Risk Factors Related to COVID-19 Reinfection and Fatality During the Omicron (BA.1/BA.2) Period in Korea.

BACKGROUND: This study aimed to investigate the deaths due to coronavirus disease 2019 (COVID-19) reinfection and related risk factors. METHODS: National cohort data were collected for a six-month period when omicron BA.1/BA.2 variant was dominant in South Korea. RESULTS: The long-term care facility residents (adjusted odds ratio, 3.11; 95% confidence interval [CI], 2.98-3.25) had significantly higher risk of reinfection than the general population. The risk of reinfection was significantly lower for persons with 2 or more vaccine doses compared to the unvaccinated. The risk of death was significantly higher in the reinfection group than in the primary infection group for persons in the 60-74 years age group (adjusted relative risk [aRR], 1.62; 95% CI, 1.19-2.20), and immunocompromised group (aRR, 4.56; 95% CI, 2.34-8.90). CONCLUSION: In these data, vaccination history was significantly related to reduced COVID-19 reinfection and severe progression, and scheduled vaccinations were important even with a history of infection.

Comparative Effectiveness of COVID-19 Bivalent Versus Monovalent mRNA Vaccines in the Early Stage of Bivalent Vaccination in Korea: October 2022 to January 2023.

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.

Risk of SARS-CoV-2 breakthrough infection following NVX-CoV2373 and BNT162b2 vaccinations in Korean Adults: A population-based observational study.

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.

Comparing the effectiveness of bivalent and monovalent COVID-19 vaccines against COVID-19 infection during the winter season of 2022-2023: A real-world retrospective observational matched cohort study in the Republic of Korea.

OBJECTIVES: To compare the effectiveness of bivalent and monovalent COVID-19 vaccines throughout the 2022-2023 winter season based on real-world data. METHODS: This retrospective observational matched cohort study used the national vaccination program and a surveillance dataset from the Republic of Korea, and included adults aged >18 years who received bivalent or monovalent COVID-19 vaccines between October 11, 2022, and December 17, 2022. Cox proportional hazard models were used to estimate the hazard ratio for COVID-19 infection between the groups. RESULTS: We included 29,245 matched individuals in the bivalent and monovalent vaccine groups, respectively. The bivalent vaccine recipients showed 12.2% (95% confidence interval [CI] 6.5-17.7%) additional protection against COVID-19 infection compared with the monovalent vaccine recipients. The additional protection provided by bivalent vaccines was significantly higher among residents of long-term care facilities (39.4%, 95% CI 21.6-53.1%). Maximum additional protection was observed 3 to 4 months after completing the vaccination (17.6%, 95% CI 6.6-27.3%). CONCLUSION: Bivalent COVID-19 vaccines showed significantly better protection against infection than monovalent vaccines among adults during the 2022-2023 winter season. Our results highlight that immunization programs with bivalent vaccines comprising recent variants can be an effective measure to prepare for seasonal COVID-19 circulation.

Estimating the number of severe COVID-19 cases and COVID-19-related deaths averted by a nationwide vaccination campaign in Republic of Korea.

BACKGROUND: The Korea Disease Control and Prevention Agency promotes vaccination by regularly providing information on its benefits for reducing the severity of coronavirus disease 2019 (COVID-19). This study aimed to analyze the number of averted severe COVID-19 cases and COVID-19-related deaths by age group and quantify the impact of Republic of Korea's nationwide vaccination campaign. METHODS: We analyzed an integrated database from the beginning of the vaccination campaign on February 26, 2021 to October 15, 2022. We estimated the cumulative number of severe cases and COVID-19-related deaths over time by comparing observed and estimated cases among unvaccinated and vaccinated groups using statistical modeling. We compared daily age-adjusted rates of severe cases and deaths in the unvaccinated group to those in the vaccinated group and calculated the susceptible population and proportion of vaccinated people by age. RESULTS: There were 23,793 severe cases and 25,441 deaths related to COVID-19. We estimated that 119,579 (95% confidence interval [CI], 118,901-120,257) severe COVID-19 cases and 137,636 (95% CI, 136,909-138,363) COVID-19-related deaths would have occurred if vaccination had not been performed. Therefore, 95,786 (95% CI, 94,659-96,913) severe cases and 112,195 (95% CI, 110,870-113,520) deaths were prevented as a result of the vaccination campaign. CONCLUSION: We found that, if the nationwide COVID-19 vaccination campaign had not been implemented, the number of severe cases and deaths would have been at least 4 times higher. These findings suggest that Republic of Korea's nationwide vaccination campaign reduced the number of severe cases and COVID-19 deaths.

Age-specific Risk of SARS-CoV-2 Reinfection During Omicron Outbreaks, South Korea.

We assessed the risk of reinfection among all residents in South Korea who tested positive for coronavirus disease 2019 from January to August 2022. Children 5-11 years [adjusted hazard ratio (aHR) = 2.20], and 12-17 years old (aHR = 2.00), were at higher risk; whereas 3-dose vaccination (aHR = 0.20) lowered the risk of reinfection.