Transferring inpatients between wards drives large nosocomial COVID-19 outbreaks, Wales, 2020-22: a matched case-control study using routine and enhanced surveillance data

Abstract

Background: The role of the hospital environment in the spread of COVID-19 is unclear.

Aim: To measure associations between ward characteristics and outbreak size to inform mitigations.

Methods: Wards with large (case wards) and small (control wards) outbreaks in three acute hospitals were compared. Cases were healthcare-associated COVID-19 inpatients (positive polymerase chain reaction test ≥8 days post admission). Case wards were adult medical/surgical wards with ≥10 cases within rolling 14-day periods, between April 1^st, 2020 and April 30^th, 2022. Control wards were equivalents with 2-9 cases. Demographic and laboratory data were extracted from routine surveillance systems. Continuous data were aggregated fortnightly and analysed as binary variables according to median values. Each case ward was compared with two control wards matched on outbreak start date (±14 days) to calculate odds ratios (ORs) and 95% confidence intervals (95% CIs) using univariable and conditional multivariable logistic regression.

Findings: From 170 outbreaks (median: 5 cases; interquartile range: 2-9), 35 case wards were identified. Community admissions were lower in case wards vs control wards (5 vs 10 median admissions; P<0.01, respectively), whereas transfers between wards within the same hospital were higher (58 vs 29 median transfers; P<0.01, respectively). Wards with more transfers in the preceding fortnight were significantly more likely to experience a large outbreak (≥35 vs <35 transfers; adjusted OR: 9.08; 95% CI: 2.5-33).

Conclusion: We recommend safely minimizing patient movements, such as by asking clinicians to record the rationale for transfer, to reduce the likelihood of disease transmission.

Keywords: COVID-19; Hospital; Nosocomial; Outbreak; Patient transfers; SARS-CoV-2; Wales.