Facility-specific epidemiological characteristics of COVID-19 clusters across SARS-CoV-2 variant waves in South Korea

Few national-scale, multi-setting studies have compared COVID-19 cluster characteristics across successive SARS-CoV-2 variant periods. We assessed how outbreak duration, cluster size, and setting distribution shifted across six facility types and variant periods using nationwide outbreak investigation data.

We analyzed 4592 COVID-19 clusters (≥10 cases) in South Korea from January 2020 to February 2022, spanning three variant periods (Before Delta, Delta, and Early Omicron) and six facility types. We compared outbreak duration and cluster size across facility types and periods using nonparametric tests and examined shifts in facility distribution. Sensitivity analyses used threshold combinations for cluster size and duration.

A total of 162,176 cases were linked to 4592 clusters. Outbreak duration decreased across variant periods (mean 17.31 days Before Delta vs 8.84 days Early Omicron), while cluster size remained stable by facility type. Religious and public-use facilities predominated during the Before Delta period, whereas educational and long-term care facilities predominated during Early Omicron. Among large and prolonged clusters (≥100 cases and ≥20 days), outbreak durations were similar across periods.

Viral characteristics shape early outbreaks, whereas facility-specific structural vulnerabilities sustain prolonged transmission. An effective response should combine variant-informed early interventions with structural and operational infection-control measures in high-risk settings.