PHWR Public Health Weekly Report

By the end of August 2022, approximately 600 million confirmed cases of coronavirus disease 2019 (COVID-19) and 6.5 million deaths from COVID-19 have been recorded [1]. Multiple COVID-19 waves have occurred in the Republic of Korea (ROK), resulting in approximately 23 million confirmed cases and 27,000 deaths [2]. In particular, approximately 50% of these cases and deaths occurred in the Seoul metropolitan area (SMA; Seoul, Incheon, Gyeonggi), a densely populated area with more than 50% of the total population and comprises well-developed transportation systems and active inter-regional exchanges.

A system to halt the import, transmission, and spread of the virus and the establishment of health care measures to treat patients are the most fundamental and critical factors in COVID-19 response. In particular, stricter infection control measures have been enforced, and numerous health care resources have been directed to the SMA compared to other non-metropolitan areas following a rapid spike in newly diagnosed patients due to concerns of a nationwide outbreak of the virus. Therefore, in this report, we sought to analyze the COVID-19 situations and characteristics of each wave in the SMA and describe the infection control measures implemented during each period.

As COVID-19 has had a significant impact on the health care systems and economies of many countries worldwide, the major characteristics of each wave must be identified and any emerging issue must be resolved to proactively respond to future public health crises. The details of trends (by region and age), changes across waves, types of infection control measures implemented depending on the type of wave, timing of these measures, and any associated factors were examined to comprehensively review the situations and responses related to COVID-19.

In this report, the characteristics of the domestic outbreak and spread of COVID-19 in the SMA during each nationwide COVID-19 wave from January 20, 2020 (index case) to August 31, 2022, were described based on the data in the COVID-19 information management system.

Six waves were identified: Wave 1 - first imported case of COVID-19 and occurrence of the wave in Daegu, Gyeongbuk, and the SMA (January 20, 2020 to August 11, 2020); Wave 2 - rapid spread throughout the SMA, including mass outbreaks in mass gatherings in central Seoul on August 15, 2020, and mass outbreaks in religious organizations (August 12, 2020 to November 12, 2020); Wave 3 - nationwide spread of a new variant of the virus (November 13, 2020 to July 6, 2021); Wave 4 - spread of the Delta variant (July 7, 2021 to January 29, 2022); Wave 5 - explosive spike in confirmed cases nationwide due to the Omicron variant (January 30, 2022 to June 25, 2022); and Wave 6 - continued emergence of new variants, including Omicron sub-variants (June 26, 2022 to August 31, 2022).

From the detection of the first COVID-19 case in January 2020 to the end of August 2022, a total of 12,122,122 confirmed cases (46,581.8 per 100,000 population in the SMA) have been recorded in the SMA, which accounts for 52.3% of the total cases in the country. By wave, the confirmed cases in the SMA accounted for 23.5% of the nationwide cases during the first wave, which mainly occurred in Daegu and Gyeongbuk. Since then, approximately 70% of the cases were identified in the SMA until the fourth wave. Since the fifth wave, in which the Omicron variant became the dominant variant, the incidence of COVID-19 skyrocketed throughout the country, with 50% of cases recorded in the SMA (Figure 1). As depicted, the SMA is vulnerable to infectious disease outbreaks in the country and is heavily influenced by such outbreaks. As a result, the local governments have promptly and flexibly adjusted and enforced infection control measures, such as social distancing, in response to the waves and increased the available infection control resources. Below, the trends of each wave in the SMA and the subsequent implemented measures are discussed.

Figure 1. Daily confirmed cases of COVID-19 in Seoul metropolitan area by wave

1. First Wave (January 20, 2020 to August 11, 2020): Situations and Characteristics of the Wave and Major Infection Control Measures in the Seoul Metropolitan Area

During the first wave, a total of 2,844 cases (10.9 per 100,000 population) were confirmed in the SMA, accounting for 23.5% of the total cases in the country (Table 1). By region, the highest number of cases were recorded in Seoul (n=1,335; 14.0 per 100,000 population), followed by Gyeonggi (n=1,201; 8.9 per 100,000 population) and Incheon (n=308; 10.4 per 100,000 population). The rate of severe infection was highest in Seoul (n=67; 5.02%), while the fatality rate was highest in Gyeonggi (n=32; 2.66%) (Supplementary Table 1). The incidence rate per 100,000 population was the highest for adults 60 years and older, with an incidence of 15.1 persons, followed by ages 19–59 years (11.0 persons) and ≤18 years (4.3 persons) (Table 1).

Table 1 Epidemiological characteristics COVID-19 confirmed cases in Seoul metropolitan area by wave

Category	1st wave	2nd wave	3rd wave	4th wave	5th wave	6th wave
Confirmed cases	2,844 (100.0)	9,166 (100.0)	88,698 (100.0)	455,137 (100.0)	9,144,856 (100.0)	2,421,421 (100.0)
Daily average confirmed cases (n)	13.9	99.6	377.4	2,209.4	62,636.0	36,688.2
Incidence rate (per 100,000)	10.9	35.2	340.8	1,749.0	35,141.1	9,304.8
Severe/critical casea) (n)	124	432	2,405	6,786	5,716	1,655
Case severe rate (%)	4.36	4.71	2.71	1.49	0.06	0.07
Death cases (n)	50	172	1,047	3,729	7,658	1,007
Case fatality rate (%)	1.76	1.88	1.18	0.82	0.08	0.04
Sex
Male	1,361 (47.9)	4,175 (45.5)	44,771 (50.5)	236,004 (51.9)	4,276,881 (46.8)	1,106,698 (45.7)
Female	1,483 (52.1)	4,991 (54.5)	43,927 (49.5)	219,133 (48.1)	4,867,975 (53.2)	1,314,723 (54.3)
Age group (yr)
0–18	172 (6.0)	763 (8.3)	9,898 (11.2)	91,134 (20.0)	2,086,612 (22.8)	455,959 (18.8)
Incidence rate (per 100,000)	4.3	19.1	247.4	2,277.6	52,147.6	11,395.1
19–59	1,784 (62.7)	5,089 (55.5)	57,425 (64.7)	273,445 (60.1)	5,589,176 (61.1)	1,533,913 (63.3)
Incidence rate (per 100,000)	11.0	31.5	355.5	1,692.9	34,602.9	9,496.5
60≤	888 (31.2)	3,314 (36.2)	21,375 (24.1)	90,558 (19.9)	1,469,068 (16.1)	431,549 (17.8)
Incidence rate (per 100,000)	15.1	56.5	364.2	1,542.8	25,028.5	7,352.3
Nationality
Korean	2,734 (96.1)	8,956 (97.7)	83,474 (94.1)	415,662 (91.3)	8,906,908 (97.4)	2,359,368 (97.4)
Foreigner	110 (3.9)	210 (2.3)	5,224 (5.9)	39,475 (8.7)	237,948 (2.6)	62,053 (2.6)

Unit: person (%). ^a)Severe/critical case: patient receiving isolated treatment through high flow therapy, respirator, ECMO (extracorporeal membrane oxygenation), and CRRT (continuous renal replacement therapy)..

During this period, mass outbreaks occurred in Daegu and Gyeongbuk primarily via a specific religious organization (Shincheonji Daegu Church). Most cases in the SMA were due to mass outbreaks in a call center in Guro, Seoul, in March 2020, an adult entertainment facilities in the SMA in April 2020, and a logistic center in Gyeonggi in May. Multiple sporadic occurrences of cases related to these mass outbreaks were reported throughout the SMA.

In response to the surge of confirmed cases, the delivery of messages stressing social distancing, including minimizing contact with others, began on February 29, 2020. As the infection spread, each city and province implemented additional measures, such as administrative orders to ban mass gathering in different facilities, mandatory screening, and stronger infection control measures [4,5]. On May 29, 2020, stricter infection control measures were implemented in the SMA, including temporary closure of public facilities and recommendations to temporary close publicly used facilities. Furthermore, starting on June 28, 2020, the “social distancing” measures were divided into stages 1–3 and implemented according to the severity of the COVID-19 wave and level of infection control measures nationwide (Figure 2, Table 2).

Figure 2. Daily confirmed cases of COVID-19 and mainly control measure in Seoul metropolitan area, first wave

Table 2 Key contents of social distancing in Seoul metropolitan area by wave

Period	Step	Date	Main contents
1st wave	Social distancing	February 29, 2020–March 21, 2020	Recommendations such as refraining from going out, minimizing contact with others, etc.. Close daycare center (Feb 27, 2020) and social welfare facilities (Feb 28, 2020), etc..
	Enhanced social distancing (1st)	March 22, 2020–April 5, 2020	Postponing or canceling non-essential outings or gatherings, returning home immediately after work, etc.. Suspend operating religious, sports, and entertainment facilities for 15 days, etc..
	Enhanced social distancing (2nd)	April 6, 2020–April 19, 2020	Additional measures. -. Strengthening infectious disease prevention and control of nursing/psychiatric hospitals and church (self-inspection by designation of a person in charge of quarantine, etc.)
	Mitigated social distancing	April 20, 2020–May 5, 2020	Some relaxation of existing measures. -. Recommend to resume operating outdoor facilities, Suspend operating high-risk facilities → Recommendation to refrain from operating.
	Distancing in daily life	May 6, 2020–	Transition of social distancing in daily life. -. Daily life is permitted in compliance with quarantine guidelines. Implement enhanced social distancing in metropolitan area (May 29, 2020–June 14, 2020). -. Recommend to refrain from operating entertainment bar, karaoke, private institute, Suspend operating public facilities, etc..
	Social distancing level 1 (1/3)	June 28, 2020–	Unification of names through social distancing, divided into three levels. -. Maintaining the ‘distancing in daily life’ stage for metropolitan area.
2nd wave	Social distancing level 2 (2/3)	August 16, 2020–	Seoul and Gyeonggi province upgraded to the level 2. -. Mandatory infectious disease prevention and control measures (restrict the number of users, movement within facilities, etc.), refrain from moving to other cities and provinces. Apply the “Level 2” to the entire metropolitan area and additional action. -. Prohibit gathering more than 50 people indoors/100 people outdoors, Only non-face-to-face worship allowed, etc..
	Enhanced social distancing level 2 (2/3)	August 30, 2020–September 13, 2020	Only take-out and delivery allowed from 21-05:00 at restaurants and cafes. Indoor sports facilities, reading rooms, and study cafes are prohibited, etc.. Only non-face-to-face classes are allowed at private institutes, Ban visiting to nursing hospitals and facilities, etc..
	Social distancing level 2 (2/3)	September 14, 2020–October 11, 2020	Maintain the “Level 2” (mitigating some control measures). -. Restaurants, cafes are allowed to operate after 9 o’clock, etc.. -. Lift of the ban on gatherings at indoor sports facilities, etc. (apply distancing rules between users, etc.).
	Social distancing level 1 (1/3)	October 12, 2020–November 6, 2020	Adjust the social distancing level 1 (partial maintained).
	Social distancing level 1 (1/5)	November 7, 2020–November 18, 2020	Apply reformed social distancing level 1 (1/1.5/2/2.5/3, total 5 stages in total). -. Apply mandatory control rules for the events gathering of 500 or more people and 23 types of intensive and general management facilities, etc..
3rd wave	Social distancing level 1.5 (2/5)	November 19, 2020–	Applied to Seoul and Gyeonggi province (Incheon will be applied from the 23rd). -. Add restriction on the number of users in major facilities and prohibition of eating and drinking, etc..
	Social distancing level 2 (3/5)	November 24, 2020–December 7, 2020	Prohibit gatherings entertainment facilities and expand restrictions on the number of users in general facilities, etc..
	Social distancing level 2.5 (4/5)	December 8, 2020–February 14, 2021	Cessation of operating movie theaters and department stores after 21:00, application of non-face-to-face principles such as worship services, etc.. Strengthen the quarantine measures during the New Year holidays, prohibit gathering of 5 or more people (Dec 24, 2021).
	Social distancing level 2 (3/5)	February 15, 2021–July 11, 2021	Downgraded and maintained on level 2. Announce strengthen control measures in the metropolitan area (Jul 4, 2021) and implementation of additional measures (Jul 7, 2021). -. Reinforcement of diagnostic tests (install additional screening centers), Strengthen preemptive tests for high-risk groups, etc..
4th wave	Social distancing level 4 (4/4)	July 12, 2021–October 31, 2021	Apply reformed criteria (level 4). -. Allow private gatherings up to four people (until 18, after that, only 2 people are allowed). -. Prohibit events and gatherings, gatherings at entertainment facilities, operate public facilities until 10 p.m..
	Step-by-step daily recovery	November 1, 2021–	Transition to step-by-step daily recovery. Strength measures for medical response in the metropolitan area (Nov 5, 2021). -. Secure the number of hospital beds, reinforce control at high risk facilities, etc..
	Enhanced social distancing	December 18, 2021–April 17, 2022	(Restrictions on private gatherings) 4 people → 6 people (Jan 17, 2022) → 8 people (Mar 21, 2022) → 10 people (Apr 4, 2022). (Operating time limit) Limited to 9 p.m. or 10 p.m. → 10 p.m. (Feb 19, 2022) → 11 p.m. (Mar 5, 2022) → 0 a.m. (Apr 4, 2022).
5th wave ‧ 6th wave	The lifting of social distancing	April 18, 2022–	The lifting of social distancing.

2. Second Wave (August 12, 2020 to November 12, 2020): Situations and Characteristics of the Wave and Major Infection Control Measures in the Seoul Metropolitan Area

During the second wave, a total of 9,166 cases (35.2 per 100,000 population) were confirmed in the SMA, which was 3.2 times more than that in the first wave and accounted for 77.5% of the total cases in the country (Table 1). By region, the highest number of cases was confirmed in Seoul (n=4,679; 49.2 per 100,000 population), followed by Gyeonggi (n=3,839; 28.3 per 100,000 population) and Incheon (n=648; 22.0 per 100,000 population). The rate of severe infection was highest in Incheon (6.33%), and the fatality rate was highest in Gyeonggi (2.24%) (Supplementary Table 2). The incidence rate per 100,000 population was the highest for adults aged 60 years and older (56.5 persons), followed by that for adults aged 19–59 years (31.5 persons) and individuals ≤18 years (19.1 persons). The percentage of adult patients aged 60 years and older increased from 31.2% in the first wave to 36.2% in the second wave. Owing to the increased number of older adult patients, the severe infection rate (4.36%→4.71%) and fatality rate (1.76%→1.88%) increased compared to those recorded in the first wave (Table 1).

During this period, most cases of COVID-19 in the ROK occurred in the SMA. Mass outbreaks occurred throughout the area owing to a mass gathering in August 2020, smaller-scale outbreaks through churches, door-to-door sales, traditional markets, and schools.

In response to the continued spread primarily throughout the SMA, stricter infection control measures were implemented in this area. The level of social distancing was increased to level 2 in Seoul and Gyeonggi from August 16, 2020, and Incheon from August 19, 2020, and the emergency response team was activated in the SMA (from August 17, 2020). From August 30, 2020, stricter infection control measures were enforced, including a ban on gathering after 21:00 in restaurants and cafe in the SMA. Further, during the special infection control period during Chuseok (September 28, 2020 to October 11, 2020), gathering was prohibited in 11 types of high-risk facilities in the SMA (Figure 3, Table 2).

Figure 3. Daily confirmed cases of COVID-19 and mainly control measure in Seoul metropolitan area, second wave

3. Third Wave (November 13, 2020 to July 6, 2021): Situations and Characteristics of the Wave and Major Infection Control Measures in the Seoul Metropolitan Area

During the third wave, a total of 88,698 cases (340.8 per 100,000 population) were confirmed in the SMA, which was 9.7 times more than that confirmed in the second wave and accounted for 69.6% of the total cases in the country (Table 1). By region, Seoul had the highest number of cases (n=44,642; 469.4 per 100,000 population), followed by Gyeonggi (n=38,449; 283.4 per 100,000 population) and Incheon (n=5,607; 190.2 per 100,000 population) (Supplementary Table 3). The incidence rate per 100,000 population was the highest for adults 60 years and older, followed by adults 19–59 years and individuals ≤18 years; however, the percentage of adult patients aged 60 years and older declined from 36.2% in the second wave to 24.1% in the second wave. As a result, the rate of severe infection (4.71%→2.71%) and fatality rate (1.88%→1.18%) decreased compared to those recorded in the second wave. The percentage of foreigners with COVID-19 increased to 6% (Table 1).

One prominent feature of this period was the continued emergence of major variants (Alpha in December 2020, Beta and Gamma in January 2021, Delta in April 2021) and their spread throughout the country. Numerous cases were increased confirmed in environments closely linked to people’s daily lives, such as long-term care facilities and health care facilities, sports and leisure facilities, workplace, and social gathering events with family and friends. Starting with a case confirmed on November 26, 2020 in the Seoul Dongbu Detention Center, the number of newly diagnosed cases skyrocketed to approximately 1,200 until the end of January 2021. Moreover, hundreds of cases were reported in areas with a high concentration of foreign workers in the Gyeonggi province.

In response to the surging COVID-19 cases in the SMA, the level of social distancing was first increased to 1.5 (from November 19, 2020) in Seoul and Gyeonggi and then to 2 throughout the SMA (from November 24, 2020). On December 8, 2020, the level of social distancing was increased to 2.5 throughout the entire SMA, and was maintained until February 2021 (Lowered to level 2 in SMA in February 2021). The COVID-19 vaccine rollout began on February 26, 2021, first with high-risk workers and residents of vulnerable facilities. A special infection control period was then established in the SMA (from March 15, 2020). During this period, administrative orders to carry out intensive inspections for violation of infection control measures and mandatory screening test for foreign workers were enforced. Foreign workers were not subjected to the infection control measures because many share work and living spaces. Thus, the Ministry of Employment and Labor launched a “Special inspection of infection control status in workplaces hiring foreign workers” (from March 4, 2021). An administrative order to mandate diagnostic testing of symptomatic individuals per physician and pharmacist recommendation was implemented in the SMA (Incheon, from April 14, 2021; Seoul and Gyeonggi, from April 15, 2021) (Figure 4, Table 2).

Figure 4. Daily confirmed cases of COVID-19 and mainly control measure in Seoul metropolitan area, third wave

4. Fourth Wave (July 7, 2021 to January 29, 2022): Situations and Characteristics of the Wave and Major Infection Control Measures in the Seoul Metropolitan Area

During the fourth wave, a total of 455,137 cases (1,749.0 per 100,000 population) were confirmed in the SMA, which was 5 times more than that confirmed in the third wave and accounted for 71.7% of the total cases in the country (Table 1). By region, the highest number of cases was confirmed in Seoul (n=213,785; 2,248.1 per 100,000 population), followed by Gyeonggi (n=199,981; 1,474.2 per 100,000 population) and Incheon (n=41,371; 1,403.2 per 100,000 population) (Supplementary Table 4). The incidence rate per 100,000 population was the highest for individuals ≤18 years, followed by adults 19–59 years and adults 60 years and older. As the percentage of patients aged 59 years or younger increased to approximately 80%, the rate of severe infection (2.71%→1.49%) and fatality rate (1.18%→0.82%) decreased compared to those in the third wave (Table 1). The percentage of foreigners with COVID-19, which first arose as a problem in the second wave, increased to 8.7%. In fact, the incidence of COVID-19 among foreign workers was high (11.5%) in Gyeonggi, which has many small-sized manufacturers that employ a high percentage of foreign workers (Supplementary Table 4).

During this period, the transmission of the Delta variant was extensive, and smaller outbreaks or infection transmissions via individual contact increased in addition to mass outbreaks in the SMA. Infections primarily occurred in facilities mainly used by school-aged children, such as private institutes, study facilities, and schools; publicly used facilities, such as karaoke rooms; and facilities enabling frequent contact, such as traditional markets in Seoul and Gyeonggi. A mass outbreak also occurred on the US military base in Pyeongtaek, Gyeonggi. At the end of January 2022, the Omicron variant became the dominant variant (detection rate: 72%), and the infection spread further, with the reproduction number exceeding 1.

In response to the increased spread among the younger generations, stricter infection control measures were implemented in the SMA, including more diagnostic testing for people in their 20s and 30s and screening test for high-risk groups (from July 7, 2021). Some local governments launched a campaign promoting at least one testing per household. From July 12, 2021, the level of social distancing was increased to 4, with private gatherings limited to four people until 18:00 and two people after 18:00, and a ban on public events, mass gatherings, and gatherings in adult entertainment facilities. In response to the increased number of cases, including among foreign workers, joint measures for infection control for foreign workers (August 29, 2021) were devised. On November 1, 2021, a transition to normalcy was attempted; however, because of another surge in COVID-19 cases, an administrative order was issued on November 5, 2021, to secure COVID-19 beds in health care facilities in the SMA. Thereafter, at-home treatment was established as the basic treatment plan for COVID-19 (from November 26, 2021). Stricter social distancing measures were then enforced, including limits on private gatherings and operations of publicly used facilities (December 18, 2021) (Table 2). As the Omicron variant began to spread more extensively, the testing system was restructured (PCR test prioritized for high-risk groups, self-test kits for all other groups) in preparation for a surge of patients in areas with the Omicron variant as the dominant variant, namely Pyeongtaek, Anseong, Gwangju, and Jeonnam (from January 26, 2022). Furthermore, the mandatory quarantine for patients treated at home was shortened from 10 days to 7 days (3 days of voluntary quarantine added for unvaccinated individuals) (from January 26, 2022) (Figure 5).

Figure 5. Daily confirmed cases of COVID-19 and mainly control measure in Seoul metropolitan area, fourth wave

5. Fifth Wave (January 30, 2022 to June 25, 2022): Situations and Characteristics of the Wave and Major Infection Control Measures in the Seoul Metropolitan Area

During the fifth wave, a total of 9,144,856 cases (35,141.1 per 100,000 population) were confirmed in the SMA, which was the highest of all the waves and accounted for 52.3% of the total cases in the country. The total case number was 20 times higher than that of the fourth wave and 3,200 times higher than that of the first wave (Table 1). By region, the highest number of cases was confirmed in Gyeonggi (n=4,727,109; 34,846.7 per 100,000 population), followed by Seoul (n=3,386,678; 35,613.8 per 100,000 population) and Incheon (n=1,031,069; 34,970.8 per 100,000 population) (Supplementary Table 5). The incidence rate per 100,000 population was the highest for individuals ≤18 years, followed by adults 19–59 years and adults aged 60 years and older. Similar to the fourth wave, the highest incidence was among school-aged individuals. In fact, the percentage of patients younger than 60 years gradually increased, accounting for 84% of the total cases. Owing to the characteristics of the virus, vaccine, and therapeutic agents, the rate of severe infection and fatality rate were markedly low compared to those of previous waves (severe infection rate 0.06%, fatality rate 0.08%). The percentage of foreigners with COVID-19 decreased to 2.6% compared to that recorded in the fourth wave (Table 1).

During this period, the Omicron variant spread quickly and resulted in a substantial increase in the number of infected patients nationwide. However, as the severe infection rate and fatality rate were lower than those associated with the previous dominant variant, Delta, response measures for the Omicron wave were focused on the prescription of drugs to prevent severe infections and death in high-risk groups and enhancement of medical response capacities.

To more efficiently respond to the rapidly increasing number of COVID-19 cases, patients with COVID-19 were subjected to a mandatory quarantine of 7 days, regardless of their vaccination status. Mandatory quarantine for contacts was applied only to unvaccinated individuals who live with the patient or close contacts in vulnerable facilities. Anti-COVID-19 pass was suspended for all facilities (from March 1, 2022), and positive results on approved rapid antigen tests began to be accepted as the basis of COVID-19 diagnosis (March 14, 2022). Any remaining social distancing measures were lifted as the wave subsided at the end of March (from April 18, 2022). Dining in publicly used facilities has been allowed since April 25, 2022, and mandatory use of face mask outdoors was lifted on May 2, 2022 (mandatory mask for gatherings of 50 people or more) (Figure 6).

Figure 6. Daily confirmed cases of COVID-19 and mainly control measure in Seoul metropolitan area, fifth wave

6. Sixth Wave (June 26, 2022 to August 31, 2022): Statistics and Characteristics of the Wave and Major Infection Control Measures in the Seoul Metropolitan Area

During the sixth wave, a total of 2,421,421 cases (9,304.8 per 100,000 population) were confirmed in the SMA, which was the second highest of all the waves and accounted for 49.4% of the total cases in the country (Table 1). Although the number of confirmed cases increased continuously from the first to the fifth wave, the number of patients in the sixth wave was 3.7 times lower than that during the fifth wave, and the cases in SMA accounted for less than 50% of the total cases in the country. By region, Gyeonggi had the highest number of cases (n=1,266,845; 9,338.8 per 100,000 population), followed by Seoul (n=903,343; 9,499.4 per 100,000 population) and Incheon (n=251,233; 8,521.1 per 100,000 population). The incidence rate per 100,000 population was still the highest in Seoul, similar to during the fifth wave. The rate of severe infection and fatality rate were similar across the cities and provinces (Supplementary Table 6). By age, the incidence per 100,000 population was the highest for individuals ≤18 years, followed by adults 19–59 years and adults 60 years and older, with a high incidence still observed among school-aged individuals. The rate of severe infection slightly increased from that in the fifth wave (0.06%→0.07%); however, the fatality rate decreased (0.08% in fifth→0.04% in sixth, Table 1).

By the end of June, the reproduction number reached 1, and the number of confirmed cases began to increase again. The infection control authorities restructured the medical response system by establishing an estimated 10,000 one-stop examination facility that can perform tests, provide in-person care, and prescribe medications (July 1, 2022). In particular, the authorities announced the enforcement of voluntary, as opposed to mandatory, social distancing measures (July 13, 2022). Measures were also taken in anticipation for an increase in confirmed cases, including the implementation of intensive infection control measures at businesses hiring foreign workers and construction sites (Establishment of an on-site inspection team and intensive infection control inspection by the Ministry of Employment and Labor from July 6 to 29, 2022) and expansion of testing centers in the SMA, in which COVID-19 cases are concentrated (July 20, 2022). The at-home treatment system was also restructured by discontinuing the separation of intensive/general management for patients under home care and instructing patients who develop symptoms to immediately seek in-person medical care (August 1, 2022) (Figure 7). PCR testing for military conscripts was resumed to prevent mass outbreaks within training camps (from August 12, 2022) [6].

Figure 7. Daily confirmed cases of COVID-19 and mainly control measure in Seoul metropolitan area, sixth wave

During the COVID-19 waves, measures, such as social distancing and restrictions on the size of gatherings, were implemented to reduce opportunities for the potential spread of infections. Population density can serve as an indicator of these interactions [7]. The SMA is a densely populated region with more than half of the total Korean population. Accordingly, this area features more frequent interactions among people than other regions and may be more heavily affected by an outbreak of a highly transmissible infectious disease.

As revealed in this report, the six COVID-19 waves in the ROK mainly occurred in the SMA, except for the first wave (Figure 1, Figure 8). Since the first wave in Daegu and Gyeongbuk, more than 70% of all COVID-19 cases in the country were concentrated in the SMA from the second to fourth waves. Although mass outbreaks were the triggers for the spread of the infection in the early stages, infections spread from facilities closely linked to people’s daily lives, and through smaller outbreaks and person-to-person contact. In response, the infection control authorities prohibited gatherings in different facilities, mandated screening tests, and implemented stricter infection control measures. In response to the spread of the infection throughout the SMA, stricter social distancing measures were implemented, along with special infection control periods designated for the SMA. From the fifth wave and onward, the Omicron variant served as the dominant variant, and the COVID-19 cases surged again nationwide regardless of region and route of infection. In response to this wave, more proactive and aggressive measures were implemented, such as concentrating the health care resources to high-risk groups in the SMA and encouraging people to participate in voluntary infection control measures.

Figure 8. Weekly confirmed, severe/critical and death cases of COVID-19 in Seoul metropolitan area by wave

Although the contents of this report do not suffice as evidence of the effectiveness of infection control measures, the measures were demonstrated to effectively prevent mass spread of the virus [8,9]. In-depth studies on the effects of pharmacological and non-pharmacological interventions will be necessary to facilitate COVID-19 responses, prepare for other pandemics, and determine the direction of the government's policies.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: None.

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: JHH, JYL, SKP. Data curation: JHH, JYL. Formal analysis: JHH, JYL. Investigation: JHH, JYL. Methodology: JHH, JYL, SKP. Supervision: SKP. Visualization: JHH. Writing – original draft: JHH, JYL, SYC. Writing – review & editing: SKP.

Supplementary data are available online.