Emergence of the Delta and Omicron Variants of COVID-19 Clusters in a Long-term Care Hospital, Seoul, Korea: Focusing on Outbreak Epidemiology, Incidence, Fatality, and Vaccination.

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.