Abstract
Background
Severe acute respiratory syndrome coronavirus 2, an RNA virus, exhibits variations in transmission power, severity rate, and vaccine effectiveness due to its mutable nature. We investigated the field epidemiological characteristics of the delta and omicron variants of coronavirus disease 2019 (COVID-19) clusters in a long-term care hospital.
Materials and Methods
This study aimed to investigate the incidence, fatality, and vaccination effectiveness of confirmed COVID-19 cases caused by delta and omicron variants. The investigation focused on patients admitted to two long-term care hospitals in a Seoul autonomous district, comparing and analyzing relevant factors.
Results
Among the COVID-19 cases, 101 (34.3%) exhibited delta variants, while 193 (65.4%) showcased omicron variants. The incidence rate of omicron variants, compared to delta variants, was 2.24 times higher (95% confidence interval [CI], 1.68–3.00). This elevation was particularly notable in women across all age groups, patients, workers, and individuals with a history of three or more vaccinations. Deaths were reported in 13 cases (52.0%) with the delta variant and 12 cases (48.0%) with the omicron variant. The fatality rate of the omicron variant, in comparison to the delta variant, was 0.09 times (95% CI, 0.44–2.26), indicating no significant difference. No discernible variations in variables were observed.
Conclusion
The noteworthy surge in outbreaks among female patients, workers engaged in outdoor activities, and the apparent ineffectiveness of vaccination against omicron mutations underscore the need for careful consideration in formulating quarantine measures.
Keywords: COVID-19, SARS-CoV-2, Long-term care hospitals, Delta and omicron variants, Incidence, Fatality
Graphical Abstract
Introduction
Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) pathogen, which was first identified in Wuhan, Hubei Province, China, in December 2019 [1]. It rapidly spread worldwide, prompting the World Health Organization to declare a Public Health Emergency of International Concern on January 30, 2020, and later escalate it to a pandemic on March 11, 2020 [2,3]. The COVID-19 pandemic was marked by the development of vaccines and the emergence of various viral variants, adding to its complexity and leading to the continuous implementation of public health measures globally [4].
The Delta variant (B.1.617.2) and Omicron variant (B.1.1.529) represented major turning points during the pandemic. The Delta variant, first reported in India in April 2021, exhibited high transmissibility and severity, accelerating the pandemic's impact and becoming the dominant strain in Korea by mid-2021 [5,6]. In contrast, the Omicron variant, identified in South Africa and Botswana in November 2021, spread rapidly with lower severity and significant immune evasion properties [7]. These variant characteristics had profound implications in high-risk settings, particularly in long-term care facilities where vulnerable populations were concentrated [8].
In Korea, the first confirmed case of COVID-19 was reported in January 2020, and the nation experienced repeated waves of large-scale outbreaks and variant-driven transmissions [9]. From July 2021, the Delta variant became the dominant strain, leading to a surge in cases and severe outcomes [10]. By December 2021, the Omicron variant emerged and replaced Delta as the dominant strain by early 2022 [11]. Long-term care facilities were among the hardest-hit settings during the pandemic, as elderly residents with underlying conditions faced heightened risks from variant-driven outbreaks [12].
According to domestic data, the Delta variant accounted for over 86% of confirmed cases from July 2021 and became the dominant strain. By December 2021, Omicron cases surged to 48.5%, and it replaced Delta as the dominant strain in January 2022, accounting for 91.4% of cases [13,14]. Throughout 2023, Omicron sublineages, including XBB.1.9.1 and XBB.1.16, continued to emerge, underscoring the need for further investigation into their transmissibility, severity, and vaccine efficacy [15].
As of July 1, 2023, XBB.1.9.1 (24.5% detection rate) and XBB.1.16 were reported in Korea, following previous Omicron sublineages such as BA.1.1, BA.2.12.1, BA.2A (Stealth Omicron), BA.4, BA.5, and BN.1 [16]. On August 31, 2023, the infectious disease classification was downgraded to Level 4, transitioning to a positive surveillance system for ongoing monitoring [6]. From the first confirmed case in January 2020 to August 30, 2023, Korea reported 34,571,876 cases (67.5% of the population), including 6,751,223 cases (72.0%) in Seoul and 237,887 cases (70.9%) in Gwangjin-gu. Nationwide deaths totaled 35,934, with a case fatality rate (CFR) of 0.10%, including 6,675 deaths (CFR 0.10%) in Seoul and 222 deaths (CFR 0.09%) in Gwangjin-gu [17].
This study aims to compare and analyze the impacts of the Delta and Omicron variants in long-term care facilities, focusing on differences in transmissibility, severity, and vaccine efficacy. The findings are intended to provide critical insights for establishing public health policies and vaccination strategies in high-risk settings. The Delta variant was associated with greater severity and reduced vaccine efficacy, while the Omicron variant exhibited lower severity but higher transmissibility [15,16,18]. By comparing these characteristics, this study seeks to provide foundational data for responding to future variants and developing effective disease control strategies.
Materials and Methods
1. Analysis target
The subjects of this study were 1,149 inpatients and staff who were admitted to two nursing hospitals located in Gwangjin-gu, Seoul, between December 29, 2021, and April 16, 2022. Among them, 294 individuals (25.6%) were diagnosed with COVID-19 through genomic testing for the Delta and Omicron variants due to a cluster outbreak. The study analyzed the incidence rate, fatality rate, and vaccine effectiveness against the Delta and Omicron variants among those diagnosed with COVID-19 at A and B nursing hospitals.
2. Research methods
1) Study setting, data collection, and overview of long-term care hospitals
Hospital A operated 348 beds across floors 2 to 5, with 11 single-occupancy rooms (3.2% of total beds) distributed among the intensive care unit on the 2nd floor and general wards on the 4th and 5th floors. At the time of the study, 301 beds (86.5%) were occupied, which limited the ability to isolate confirmed COVID-19 cases in single rooms or by ward units. The hospital provided nursing care, dialysis, and rehabilitation therapy. It employed 84 caregivers, of whom 66 (78.6%) were foreign nationals, primarily from China, working in shared caregiving arrangements. Although an infection control office was operational, its staff handled multiple responsibilities, which limited its functionality. After a large-scale outbreak, the Seoul Metropolitan Government offered infection control consulting to support the hospital.
Hospital B, with no Delta variant cases, experienced a cluster outbreak due to the rapid spread of the Omicron variant. The hospital operated 147 beds across floors 3 to 6, with seven single-occupancy rooms (4.8%) located on the 2nd, 5th, and 6th floors. At the time, 113 beds (76.9%) were occupied, similarly complicating isolation measures. The hospital provided patient care and rehabilitation therapy and employed 34 caregivers, of whom 32 (94.1%) were foreign nationals, primarily from China, working in shared caregiving arrangements.
2) Study design
This study analyzed confirmed COVID-19 cases to compare the characteristics of the Delta (B.1.617.2) and Omicron (B.1.1.529) variants. Data collection spanned from December 29, 2021, when the Omicron-driven cluster outbreak began, to April 16, 2022, when the outbreak subsided.
3) Diagnostic criteria and variant identification
Suspected COVID-19 cases underwent nasopharyngeal specimen collection at municipal public health centers, and reverse transcription polymerase chain reaction (RT-PCR) testing was conducted by public health laboratories. Positive RT-PCR results qualified patients for inclusion in the study. Viral samples from confirmed cases were analyzed using next-generation sequencing technology provided by the Korea Disease Control and Prevention Agency (KDCA). Variants were classified into categories such as Variants of Concern, Variants of Interest, and Variants Under Monitoring based on Spike protein mutations identified using the GISAID database and SARS-CoV-2 mutation analysis tools.
4) Epidemiological and vaccination data
Epidemiological data, including demographic characteristics (gender, age, and classification as staff or inpatient), clinical attributes (index case status, confirmation status, and mortality), and vaccination history, were retrieved from the KDCA COVID-19 information system. Vaccination status was categorized as unvaccinated, partially vaccinated (1st dose), fully vaccinated (2nd dose), or booster vaccinated (3rd dose) based on timing and completion.
5) Statistical analysis
Descriptive statistics and logistic regression analyses were performed using Excel and SPSS (version 29.0; Statistical Package for Social Sciences, SPSS Inc., Chicago, IL, USA). These methods were used to assess correlations between demographic characteristics, clinical features, vaccination status, and infection severity, enabling a comprehensive comparison of the Delta and Omicron variants.
Results
1. General status of delta variants and omicron variants outbreaks
There were two consecutive confirmed cases of the Delta variant and the Omicron variant at “A” Nursing hospital, which is a 6-story building with 348 beds, of which 4 floors are used as wards. The period during which the Delta variant outbreak at “A” Nursing hospital was from December 29, 2021 to February 11, 2022, lasting for (a total of) 46 days. At that time there were 544 people in the nursing hospital, 301 residents (55.3%) and 243 workers (44.7%). The index patient showed symptoms on December 24, 2021, tested positive on December 29 after a nasopharyngeal PCR test on December 28. The total number of confirmed cases was 101 (18.6%), including 88 residents (87.1%) and 13 workers (12.9%). Of the confirmed cases, 13 (12.9%) died, resulting in a mortality rate of 12.9%. The next period during which the Omicron variant outbreak at “A” Nursing hospital was from February 18, 2022 to March 11, 2022, lasting for a total of 21 days. Out of the total 392 individuals of “A” Nursing hospital, there were 195 inpatients (49.7%) and 197 staff members (50.3%). There were total of 88 confirmed cases (22.4%), with 51 inpatients (58.0%), and 37 staff members (42.0%), among them. Out of the confirmed cases, 5 inpatients (5.7%) died. In addition, the only occurrence of the Omicron variant outbreak at “B” Nursing hospital was from March 11, 2022 to April 16, 2022, lasting for a total of 36 days. At that time there were 213 individuals in the hospital, with 123 residents (57.7%) and 90 staff members (42.3%). There were a total of 105 confirmed cases (49.3%), of which 64 (61.0%) were inpatients and 41 (39.0%) were staff members, and among the confirmed cases, 7 inpatients (6.7%) died.
The characteristics of the Delta variant outbreak at “A” nursing hospital included a higher proportion of women at 355 (65.3%) compared to men at 189 (34.7%). Regarding age distribution, individuals aged 59 or younger accounted for 189 (34.7%), those between 60 and 74 years old accounted for 168 (30.9%), and those aged 75 or older accounted for 187 (34.4%). Compared to the age group of 60 and older, the proportion of individuals aged 75 or older was relatively higher. Looking at the participants, inpatients accounted for 301 (55.3%), which was higher than the proportion of staff members 243 (44.7%). In terms of vaccination history, the proportion of those who completed the 3rd dose was the highest at 285 (52.4%), with recipients who received an mRNA vaccine after viral-vector vaccination comprising the highest proportion at 244 (85.6%). Those who completed the second vaccination were 154 (28.3%), and among those who completed the second vaccination, 332 (75.6%) received the viral-vector vaccine and 100 (22.8%) received the mRNA vaccine.
Those who had passed 90 days since the second vaccination accounted for 85 (55.2%), while those who had passed less than 90 days after the second vaccination were 69 (44.8%), which corresponded to the vaccination completion standard.
In the case of a cluster outbreak of the Omicron variant at A Nursing hospital, the characteristics were that the proportion of females was relatively higher at 62.8% compared to males at 146 (37.2%). The age distribution was slightly higher for those aged 59 and under at 149 (38.0%) compared to 60-74 age group at 123 (31.4%) and 75 years and older at 120 (30.6%). In terms of composition, the proportion of staff members was 197 (50.3%), showing a slight difference from inpatients at 195 (49.7%). The vaccination status showed that the proportion of individuals who completed the third dose was 247 (63.0%), which was 40.3 percentage points higher than those who completed the second dose at 22.7%. Among the third dose recipients, the majority (82.2%) received mRNA vaccines after viral-vector vaccinations.
The percentage of individuals completed their second dose of vaccination was 89 (22.7%). Among those who have completed their second dose, 55 (61.8%) received viral-vector vaccines and 32 (36.0%) received mRNA vaccines. After the second dose, 92.1% had elapsed 90 days, while those with less than 90 days since the second dose accounted for 7.9% of cases; this was in accordance with the criteria for completed vaccination. (Criteria for completed vaccination: Individuals who have completed their second dose and have passed 14 days after the second dose, within 90 days from the date of vaccination).
The characteristics of the Omicron variant cluster outbreak at “B” Nursing hospital were different from the above two cases as it occurred in a different nursing home. The proportion of females was relatively higher at 161 (75.6%) compared to males at 52 (24.4%), and the age group of 75 years and older accounted for 48.4%, which was higher than the 60-74 age group at 62 (29.1%) and the under 59 age group at 48 (22.5%).
Therefore, the proportion of confirmed cases among the elderly was relatively high. Among confirmed cases, the proportion of inpatients was 123 (57.7%), while staff accounted for 90 (42.3%), with inpatients cases being 15.4% higher. The vaccination rate showed that the proportion of those who completed the third dose was 181 (85.0%), significantly higher than the 11 (5.2%) of those who completed the second dose. Among those who completed the third dose, majority 153 (84.5%) received mRNA vaccines after viral-vector vaccination. Among those who completed the second dose, 7 (63.6%) were dosed with mRNA vaccines whereas 4 (36.4%) received viral-vector vaccines. Those who had passed 90 days after the second dose accounted for 81.8%, while those within 90 days were 18.2%, meeting the criteria for completed vaccination (Criteria for completed vaccination after the second dose: 14 days after the second dose and within 90 days from the vaccination date) (Table 1). The epidemic curves for each outbreak period are shown in Figure 1.
Table 1. General characteristics and brief of a cluster outbreak of delta and omicron variants in long-term care hospitals.
| Variables | Total | Hospital A | Hospital A | Hospital B | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Variant type | Delta & Omicron | Delta | Omicron | Omicron | |||||||||
| Epidemic period | 29 December 2021–16 April 2022 | 29 December 2021–11 February 2022 | 18 February 2022–11 March 2022 | 11 March 2022–16 April 2022 | |||||||||
| Symptom onset date | 24 December 2021– | 24 December 2021 | 16 February 2022 | 10 March 2022 | |||||||||
| Diagnosis date | 28 December 2021–16 April 2022 | 28 December 2021 | 18 February 2022 | 11 March 2022 | |||||||||
| Variables | Subtotal of inpatients & staff members, n (%)a | Confirmed cases, n (attack rate, %) | Death cases, n (fatality, %) | Subtotal of inpatients & staff members, n (%) | Confirmed cases, n (attack rate, %) | Death cases, n (fatality, %) | Subtotal of inpatients & staff members, n (%) | Confirmed cases, n (attack rate, %) | Death cases, n (fatality, %) | Subtotal of inpatients & staff members, n (%) | Confirmed cases, n (attack rate, %) | Death cases, n (fatality, %) | |
| Totalb | 1,149 (100) | 294 (25.6) | 25 (8.5) | 544 (100) | 101 (18.6) | 13 (12.9) | 392 (100) | 88 (22.4) | 5 (5.7) | 213 (100) | 105 (49.3) | 7 (6.7) | |
| Gender | |||||||||||||
| Male | 387 (33.7) | 133 (34.4) | 15 (11.3) | 189 (34.7) | 61 (32.3) | 7 (11.5) | 146 (37.2) | 39 (26.7) | 4 (10.3) | 52 (24.4) | 33 (63.5) | 4 (12.2) | |
| Female | 762 (66.3) | 161 (21.1) | 10 (6.2) | 355 (65.3) | 40 (11.3) | 6 (15.0) | 246 (62.8) | 49 (19.9) | 1 (2.0) | 161 (75.6) | 72 (44.7) | 3 (4.2) | |
| Age, years | |||||||||||||
| ≤59 | 386 (33.6) | 55 (14.3) | 3 (5.5) | 189 (34.7) | 10 (5.3) | 2 (20.0) | 149 (38.0) | 31 (20.8) | 1 (3.2) | 48 (22.5) | 14 (29.2) | 0 | |
| 60–74 | 353 (30.7) | 115 (32.6) | 8 (7.0) | 168 (30.9) | 48 (28.6) | 4 (8.3) | 123 (31.4) | 28 (22.8) | 3 (10.7) | 62 (29.1) | 39 (62.9) | 1 (2.6) | |
| ≥75 | 410 (35.7) | 124 (30.2) | 14 (11.3) | 187 (34.4) | 43 (22.9) | 7 (16.3) | 120 (30.6) | 29 (24.2) | 1 (3.4) | 103 (48.4) | 52 (50.5) | 6 (11.5) | |
| Group | |||||||||||||
| Inpatients | 619 (53.9) | 204 (33.0) | 25 (12.3) | 301 (55.3) | 88 (29.2) | 13 (14.8) | 195 (49.7) | 51 (26.2) | 5 (9.8) | 123 (57.7) | 65 (52.8) | 7 (10.8) | |
| Staff members | 530 (46.1) | 90 (17.0) | 0 | 243 (44.7) | 13 (5.3) | 0 | 197 (50.3) | 37 (18.8) | 0 | 90 (42.3) | 40 (44.4) | 0 | |
| Vaccination status | |||||||||||||
| Unvaccinated | 145 (12.6) | 49 (33.8) | 9 (18.4) | 87 (16.0) | 28 (32.2) | 6 (21.4) | 41 (10.5) | 12 (29.3) | 2 (16.7) | 17 (8.0) | 9 (5.3) | 1 (11.1) | |
| First | 37 (3.2) | 14 (37.8) | 0 | 18 (3.3) | 9 (50.0) | 0 | 15 (3.8) | 3 (20.0) | 0 | 4 (1.9) | 2 (50.0) | 0 | |
| Second | 254 (22.1) | 61 (24.0) | 7 (11.5) | 154 (28.3) | 36 (23.4) | 5 (13.9) | 89 (22.7) | 20 (22.5) | 1 (5.0) | 11 (5.2) | 5 (45.5) | 1 (20.0) | |
| ≤90 daysc | 176 (69.3) | 36 (20.5) | 5 (13.9) | 85 (55.2) | 20 (23.5) | 4 (20.0) | 82 (92.1) | 13 (15.9) | 1 (7.7) | 9 (81.8) | 3 (33.3) | 0 | |
| ≥90 daysc | 78 (30.7) | 25 (32.1) | 2 (8.0) | 69 (44.8) | 16 (23.2) | 1 (6.3) | 7 (7.9) | 7 (100) | 0 | 2 (18.2) | 2 (100) | 1 (50.0) | |
| Third | 713 (62.1) | 170 (23.8) | 9 (5.3) | 285 (52.4) | 28 (9.8) | 2 (7.1) | 247 (63.0) | 53 (21.5) | 2 (3.8) | 181 (85.0) | 89 (49.2) | 5 (5.6) | |
| Vaccines typed | 967 (100) | 231 (23.9) | 16 (6.9) | 439 (100) | 64 (14.6) | 7 (10.9) | 336 (100.0) | 73 (21.7) | 3 (4.1) | 192 (100.0) | 94 (49.0) | 6 (6.4) | |
| Second vaccination | |||||||||||||
| Mixed | 16 (1.7) | 3 (18.8) | 0 | 7 (1.6) | 0 | 0 | 7 (2.1) | 1 (14.3) | 0 | 2 (1.0) | 2 (100) | 0 | |
| mRNA | 204 (21.1) | 45 (22.1) | 4 (8.9) | 100 (22.8) | 13 (13.0) | 2 (15.4) | 71 (21.1) | 16 (22.5) | 0 (0.0) | 33 (17.2) | 16 (48.5) | 2 (12.5) | |
| Viral-vector | 747 (77.2) | 183 (24.5) | 12 (6.6) | 332 (75.6) | 51 (15.4) | 5 (9.8) | 258 (76.8) | 56 (21.7) | 3 (5.4) | 157 (81.8) | 76 (48.4) | 4 (5.3) | |
| Third vaccination | 712 (100) | 170 (23.9) | 9 (5.3) | 285 (100) | 28 (9.8) | 2 (7.1) | 246 (100) | 53 (21.5) | 2 (3.8) | 181 (100) | 89 (49.2) | 5 (5.6) | |
| Mixed-mRNA | 13 (1.8) | 2 (15.4) | 0 | 6 (2.1) | 0 | 0 | 5 (2.0) | 0 | 0 | 2 (1.1) | 2 (100) | 0 | |
| mRNA-mRNA | 99 (13.8) | 21 (21.2) | 1 (4.8) | 35 (12.3) | 2 (5.7) | 0 | 38 (15.4) | 7 (18.4) | 0 | 26 (14.4) | 12 (46.2) | 1 (8.3) | |
| Viral-vector-mRNA | 600 (84.3) | 147 (24.5) | 8 (5.4) | 244 (85.6) | 26 (10.7) | 2 (7.7) | 203 (82.5) | 46 (22.7) | 2 (4.3) | 153 (84.5) | 75 (49.0) | 4 (5.3) | |
aColumn percentage.
bRow percentage.
cAfter the second vaccination.
dViral-vector vaccines included AstraZeneca and Janssen vaccine, mRNA vaccines included Moderna and Pfizer vaccine.
Figure 1. Epidemic curves for delta and omicron variants outbreak in long-term care hospitals.
Illustration of COVID-19 epidemics in Gwangjin-gu, Seoul: (A) Symptomatic COVID-19 local cases during 29, December 2021–16, April 2022 (Delta and Omicron variants), (B) Symptomatic COVID-19 local cases during 29, December 2021–11, February 2022 (Delta variants dominated), (C) Symptomatic COVID-19 local cases during 18, February 2022–11, March 2022 (Omicron variants dominated), (D) Symptomatic COVID-19 local cases during 14, March 2022–16, April 2022 (Omicron variants dominated).
2. Comparison of incidence rates of omicron variant compared to delta variant
To compare the incidence rates of the Delta variant versus the Omicron variant, a logistic regression analysis was used to analyze the comparative risk using adjusted factors such as gender, age, household composition, and vaccination history. The comparative risk of Omicron variant incidence rate compared to the Delta variant among all subjects was 2.24 times higher (95% confidence interval [CI], 1.68–3.00). When comparing the incidence rates of the Delta variant versus the Omicron variant by general characteristics, it was found that the relative risk was 3.52 times higher for females (95% CI, 2.35–5.29), 6.43 times higher for those aged 59 or younger (95% CI, 3.02–13.68), 7.10 times higher for staff members (95% CI, 3.75–13.46), 1.19 times higher for unvaccinated individuals (95% CI, 0.58–2.43), and 4.49 times higher for those who completed their third dose vaccination (95% CI, 2.86–7.04) (Table 2).
Table 2. The attack rates of the omicron variant compared to the delta variant.
| Variables | Delta | Omicron | Delta vs. Omicron | ||||
|---|---|---|---|---|---|---|---|
| Subtotal of inpatients & staff members, n (%) | Confirmed cases, n (attack rate, %) | Subtotal of inpatients & staff members, n (%) | Confirmed cases, n (attack rate, %) | Relative risk (95% CI) | Adjusted relative risk (95% CI) | ||
| Total | 544 (100) | 101 (18.6) | 605 (100) | 193 (31.9) | 2.05 (1.56–2.71) | 2.24 (1.68–3.00) | |
| Gender | |||||||
| Male | 189 (34.7) | 61 (32.3) | 198 (32.7) | 72 (36.4) | 1.20 (0.79–1.83) | 1.29 (0.83–2.00) | |
| Female | 355 (65.3) | 40 (11.3) | 407 (67.3) | 121 (29.7) | 3.33 (2.25–4.93) | 3.52 (2.35–5.29) | |
| Age, years | |||||||
| ≤59 | 189 (34.7) | 10 (5.3) | 197 (32.6) | 45 (22.8) | 5.30 (2.58–10.87) | 6.43 (3.02–13.68) | |
| 60–74 | 168 (30.9) | 48 (28.6) | 185 (30.6) | 67 (36.2) | 1.42 (0.91–2.22) | 1.59 (1.00–2.54) | |
| ≥75 | 187 (34.4) | 43 (23.0) | 223 (36.9) | 81 (36.3) | 1.91 (1.23–2.96) | 2.01 (1.27–3.17) | |
| Group | |||||||
| Inpatients | 301 (55.3) | 88 (29.2) | 318 (52.6) | 116 (36.5) | 1.39 (0.99–1.95) | 1.45 (1.02–2.06) | |
| Staff members | 243 (44.7) | 13 (5.3) | 287 (47.4) | 77 (26.8) | 6.49 (3.50–12.02) | 7.10 (3.75–13.46) | |
| Vaccination status | |||||||
| Unvaccinated | 87 (16.0) | 28 (32.2) | 58 (9.6) | 21 (36.2) | 1.20 (0.59–2.41) | 1.19 (0.58–2.43) | |
| First | 18 (3.3) | 9 (50.0) | 19 (3.1) | 5 (26.3) | 0.36 (0.09–1.42) | 0.35 (0.08–1.48) | |
| Second | 154 (28.3) | 36 (23.4) | 100 (16.5) | 25 (25.0) | 1.09 (0.61–1.96) | 0.99 (0.54–1.81) | |
| Third | 285 (52.4) | 28 (9.8) | 428 (70.7) | 142 (33.2) | 4.56 (2.94–7.07) | 4.49 (2.86–7.04) | |
CI, confidence interval.
3. Analysis of the fatality rate of the omicron variant compared to the delta variant
To analyze the fatality rate of the Omicron variant compared to the Delta variant, logistic regression analysis was used to estimate the relative risk, adjusting for gender, age, household composition, and vaccination status. The fatality rate of the Omicron variant compared to the Delta variant in the overall study population was 0.99 times (adjusted rate ratio [aRR], 0.99; 95% CI, 0.44–2.26). When stratified by general characteristics, the fatality rate comparison analysis of the Omicron variant compared to the Delta variant showed a relative risk of 1.32 times for males (aRR, 1.32; 95% CI, 0.46–3.81), 1.16 times for individuals aged 60-74 (aRR, 1.16; 95% CI, 0.28–4.91), and 1.0 times for inpatients (aRR, 1.0; 95% CI, 0.44–2.28). In the unvaccinated group, the fatality rate of the Omicron variant compared to the Delta variant was 0.7 times (aRR, 0.70; 95% CI, 0.16–2.95), while in those who completed the third dose of vaccination, it was 2.12 times (aRR, 2.12; 95% CI, 0.43–10.55) (Table 3).
Table 3. The fatality rates of omicron variant compared to delta variants.
| Variables | Delta | Omicron | Delta vs. Omicron | ||||
|---|---|---|---|---|---|---|---|
| Subtotal of inpatients & staff members, n (%) | Confirmed cases, n (attack rate, %) | Subtotal of inpatients & staff members, n (%) | Confirmed cases, n (attack rate, %) | Relative risk (95% CI) | Adjusted relative risk (95% CI) | ||
| Total | 101 (100) | 13 (12.9) | 193 (100) | 12 (6.2) | 0.83 (0.37–1.83) | 0.99 (0.44–2.26) | |
| Gender | |||||||
| Male | 61 (60.4) | 7 (11.5) | 72 (37.3) | 8 (11.1) | 1.09 (0.39–3.08) | 1.32 (0.46–3.81) | |
| Female | 40 (39.6) | 6 (15.0) | 121 (62.7) | 4 (3.3) | 0.58 (0.16–2.06) | 0.66 (0.18–2.47) | |
| Age, years | |||||||
| ≤59 | 10 (9.9) | 2 (20.0) | 45 (23.3) | 1 (2.2) | 0.48 (0.04–5.30) | 0.69 (0.06–8.27) | |
| 60–74 | 48 (47.5) | 4 (8.3) | 67 (34.7) | 4 (6.0) | 0.91 (0.22–3.68) | 1.16 (0.28–4.91) | |
| ≥75 | 43 (42.6) | 7 (16.3) | 81 (42.0) | 7 (8.6) | 0.83 (0.29–2.42) | 1.02 (0.33–3.08) | |
| Group | |||||||
| Inpatients | 88 (87.1) | 13 (14.8) | 116 (60.1) | 12 (10.3) | 0.87 (0.38–1.94) | 1.00 (0.44–2.28) | |
| Staff members | 13 (12.9) | 0 | 77 (39.9) | 0 | - | - | |
| Vaccination status | |||||||
| Unvaccinated | 28 (27.7) | 6 (21.4) | 21 (10.9) | 3 (14.3) | 0.74 (0.18–3.07) | 0.70 (0.16–2.95) | |
| First | 9 (8.9) | 0 | 5 (2.6) | 0 | - | - | |
| Second | 36 (35.6) | 5 (13.9) | 25 (13.0) | 2 (8.0) | 0.61 (0.12–3.20) | 0.49 (0.09–2.64) | |
| Third | 28 (27.7) | 2 (7.1) | 142 (73.6) | 7 (4.9) | 2.35 (0.49–11.4) | 2.12 (0.43–10.6) | |
CI, confidence interval.
4. Comparison of incidence and fatality rates according to unvaccinated vaccination history
To compare the incidence and fatality rates based on vaccination status compared to unvaccinated individuals, logistic regression analysis was conducted to estimate the relative risk. The analysis model adjusted for the type of variant, age, gender, household composition, and vaccination history. Comparisons were made between unvaccinated individuals and those who completed their 1st and 2nd doses, and 3rd doses.
Among the confirmed cases, the completion rates for the 1st and 2nd doses compared to unvaccinated individuals were 37% (aRR, 0.63: 95% CI, 0.40–0.99) and for the 3rd dose completion, before adjustment, was 39% (aRR, 0.61: 95% CI, 0.42–0.90), indicating vaccine effectiveness. However, after adjusting for gender, age, household members, and vaccination history, there was no significant statistical difference. Additionally, the relative risk was estimated by stratifying according to the type of variant, age, gender, and household members. In the Delta variant group, the effectiveness of vaccination completion rates of the 1st and 2nd doses were 30% (aRR, 0.70; 95% CI, 0.39–1.28) and 3rd doses were 58% (aRR, 0.42; 95% CI, 0.22–0.81), respectively. However, in the Omicron variant group, the completion rates of the 1st and 2nd doses were 44% (aRR, 0.56; 95% CI, 0.28–1.11) and there was no significant statistical effect for 3rd dose completion. For the age group, vaccine effectiveness after completing the 1st and 2nd doses was 56% for those under 65 years old (aRR, 0.44; 95% CI, 0.18–1.08) and 27% for those 65 years and older (aRR, 0.73; 95% CI, 0.43–1.24). However, there was no significant statistical difference in vaccine effectiveness across all age groups after completing the 3rd dose. In terms of gender and household composition, the effectiveness of the 3rd dose decreased compared to the 1st and 2nd doses for those unvaccinated. Among genders, males and among household members, inpatients had lower relative vaccine effectiveness in preventing infection compared to unvaccinated individuals after completing the 3rd dose than for completion of the first and second doses.
Among the deceased, there was a vaccine effectiveness of 65% for completion of the first and second doses compared to unvaccinated individuals (aRR, 0.35; 95% CI, 0.13–0.97), and 55% for completion of the third dose (aRR, 0.45; 95% CI, 0.17–1.19). Stratified comparisons of relative risks were estimated by variant type, age, gender, and household composition. In the Delta variant group, the effectiveness of the vaccine was 62% (aRR, 0.38; 95% CI, 0.11–1.30) was 62% for those fully vaccinated with the first and second doses (aRR, 0.38; 95% CI, 0.11–1.30), and 77% for those fully vaccinated with the third dose (aRR, 0.23; 95% CI, 0.04–1.17). However, in the Omicron variant group, the effectiveness of the vaccine was 73% for those fully vaccinated with the 1st and 2nd doses (aRR, 0.27; 95% CI, 0.04–1.72). Nonetheless, for those fully vaccinated with a 3rd dose, the effectiveness dropped to 30% (aRR, 0.70; 95% CI, 0.17–2.89), indicating a lower vaccine efficacy with the third dose. For individuals under 65 years old, there was a difference in vaccine effectiveness between those fully vaccinated with the 1st and 2nd doses at 90% (aRR, 0.10; 95% CI, 0.01–1.10) and those fully vaccinated with a 3rd dose at 89% (aRR, 0.11; 95% CI, 0.01–1.11). However, for those aged 65 and older, the vaccine effectiveness differed with 52% for those fully vaccinated with the 1st and 2nd doses (aRR, 0.48; 95% CI, 0.15–1.53) and 37% for those fully vaccinated with the 3rd dose (aRR, 0.63; 95% CI, 0.21–1.95). Among the genders, it cannot be said that vaccination status in females and composition of members of group had an impact on death, but males had a vaccine effectiveness of 53% for completion of 1st and 2nd doses (aRR, 0.47; 95% CI, 0.12–1.87), and 66% for completion of the 3rd dose (aRR, 0.34; 95% CI, 0.08–1.43). Only inpatients had deaths among composition of the members, in terms of vaccination status compared to confirmed cases, the impact on the incidence rate compared to non-vaccination was relatively lower in completion of the 3rd dose than in completion of the 1st and 2nd doses, and there were no practitioners (Table 4).
Table 4. Effect of unvaccinated vaccination history on incidence and fatality rate.
| Variables | 1st and 2nd vaccination | 3rd vaccination | ||||
|---|---|---|---|---|---|---|
| Relative risk (95% CI) | Adjusted relative risk (95% CI) | Relative risk (95% CI) | Adjusted relative risk (95% CI) | |||
| Attack rate (n=294) | ||||||
| Total | 0.68 (0.44–1.05) | 0.63 (0.40–0.99) | 0.61 (0.42–0.90) | 0.77 (0.51–1.18) | ||
| Type of variants | ||||||
| Delta | 0.75 (0.42–1.31) | 0.70 (0.39–1.28) | 0.23 (0.13–0.42) | 0.42 (0.22–0.81) | ||
| Omicron | 0.59 (0.30–1.17) | 0.56 (0.28–1.11) | 0.87 (0.49–1.55) | 1.14 (0.63–2.08) | ||
| Age, years | ||||||
| <65 | 0.51 (0.23–1.16) | 0.44 (0.18–1.08) | 0.51 (0.26–1.00) | 0.68 (0.32–1.46) | ||
| ≥65 | 0.76 (0.45–1.27) | 0.73 (0.43–1.24) | 0.87 (0.54–1.40) | 0.87 (0.52–1.45) | ||
| Gender | ||||||
| Male | 0.58 (0.31–1.09) | 0.59 (0.31–1.12) | 0.56 (0.31–1.01) | 0.82 (0.43–1.56) | ||
| Female | 0.73 (0.39–1.35) | 0.70 (0.37–1.34) | 0.76 (0.45–1.29) | 0.73 (0.41–1.31) | ||
| Group | ||||||
| Inpatients | 0.73 (0.45–1.17) | 0.71 (0.44–1.15) | 1.00 (0.63–1.58) | 0.98 (0.61–1.57) | ||
| Staff members | 0.37 (0.11–1.19) | 0.29 (0.08–1.04) | 0.56 (0.24–1.31) | 0.34 (0.13–0.88) | ||
| Fatality rate (n=25) | ||||||
| Total | 0.37 (0.14–1.02) | 0.35 (0.13–0.97) | 0.19 (0.08–0.50) | 0.45 (0.17–1.19) | ||
| Type of variants | ||||||
| Delta | 0.40 (0.12–1.36) | 0.38 (0.11–1.30) | 0.10 (0.02–0.48) | 0.23 (0.04–1.17) | ||
| Omicron | 0.31 (0.05–1.93) | 0.27 (0.04–1.72) | 0.30 (0.08–1.21) | 0.70 (0.17–2.89) | ||
| Age, years | ||||||
| <65 | 0.16 (0.02–1.54) | 0.10 (0.01–1.10) | 0.03 (0.00–0.33) | 0.11 (0.01–1.11) | ||
| ≥65 | 0.48 (0.15–1.54) | 0.48 (0.15–1.53) | 0.45 (0.15–1.32) | 0.63 (0.21–1.95) | ||
| Gender | ||||||
| Male | 0.43 (0.12–1.54) | 0.47 (0.12–1.87) | 0.28 (0.08–1.01) | 0.34 (0.08–1.43) | ||
| Female | 0.26 (0.05–1.45) | 0.16 (0.02–1.08) | 0.16 (0.04–0.65) | 0.52 (0.10–2.82) | ||
| Group | ||||||
| Inpatients | 0.35 (0.13–0.98) | 0.35 (0.13–0.97) | 0.42 (0.16–1.08) | 0.45 (0.17–1.19) | ||
| Staff members | - | - | - | - | ||
Among the dead, there were no workers or anyone who had completed the first vaccination.
CI, confidence interval.
Discussion
This study analyzed the impact of variant-specific incidence rates, fatality rates, and vaccination status among 294 individuals (25.6%). These individuals were confirmed through genome testing for COVID-19 cluster outbreaks out of 1,149 inpatients and staff from two long-term care hospitals in Gwangjin-gu, Seoul, between December 29, 2021, and April 16, 2022. The results showed that the incidence rate of the Omicron variant was 2.24 times higher than that of the Delta variant. The fatality rate, however, was 0.99 times lower, which is consistent with previous domestic studies [19,20]. While earlier studies reported higher incidence rates among males for both Delta and Omicron variants, this study found higher incidence rates among individuals aged 60–74, females rather than males, and staff rather than inpatients. These findings can be attributed to factors such as the higher female population ratio in Gwangjin-gu, the high transmissibility of the Omicron variant, and increased external mobility among staff.
Regarding vaccine efficacy, earlier studies reported that individuals who completed three doses of vaccination had higher prevention effectiveness compared to unvaccinated or partially vaccinated individuals [19,20]. However, this study found significant preventive effects in individuals who completed one or two doses, but no statistically significant difference between those who completed two and three doses. For the Delta variant, individuals with three doses showed greater protection than those with two doses, whereas for the Omicron variant, significant protection was observed only up to the second dose. These findings suggest a decline in vaccine efficacy after the third dose, particularly against Omicron, and underscore the need for further research on vaccine-variant interactions.
Different vaccine types and administration schedules likely influenced the preventive efficacy against variants. mRNA vaccines (Pfizer, Moderna) and viral vector vaccines (AstraZeneca, Janssen) demonstrated differing effectiveness against variants, with significant reductions in neutralizing antibody production reported for Omicron [12]. Optimizing vaccination intervals and booster timing may help maintain antibody levels and strengthen infection prevention effects [3]. The neutralizing antibody escape capability of the Omicron variant was identified as a major factor limiting vaccine effectiveness. Compared to the Delta variant, Omicron showed stronger immune escape properties, leading to reduced preventive efficacy even after full vaccination [4,5]. These findings emphasize the necessity for variant-specific vaccines or new platforms capable of eliciting broad neutralizing antibody responses [6].
International studies have shown that infection prevention effectiveness declined from approximately 83% one month after full vaccination to 22% after more than five months, with immune-evading variants identified as a major contributing factor [21]. Delta variants were associated with higher severity and Omicron variants with lower severity, but both variants exhibited high transmissibility, resulting in surges in confirmed cases. Vaccination continued to provide strong protection against severe infections and mortality over time [9]. This study confirmed that vaccination effectively reduced the spread and fatality rates associated with Omicron.
This study has several limitations. First, clinical information registered in the COVID-19 Information Management System may not have fully adhered to reporting standards or may have been delayed due to early detection issues. Additionally, transferring case management to local public health centers may have resulted in reporting delays, and early discharges might have led to underreporting of fatalities. Second, vaccination records were based on self-reported information, which may have included inaccuracies in vaccine type or administration history, leading to potential misclassification of vaccination status. Third, data collection for some severe cases relied on caregivers or facility staff, which may have introduced errors, especially for cognitively impaired patients who might have faced difficulties understanding and responding to the questions accurately. Fourth, differences in facility size and management approaches between the two long-term care hospitals in Gwangjin-gu might have introduced selection and information bias during participant selection. Lastly, most patients in long-term care facilities were elderly and had underlying conditions, which may limit the generalization of these findings to other populations.
The findings indicated that high external mobility among staff contributed to increased incidence rates within long-term care facilities. This highlights the need for strengthened infection control measures. Tailored strategies, including regular PCR testing, use of personal protective equipment, minimizing movement in high-risk areas, and restricting close contact, are essential for infection prevention. Ongoing monitoring of staff vaccination status and enhancing booster campaigns for emerging variants are equally important. Vaccination strategies should focus on protecting the elderly and individuals with underlying conditions by optimizing vaccination schedules and vaccine types. Evaluating vaccine efficacy against new variants and designing tailored vaccination schedules to maximize immune responses in high-risk populations are also crucial.
Future research should include quantitative analyses, such as regression models, to explore gender- and age-specific differences in incidence rates. Additionally, comparative analyses of infection control measures and the impacts of vaccination within long-term care facilities should assess their influence on infection rates and severity to develop effective public health policies. Continuous analysis of transmissibility and vaccine effectiveness against new variants will provide practical and sustainable strategies to combat emerging challenges.
During the prolonged COVID-19 pandemic, social distancing and vaccination have been instrumental in reducing transmission and preventing severe cases. Nevertheless, the Omicron variant's high transmissibility and lower fatality rates have led to severe cases and deaths being concentrated among the elderly. This underscores the critical need for enhanced infection control measures in high-risk facilities. This study shows that the decline in vaccine efficacy after the third dose was likely due to reduced antibody levels and the immune evasion properties of Omicron. These findings suggest the need for additional booster strategies or the development of variant-specific vaccines. Future studies should analyze differences among Omicron sublineages, such as BA.1 and BA.2, to enhance variant-specific prevention strategies. Proactive testing and strengthened systems to prevent severe cases in the elderly are essential in long-term care facilities. Additionally, broader studies, including the general population, are necessary to comprehensively analyze vaccine efficacy and the impact of variants. These insights can help establish inclusive public health strategies and minimize transmission through community-based education campaigns and policies to protect healthcare workers.
By analyzing cluster outbreaks in long-term care facilities in Gwangjin-gu since the first confirmed case in February 2020, this study highlights the importance of monitoring vaccine efficacy, immune evasion by variants, reinfection risks, and sustained observation of asymptomatic infections. These findings are expected to serve as foundational data for improving COVID-19 response measures and strengthening healthcare systems.
This study analyzed COVID-19 cluster outbreaks in two long-term care hospitals in Gwangjin-gu, Seoul, from December 2021 to April 2022, evaluating the incidence rates, fatality rates, and vaccine efficacy of different variants. The findings revealed that the Omicron variant had an incidence rate 2.24 times higher than that of the Delta variant but showed lower fatality rates, indicating increased transmissibility and reduced severity, consistent with previous studies.
Higher incidence rates were observed among individuals aged 60–74, females, and staff, which were likely influenced by regional factors and increased external mobility. Additionally, vaccine efficacy was significant up to the second dose but showed a decline after the third dose, highlighting the need for updated booster strategies and the development of variant-specific vaccines.
Future research should focus on analyzing differences among Omicron sublineages and assessing vaccine efficacy in the general population to enhance public health strategies. In high-risk facilities, proactive testing and reinforced systems to prevent severe cases should be prioritized.
ACKNOWLEDGEMENT
We would like to thank Moo-Sik Lee for generous support and encouragement to infectious disease investigators.
Footnotes
Funding: None.
Conflict of Interest: No conflict of interest.
- Conceptualization: GHO.
- Data curation: GHO.
- Formal analysis: GHO, PK.
- Investigation: GHO, JMP, MSL.
- Methodology: GHO, JMP, MSL.
- Visualization: GHO, MSL.
- Writing - original draft: GHO, PK, MSL.
- Writing - review & editing: GHO, JMP, PK, MSL.
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