years (SD, 15.6 years) in 2022 and 73.8 years (SD, 15.4 years) in 2023. Notably, except for 2021, the average age consistently remained in the 70s.

In terms of age distribution, individuals aged ≥80 years constituted the largest group, accounting for 34.7% (13,218 cases), followed by those aged 70 to 79 years at 26.4% (10,072 cases), and 60 to 69 years at 20.7% (7,893 cases). Notably, individuals aged ≥60 years comprised 81.8% (31,183 cases) of all severe/critical patients (Table 1).

Over half of the severe/critical cases (61.7%, 23,510 cases) were reported in metropolitan areas (Seoul, Gyeonggi, and Incheon) (Table 1). Non-metropolitan areas contributed 38.0% (14,476 cases) of the cases, with only a negligible 0.3% (126 cases) identified through quarantine procedures. Within the metropolitan regions, Seoul accounted for the highest proportion of cases (27.3%, 10,401 cases), followed by Gyeonggi (26.9%, 10,260 cases) and Incheon (7.5%, 2,849 cases). Among non-metropolitan areas, Busan recorded the highest proportion of cases (6.2%, 2,366 cases), followed by Daegu (4.8%, 1,820 cases) and Chungnam (4.0%, 1,510 cases) (Table 1).

Clinical Characteristics

The average duration from diagnosis to the onset of severe/critical illness was 6.6 days. This duration has decreased over time: from 8.2 days in 2020, to 7.7 days in 2021, 6.4 days in 2022, and down to 5.0 days in 2023, as shown in Table 2. Children aged 0 to 9 years experienced the quickest progression to severe/critical illness, averaging 4.5 days, while patients aged 30 to 39 years had the slowest progression, averaging 7.5 days.

The average duration of isolated treatment for severe/critical cases was 9.3 days. This duration decreased over time, similar to the progression to severe/critical illness: from 14.0 days in 2020 to 10.7 days in 2021, 8.7 days in 2022, and 7.8 days in 2023 (Table 2). The shortest average isolated treatment period was observed in individuals aged 10 to 19 years (6.6 days), while the longest was in those aged 70 to 79 years (9.9 days).

Trends in the Severity and Fatality Rates

The overall severity rate of COVID-19 throughout the study period was 0.19%. There was a significant annual decrease, with the rate dropping from 4.33% in 2020 to 0.14% in 2022 and 0.15% in 2023, compared to 2.25% in 2021. The standardized severity rate remained consistent at 0.19% during the study period. Annually, the rates were 3.19% in 2020, 2.23% in 2021, 0.15% in 2022, and 0.12% in 2023, demonstrating a decreasing trend similar to that of the severity rate.

The overall fatality rate during the outbreak was 0.10%. The severity of the outbreak showed a consistent decline, with fatality rates decreasing to 2.16% in 2020, 0.91% in 2021, 0.09% in 2022, and 0.06% in 2023. Throughout the study period, the standardized fatality rate remained at 0.10%. Over time, this rate also showed a downward trend, falling from 1.58% in 2020 to 0.95% in 2021, 0.10% in 2022, and finally to 0.05% in 2023.

Both severity and fatality rates generally increased with age across all years. However, deviations from this trend were observed in 2021 compared to 2020. Specifically, the severity rate for individuals aged ≤59 years rose in 2021, while it declined for those aged ≥60 years. Similarly, the fatality rate in 2021 decreased relative to 2020 for all age groups, except for those aged 0 to 9, 20 to 29, and 40 to 49 years. These findings indicate that age-related changes in severity may exhibit slight annual variations (Table 3).

Discussion

This study represents the first comprehensive analysis of data on all severe/critical COVID-19 patients in the Republic of Korea, spanning from the initial confirmed case to the conclusion of mandatory surveillance. In 2022, the rise of the Omicron variant precipitated a marked escalation in confirmed cases, especially among high-risk individuals in vulnerable facilities [11,12]. This increase underscores the potential for a corresponding rise in severe/critical cases among older adults.

To prepare for this scenario, researchers analyzed the severity and fatality rates of Omicron cases using data from the ongoing monitoring of severe/critical patients [13]. This scientific evidence enabled the government to quickly communicate the necessity for preventive measures, including border restrictions and stricter quarantine protocols, to curb the spread of the variant [14]. The monitoring system implemented for severe/critical cases played a key role in an effective response to infectious diseases, ensuring preparedness and protecting citizens. We also consider it to be a contributing factor in maintaining a relatively stable proportion of older adults among these high-risk patients over time.

An analysis of critically ill patients by age revealed that most were older adults (≥60 years). The proportion of patients aged ≥80 years increased starting in 2022, and pediatric cases (<19 years) began to emerge in 2021. Notably, while individuals aged ≥60 years accounted for only about 20% of the total COVID-19 cases throughout the study period, they represented 70% to 80% of the critically ill patients. This disparity became even more pronounced from 2022 to 2023, with this age group accounting for over 85% of critically ill cases. Interestingly, a similar trend was observed in the United States, where 62.5% of hospitalized patients with COVID-19 from January to August 2023 were aged ≥65 years [15].

The average time from diagnosis to the onset of severe/critical illness was 6.6 days, with the youngest age group (under 19 years) experiencing the shortest duration, averaging 5.1 days. This duration has decreased over time, falling from 8.2 days in 2020 to 5.0 days in 2023 (Figure 1, Figures S1, S2).

The average duration of isolated treatment for severe/critical cases was 9.3 days. Individuals aged ≥60 years experienced the longest isolation period, averaging 9.5 days. This prolonged duration may be attributed in part to policy changes, including modifications to isolation criteria in response to new variants and changes in the severity of the disease over the course of the pandemic.

Additional analyses are necessary to comprehensively assess the factors influencing disease progression. This involves considering patient factors, including underlying conditions, and clinical factors, such as vaccination status and the use of COVID-19 therapeutics. Such analysis is crucial for evaluating the effectiveness of vaccination programs.

This study underscores the importance of rapid and efficient responses to future pandemics, though several limitations persist. First, the current surveillance system for patients with severe COVID-19 depends on data reported by public health centers and medical institutions. Delays in reporting, particularly during spikes in severe cases or patient deaths, can result in an underestimation of the true extent of severe illness. While the Republic of Korea employs various systems to ensure accurate monitoring, challenges in data linkage highlight the necessity for enhanced monitoring systems in future pandemics. Second, this study utilized descriptive statistics to explore overall trends in severity across the entire population. However, the absence of detailed information necessary for comparing urban and rural areas or for analyzing specific types of medical institutions limited the scope of the analysis. Moreover, variations in the definitions of severe/critical cases across different countries complicate international comparisons. For example, while the Republic of Korea uses specific criteria such as intensive care unit admission or ventilator use to classify severe cases, some countries consider all hospitalized cases as severe. These discrepancies hinder uniform analysis and make cross-country comparisons challenging. These constraints underscore the need for integrating diverse datasets and conducting more standardized, in-depth investigations to enable more refined analyses.

Despite the decreasing trends in severity and fatality rates, our findings highlight the ongoing vulnerability of older adults. It is crucial to focus on protective and management strategies, particularly prioritizing vaccination efforts for this age group, to prepare for future outbreaks.

HIGHLIGHTS

• We collected demographic and clinical characteristics of confirmed coronavirus disease 2019 cases from January 20, 2020 to August 30, 2023 to assess trends in health status related to the virus.

• The severity and fatality rates were 0.19% and 0.10%, respectively.

• The majority of severe/critical cases occurred in individuals aged ≥60 years, with more than half (52.8%, 20,136 cases) identified during the 2022 surveillance period.

• Intensive protection and management of older adults and high-risk groups, as well as prompt and accurate surveillance and analysis of severe/critically ill patients, are crucial for reducing severity and fatality rates.

Supplementary Material

Article information

Ethics Approval

The study protocol was approved by the Institutional Review Board of the KDCA (IRB-2022-10-03-PE-A). Obtaining informed consent was exempted by the IRB as there was no personal information in the study.

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: SJJ; Data curation: SJJ, SYP; Formal analysis: SJJ, JHJ; Investigation: SJJ, MS, BR, SYC; Methodology: SJJ, SYP; Project administration: SJJ; Resources: SJJ; Software: SJJ; Supervision: SSK; Validation: SJJ, SYP, JHJ; Visualization: SJJ, JHJ; Writing–original draft: SJJ; Writing–review & editing: all authors. All authors read and approved the final manuscript.

Figure 1.

Trends in monthly confirmed cases, severe/critical cases, and deaths due to COVID-19 during the study period. The solid lines represent monthly averages, while the peak values provided above the graphs correspond to the single highest daily value recorded within each month. For example, the maximum number of daily cases was recorded on March 17, 2022, within the monthly trend for March 2022, is represented as “March 17, 2022.” This distinction between monthly averages and daily peak values is intended to provide both overall trends and critical peak days during the pandemic.

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References

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