Evaluation of ward-level risk factors for nosocomial COVID-19 outbreaks: A matched case-control study

Introduction: The prevention of SARS-CoV-2 outbreaks in acute care hospitals is a major challenge. In the second wave of the pandemic, we observed several wards with outbreaks whereas others were spared. Objectives: We aimed to investigate risk factors for nosocomial COVID-19 outbreaks on a ward level. Methods: We conducted a matched case-control study in our tertiary care centre with over 700 beds by defining outbreak (≥ 2 nosocomial patients infected within a 14-day period) and control wards. Nosocomial infection was defined if a patient tested positive for COVID-19 on day 5 or later of hospitalisation. Matching was done 1:1 for approximate number of beds (± 10) and the time of the outbreak. The beginning of the outbreak was defined as the day of the first positively tested nosocomial COVID-19 case on the ward. Intensive care units and designated COVID- 19 wards were excluded. Presumed ward-, patient- and staff-specific variables were investigated. Paired Wilcoxon signed-rank test was used to compare variables between outbreak- and control wards. Results: From July to December 2020, we observed 9 outbreak wards (surgical and medical, range of beds 17 - 31, range nurses 19 - 41 per ward) with a total of 40 patients infected (range 2-7 per ward). The percentage of healthcare workers (HCW) tested positive within a period of 14 days prior until 2 days after the start of the outbreak was the only significant risk factor (9.7% vs 2.7%, p = 0.04). No difference in the percentage of infected HCW was observed in a time period further preceding the start of the outbreak (3 months to 2 weeks). Outbreak wards trended towards a higher number of beds per room (2.22 vs 1.97, p = 0.09) and a younger HCW age (33.3 vs 36.2 years, p = 0.17) compared to control wards. No association was found for factors reflecting work-load, patient turnover, or work experience of HCW (Table). Conclusion: Increased numbers of infected HCW shortly before the outbreak seem to be a risk factor for nosocomial SARS-CoV-2 outbreaks. This supports the notion that infected HCW are an important source of nosocomial COVID-19 and underscores the importance of adequate infection control- and prevention measures of HCW in- and outside the hospital. (Figure Presented).