Factors associated with extent of COVID-19 outbreaks: A prospective study in a large hospital network

The COVID-19 pandemic has generated numerous hospital outbreaks. This study aimed to identify factors related to the extent of nosocomial COVID-19 outbreaks in the largest French public health institution.

An observational study was conducted from July 2020 to September 2021. Outbreaks were defined as at least 2 cases, patients and/or health care workers (HCWs), linked by time and geographic location. Logistic regression was performed to identify risk factors for large outbreaks among nine variables: variant, medical ward, COVID-19 vaccination rate and incidence among HCWs and Paris population, number of weekly COVID-19 tests among HCWs and the positivity rate, epidemic waves.

Within 14 months, 799 outbreaks were identified: 450 small ones (≤6 cases) and 349 large ones (≥7 cases), involving 3,260 patients and 3,850 HCWs. In univariate analysis, large outbreaks were positively correlated to geriatrics wards, COVID-19 incidence, and rate of weekly positive tests among HCWs; and negatively correlated to intensive care units, variant Delta, fourth wave, vaccination rates of the Paris region's population and that of the HCWs. In multivariate analysis, factors that remained significant were the type of medical ward and the vaccination rate among HCWs.

Intensive care unit and high vaccination rates among HCWs were associated with a lower risk of large COVID-19 outbreaks, as opposed to geriatric wards, which are associated with a higher risk.