COVID-19, an unprecedented global pandemic, has caused successive waves that pose unique challenges to public health and epidemiological research. Traditional Susceptible-Infected-Recovered (SIR) models often struggle to capture these complex dynamics, especially given that the virus has spawned multiple sub-variants. To tackle these challenges, we adopt an empirical modeling approach by integrating real-world data from Our World in Data (daily confirmed COVID-19 cases) and GISAID (variant prevalence) into a newly proposed integrated model. Specifically, we sum multiple sigmoidal (logistic) curves, each representing the cumulative infections of a distinct dominant variant, and recalibrate the model whenever a new variant emerges. By incorporating variant-specific parameters, our framework effectively captures the biological and epidemiological characteristics of COVID-19 in a dynamic, data-driven manner. Most importantly, we employ the Pruned Exact Linear Time (PELT) algorithm to provide rigorous mathematical justification for when separate variant models can be legitimately summed: specifically, when variant dominance reaches approximately 50%. This establishes the theoretical foundation that separate logistic models can be additively combined under specific dominance conditions. We evaluate our model using Mean Absolute Percentage Error (MAPE), Root Mean Square Error (RMSE), and Mean Absolute Error (MAE). Our empirical findings confirm that integrating dominant variants is associated with markedly improved accuracy compared to a single-strain approach, based on data from fourteen countries (including South Korea, the United States, and the United Kingdom) and global aggregates. We also provide theoretical motivation for approximating SIR dynamics via logistic functions and discuss how this integrated framework can be refined to enhance predictive performance. In doing so, we demonstrate the feasibility and advantages of iteratively updating epidemiological models in response to emerging variants, thereby offering actionable insights for ongoing and future pandemic management.

Objectives This study aimed to comprehensively outline the methodological approaches used in published research comparing the vaccine effectiveness (VE) of coronavirus disease 2019 (COVID-19) vaccines. Methods A systematic search was conducted on June 13, 2024, to identify comparative studies evaluating the effectiveness of mRNA versus non-mRNA and monovalent versus bivalent COVID-19 vaccines. We screened titles, abstracts, and full texts, collecting data on publication year, country, sample size, study population composition, study design, VE estimates, outcomes, and covariates. Studies that reported relative VE (rVE) were analyzed separately from those that did not. Results We identified 25 articles comparing rVE between mRNA and non-mRNA COVID-19 vaccines, as well as between monovalent and bivalent formulations. Among the studies assessing VE by vaccine type, 126 did not provide rVE estimates. Comparative VE studies frequently employed retrospective cohort designs. Among the definitions of rVE used, the most common were hazard ratio and absolute VE, calculated as (1−odds ratio)×100. Studies were most frequently conducted in the United Kingdom and the United States, and the most common outcome was infection. Most targeted the general population and assessed the VE of mRNA vaccines using the AstraZeneca vaccine as a reference. A small proportion, 7.3% (n=11), did not adjust for any variables. Only 3 studies (2.0%) adjusted for all core confounding variables recommended by the World Health Organization. Conclusion Few comparative studies of COVID-19 vaccines have incorporated rVE methodologies. Reporting rVE and employing a consistent set of covariates can broaden our understanding of COVID-19 vaccines.

The importance of a public health strategy for mass gatherings has been consistently emphasized internationally in the past. In the Republic of Korea, the Korea Disease Control and Prevention Agency (KDCA) has established guidelines and standard operation procedures (SOPs) for preparations to address and respond to infectious disease outbreaks due to mass gathering events. The most recent example of the KDCA's response in line with these guidelines and SOPs was that for the 「Winter Youth Olympic Games Gangwon 2024」 (Gangwon 2024), for which a proactive response and cooperation system, quarantine measures, and an infectious disease surveillance system were implemented. During Gangwon 2024, 14 cases of infectious diseases were confirmed: 3 cases of norovirus infection (1 outbreak case), 1 case of chickenpox, 4 cases of coronavirus disease 2019, and 6 cases of influenza. There were no severe infectious disease outbreaks. We expect to provide a reference for responding to infectious disease outbreaks due to future mass gathering events by evaluating the experience of infectious disease prevention activities at Gangwon 2024.

It is crucial to ascertain the epidemiological characteristics of emerging infectious diseases with pandemic potential through the investigation of the First Few X (FFX) cases and contacts, coupled with the estimation of epidemiological parameters. The primary objective of FFX investigations is to gather epidemiological, clinical, and virological data from a limited initial pool of laboratory-confirmed cases and contacts, aiming to delineate the actual transmission dynamics of a novel pathogen. These identified characteristics from FFX investigations can yield epidemiological parameters indicative of the transmissibility and severity of infectious diseases. Moreover, they can inform the development of targeted strategies for disease prevention.

Objectives: Most COVID-19 outbreaks in South Korea occurred through close contact in dense and closed indoor environments. However, a COVID-19 outbreak occurred in a group that camped together at an outdoor camping site in 2020, raising the need to observe precautions even when outdoors. We aimed to investigate the epidemiological characteristics and the risk factors for transmission for developing precautionary measures during outdoor activities. Methods: A study was conducted among 10 patients during the outbreak that occurred at a domestic camping site in July 2020. The demographic and epidemiological characteristics were obtained through an epidemiological investigation; the transmission risk was evaluated through additional site visits and CCTV analysis. Results: The incidence rate of this outbreak was 55.6%. No other common sources of exposure other than that within the campsite were identified; the outbreak possibly occurred due to transmission via close contact with infected people during mealtime and conversation throughout the camping period without implementation of adequate prevention and control measures. Conclusion: It is the first COVID-19 outbreak that occurred at an outdoor camping site in South Korea. COVID-19 can spread even when outdoors, prevention and control measures such as maintaining social distancing and wearing a mask should be observed when a large number of people gather outdoors. To ensure safe camping, facility managers should keep the cleaning and hygiene, and users should minimize the risk of infection by following the recommended precautions while staying in public spaces.

South Korea experienced a low prevalence of SARS-CoV-2 until the emergence of the omicron in early 2022, triggering a major community epidemic. To evaluate effectiveness of NVX-CoV2373 and BNT162b2 vaccines in Korean population, we conducted an observational study utilizing individual-level case data on laboratory-confirmed SARS-CoV-2 infection, along with vaccination record. A total of 47,078 recipients of NVX-CoV2373 vaccine and 7,561 recipients of BNT162b2 vaccine were eligible for the study. Thirty days post-second doses, COVID-19 rates were 7.9% (595 out of 7561) of NVX-CoV2373 recipients and 8.6 % (647 out of 7561) of BNT162b2 recipients experienced COVID-19. NVX-CoV2373 rates increased to 9.8 % and 11.2 % at 60 and 90 days, while BNT162b2 rates were 10.5 % and 11.3 % at the same intervals. The 22-weeks risk ratios for recipients of the NVX-CoV2373 vaccine as compared with recipients of the BNT162b2 vaccine were 1.11 (95 % CI, 0.99 to 1.25) for laboratory-confirmed SARS-CoV-2 infection. Continued monitoring is essential to evaluate the duration of protection across different vaccine platforms and schedules.

BACKGROUND:This retrospective observational matched cohort study assessed the differences in critical infections caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during the omicron-predominant period of the coronavirus disease 2019 (COVID-19) pandemic. We evaluated the vaccine effectiveness of bivalent mRNA vaccine compared to unvaccinated individuals. METHODS:We collected COVID-19 case data from the Korean COVID-19 vaccine effectiveness cohort. We calculated the probability of critical COVID-19 cases by comparing the vaccinated and unvaccinated groups. RESULTS:The risk of being critically infected due to SAR-CoV-2 infection was 5.96 times higher (95% confidence interval, 5.63-6.38) among older individuals who were unvaccinated compared to those who received the bivalent COVID-19 vaccine. CONCLUSION:Our findings indicate that the bivalent vaccine reduces the disease burden of the SARS-CoV-2 omicron variant, particularly among the older population. Further studies are warranted to determine the effectiveness of booster doses of vaccines for SARS-CoV-2 infection.

Background As the population acquires immunity through vaccination and natural infection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), understanding the intrinsic severity of coronavirus disease (COVID-19) is becoming challenging. We aimed to evaluate the intrinsic severity regarding circulating variants of SARS-CoV-2 and to compare this between vaccinated and unvaccinated individuals. Methods With unvaccinated and initially infected confirmed cases of COVID-19, we estimated the case severity rate (CSR); case fatality rate (CFR); and mortality rate (MR), including severe/critical cases and deaths, stratified by age and compared by vaccination status according to the period regarding the variants of COVID-19 and vaccination. The overall rate was directly standardized with age. Results The age-standardized CSRs (aCSRs) of the unvaccinated group were 2.12%, 5.51%, and 0.94% in the pre-delta, delta, and omicron period, respectively, and the age-standardized CFRs (aCFRs) were 0.60%, 2.49%, and 0.63% in each period, respectively. The complete vaccination group had lower severity than the unvaccinated group over the entire period showing under 1% for the aCSR and 0.5% for the aCFR. The age-standardized MR of the unvaccinated group was 448 per million people per month people in the omicron period, which was 11 times higher than that of the vaccinated group. In terms of age groups, the CSR and CFR sharply increased with age from the 60 s and showed lower risk reduction in the 80 s when the period changed to the omicron period. Conclusions The intrinsic severity of COVID-19 was the highest in the delta period, with over 5% for the aCSR, whereas the completely vaccinated group maintained below 1%. This implies that when the population is vaccinated, the impact of COVID-19 will be limited, even if a new mutation appears. Moreover, considering the decreasing intrinsic severity, the response to COVID-19 should prioritize older individuals at a higher risk of severe disease.

This retrospective matched cohort study evaluated the efficacy of 2 doses of NVX-CoV2373 compared with that of BNT162b2 vaccines in preventing severe acute respiratory syndrome coronavirus 2 infection in adolescents. We analyzed 13-week risk differences and ratios between these 2 vaccines. The study included 465 NVX-CoV2373 and 465 BNT162b2 recipients. Throughout the follow-up period, 4.1% of NVX-CoV2373 recipients and 2.8% of BNT162b2 recipients contracted the severe acute respiratory syndrome coronavirus 2 infection. The incidence risk ratio for NVX-CoV2373 compared with that for BNT162b2 was calculated at 1.46 (95% CI 0.68-3.22; P = 0.296). While our findings suggest noninferiority between the 2 vaccines, further research is needed to comprehensively assess their effectiveness in real-world settings. Our study highlights the critical need for vigilant vaccine surveillance and monitoring efforts to ensure informed decision-making and public health protection.

This study aimed to identify factors influencing compliance with social distancing, a key nonpharmaceutical intervention during the early stages of the coronavirus disease (COVID-19) pandemic. The study population comprised 182 758 Koreans who participated in the 2020 Community Health Survey. Personal characteristics were classified into sociodemographic, health behavioral, and psychosocial factors, and factors associated with social distancing compliance were identified. Health behaviors and psychosocial factors were highly related to compliance with social distancing. Approximately 13% of smokers were less likely to practice physical distancing and 50% of high-risk drinkers were less likely to limit going out or attending gatherings and events. Higher concern about COVID-19 and a more positive perception of the government’s response policy were associated with a higher compliance with social distancing. Strategic public health policies considering the characteristics of the public are needed to enhance compliance with nonpharmaceutical interventions during disease outbreaks lacking effective treatments and vaccines.

Because of the COVID-19 pandemic, wastewater-based surveillance has garnered extensive attention in many countries, including the United States and those in Europe, by pre-emptively monitoring and responding to infectious diseases in the community. Additionally, it is actively used in national infectious disease monitoring policies. From 2021, the Korea Disease Control and Prevention Agency conducted a pilot project to prove the effectiveness of wastewater-based surveillance system in the Republic of Korea and laid the foundation to introduce a national wastewater-based surveillance system. In 2023, KOrea WAstewater Surveillance (KOWAS) project is being conducted in cooperation with 17 cities or provinces and 18 Institute of Health and Environment Researches. Henceforth, we will expand the scope of monitoring infectious diseases by cooperating with related ministries and prepare the crisis response for a potential pandemic by upgrading experimental techniques.

BACKGROUND:We aimed to analyze the risk factors for sudden death after diagnosis of coronavirus disease 2019 (COVID-19) in South Korea and to provide evidence for informing prevention and control interventions for patients at risk of sudden death.METHODS:We included 30,302 COVID-19 related deaths registered in the patient management information system (Central Disease Control Headquarters) between January 1, 2021, and December 15, 2022. We collected their epidemiological data recorded by the reporting city, province, or country. We performed multivariate logistic regression analysis to identify risk factors for sudden death after diagnosis of COVID-19.RESULTS:Among the 30,302 deaths, there were 7,258 (24.0%) and 23,044 (76.0%) sudden and non-sudden deaths, respectively. Sudden death means a person who died within 2 days of diagnosis and who did not receive inpatient treatment. Underlying condition, vaccination status, and place of death were significantly associated with the survival period in all age groups. Moreover, region, sex, and prescription were significantly associated with the survival period only in certain age groups. However, reinfection was not significantly associated with the survival period in any age group.CONCLUSION:To our knowledge, this is the first study on the risk factors for sudden death after a diagnosis of COVID-19, which included age, underlying condition, vaccination status, and place of death. Additionally, individuals aged < 60 years without an underlying condition were at high risk for sudden death. However, this group has relatively low interest in health, as can be seen from the high non-vaccination rate (16.1% of the general population vs. 61.6% of the corresponding group). Therefore, there is a possibility for the presence of an uncontrolled underlying disease in this population. In addition, many sudden deaths occurred due to delayed hospital visits to continue economic activities even after the onset of COVID-19 symptoms (7 days overall vs. 10 days average for the group). In conclusion, 'continued interest in health' is a key factor in avoiding sudden death in the economically active group (under 60 years of age).

Objectives: Most COVID-19 outbreaks in South Korea occurred through close contact in dense and closed indoor environments. However, a COVID-19 outbreak occurred in a group that camped together at an outdoor camping site in 2020, raising the need to observe precautions even when outdoors. We aimed to investigate the epidemiological characteristics and the risk factors for transmission for developing precautionary measures during outdoor activities.

As the coronavirus disease 2019 pandemic continues, the suspected reinfection cases are increasing due to the decrease in immune antibodies and immune avoidance reactions to new variants. We analyzed the characteristics of suspected reinfection cases identified by the surveillance system from January 20, 2020 to July 22, 2023. Moreover, we assessed the vaccination history to evaluate vaccine effectiveness. Among the suspected reinfection cases (7.8%), second infected cases were 98.5%, third infected cases were 1.4%, and more than fourth infected cases were 0.02%. Secondary infections almost were in the Omicron waves, especially with members of long-term care facilities during the Omicron variant period (5.9%) higher than the pre-Omicron period (1.6%). The fatality rate of the second infected cases (0.06%) was lower than the fatality rate of the one-time infected cases (0.12%). The critical infection rate of second infected cases aged 60 or older was 0.54% and the fatality rate was 0.29%. Among second infected cases aged 60 or over, the vaccine effectiveness against critical infection compared to unvaccinated was 68.4% for monovalent vaccine, 77.1% for bivalent vaccine, and the vaccine effectiveness against death compared to unvaccinated was 69.7% for the monovalent vaccine, 81.2% for the bivalent vaccine. Therefore, even in the suspected reinfection cases with the history of previous infection, it is necessary to recommend regular vaccinations to prevent severity and death for the elderly group with a high fatality rate.

Background: This retrospective observational matched-cohort study of 2,151,216 individuals from the Korean coronavirus disease 2019 (COVID-19) vaccine effectiveness cohort aimed to evaluate the comparative effectiveness of the COVID-19 bivalent versus monovalent vaccines in providing additional protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, critical infection, and death in Korea.Methods: Among individuals, those vaccinated with COVID-19 bivalent vaccines were matched in a 1:1 ratio with those who were vaccinated with monovalent vaccines (bivalent vaccines non-recipients) during the observation period. We fitted a time-dependent Cox proportional-hazards model to estimate hazard ratios (HRs) of COVID-19 outcomes for infection, critical infection, and death, and we defined vaccine effectiveness (VE) as 1-HR.Results: Compared with the bivalent vaccination group, the incidence proportions in the monovalent vaccination group were approximately three times higher for infection, nine times higher for critical infection, and 11 times higher for death. In the early stage of bivalent vaccination, relative VE of bivalent vaccine against monovalent vaccine was 42.4% against SARS-CoV-2 infection, 81.3% against critical infection, and 85.3% against death. In addition, VE against critical infection and death according to the elapsed period after bivalent vaccination was maintained at > 70%.Conclusion: The bivalent booster dose provided additional protection against SARS-CoV-2 infections, critical infections, and deaths during the omicron variant phase of the COVID-19 pandemic.

The Korea Disease Control and Prevention Agency has been conducting a complete investigation of coronavirus disease 2019 (COVID-19) confirmed patients and their contacts. In January 2022, the number of confirmed cases increased rapidly due to the development of dominant Omicron variants, which demanded a new quarantine response strategy. On February 7, 2022, a self-report COVID-19 case reporting form was introduced, allowing confirmed patients to enter their basic epidemiological information in the questionnaire. In the fourth quarter of 2022, the registration status of the COVID-19 case reporting form was 98.9%. The registration rate using the self-report COVID-19 case reporting form continued to rise monthly. In the status of each major item, no differences were found in terms of presence of symptoms, underlying diseases, time taken for the diagnosis of confirmed diseases, and the ratio of members of facilities vulnerable to infection. With respect to the status of cohabitants, no significant change in the number of cohabitants per index patient was observed; however, the secondary incidence rate tended to decrease by a small degree every month. There may be limitations in generalizing the monitoring results because input values are not verified for COVID-19 case reporting form data. Therefore, it is necessary to verify the accuracy of the data registered in COVID-19 case reporting form by comparing each item with other objectified data sources. It will be used as evidence for policy preparation through additional analysis of each item of the COVID-19 case reporting form.

Healthcare personnel (HCP) are vulnerable to COVID-19 infection due to their higher risk of contact with infected persons. The numbers of cases and deaths among HCP in Korea were divided into four periods associated with different major variants of SARS-CoV-2: GH clade, Alpha, Delta, and Omicron. To evaluate the implication of HCP infection in Korea, we overviewed the pandemic status in Korea and in other countries: the cases, deaths, excess mortality, and vaccination rates in Germany, Israel, Italy, Japan, the United Kingdom, and the United States. In about two years, there were 10,670 HCP cases among all COVID-19 cases (1.15% of 925,975 cases). HCP cases had a lower death rate (%) compared to that for all cases (0.14 versus 0.75). Nurses were the most infected (55.3%), followed by HCP of other categories (28.8%) and doctors (15.9%), while deaths were mostly reported among doctors (9 out of 15, 60%). Cases among HCP gradually increased, but the death rate decreased as the pandemic progressed. Compared to five of the other countries examined, Korea had a higher incidence of cases but a lower mortality, lower excess mortality, and a higher vaccination rate.

During Caesarean sections (CS), hypotension is a common complication that can lead to adverse fetal outcomes. Early prediction and intervention can help prevent intraoperative hypotension, but there is currently no available method to accurately predict hypotension. This study aims to predict intraoperative hypotension during CS with deep learning-based models from intraoperative non-invasive hemodynamic data which are routinely monitored. A multicentre retrospective study was conducted on pregnant women who underwent CS under spinal anesthesia: (1) Seoul National University Hospital (SNUH, n = 987) and (2) Boramae Medical Center (BMC, n = 180). Intraoperative hypotension was defined as a mean arterial pressure of less than 65mm Hg at any point during surgery. A deep learning-based prediction models (DLPM) for intraoperative hypotension was developed, using preoperative variables and non-invasive hemodynamic monitoring data. The prediction model calculated the risk of intraoperative hypotension after 1 and 5 minutes in real-time. In SNUH, 39,239 and 37,852 were extracted at 1 and 5 minutes prior to the event, respectively, and the incidence of hypotension for each segment was 8.54% and 8.77%. In BMC, the incidence of hypotension was found to be 3.58% and 3.56% from 5,643 and 5,614 segments analysed at 1 and 5 minutes prior to the event, respectively. The DLPM successfully predicted the risk of hypotension at 1 and 5 minutes prior to occurrence, with areas under the receiver operating characteristic curve of 0.80 and 0.74 in SNUH internal validation, and 0.76 and 0.72 in BMC external validation set, respectively. The deep learning-based algorithm has been successful in predicting intraoperative hypotension during CS and might be used for early intervention. However, further studies are required to explore the clinical usefulness of prediction model for prevention of adverse outcomes.

Objectives: Healthcare facilities are high-risk sites for infection. This study analyzed the epidemiological characteristics of a coronavirus disease 2019 (COVID-19) outbreak in a tertiary hospital after COVID-19 vaccination had been introduced in Republic of Korea. Vaccine effectiveness (VE) and shared anti-infection strategies are also assessed.Methods: The risk levels for 4,074 contacts were evaluated. The epidemiological characteristics of confirmed cases were evaluated using the chi-square test. The “1 minus relative risk” method was used to determine VE in preventing infection, progression to severe disease, and death. In the largest affected area (the 8th floor), a separate relative risk analysis was conducted. A multivariate logistic regression analysis (with 95% confidence interval [CIs]) was used to identify transmission risk factors with a significance level <10% via the backward elimination method.Results: In total, 181 cases of COVID-19 were confirmed, with an attack rate of 4.4%. Of those cases, 12.7% progressed to severe disease, and 8.3% died. In the cohort isolation area on the 8th floor, where 79.0% of the confirmed cases occurred, the adjusted odds ratio was 6.55 (95% CI, 2.99–14.33) and 2.19 (95% CI, 1.24–3.88) for caregivers and the unvaccinated group, respectively. VE analysis revealed that 85.8% of the cases that progressed to severe disease and 78.6% of the deaths could be prevented by administering a second vaccine.Conclusion: Caregiver training for infection prevention and control is necessary to reduce infection risk. Vaccination is an important intervention to reduce the risk of progression to severe disease and death.