# From wild-type to Omicron: changes in SARS-CoV-2 hospital cluster dynamics. Observations from a German tertiary care hospital

## TL;DR

The study compares SARS-CoV-2 hospital clusters caused by wild-type/Alpha/Delta and Omicron variants in a German hospital, finding Omicron clusters had faster dynamics but stable characteristics.

## Contribution

The paper provides insights into how SARS-CoV-2 cluster dynamics changed with the emergence of the Omicron variant in a healthcare setting.

## Key findings

- Omicron clusters had a significantly shorter time from onset to case detection compared to wild-type/Alpha/Delta clusters.
- Omicron clusters showed rapid dynamics but stable characteristics, suggesting improved compliance with infection control measures.
- No significant differences were found in cluster size, duration, or healthcare worker involvement between variant groups.

## Abstract

SARS-CoV-2 hospital clusters are a challenge for healthcare systems. There is an increased risk of infection for both healthcare workers (HCWs) and patients; cluster countermeasures are also a drain on resources for the wards affected. We analysed to which extent characteristics and dynamics of SARS-CoV-2 clusters varied throughout the pandemic at a German university hospital.

Patient and/or HCW clusters from 10/2020 to 04/2022 were included in the study and grouped by virus variant into i.) clusters comprised of the presumably predominant wild-type, Alpha or Delta (WAD) SARS-COV-2 variants, and ii.) clusters comprised predominantly of Omicron subtype cases. The two groups were compared for specific characteristics and dynamics.

Forty-two SARS-CoV-2 clusters and 528 cases were analysed. Twenty-one clusters and 297 cases were attributed to the WAD and 21 clusters and 231 cases to the Omicron group. There were no significant differences in median size (8 vs. 8 cases, p=0.94) or median duration (14 vs. 12 days; p=0.48), nor in the percentage of HCWs involved (46.8% vs. 50.2%; p=0.48). Patients in the WAD group were older (median 75 vs. 68 years of age; p≤0.05). The median time from cluster onset to case onset was significantly shorter for the Omicron group (median 6 vs. 11 days; p≤0.05).

Omicron clusters exhibited a more rapid dynamic, forcing all parties involved to adapt to the increased workload. Compared to excessive community case counts, constant Omicron cluster-affiliated case counts and stable cluster characteristics suggest an improved compliance with IPC countermeasures.