Nosocomial COVID-19 infection in the era of vaccination and antiviral therapy

Coronavirus disease 2019 (COVID-19) vaccination and antiviral therapies have altered the course of the COVID-19 pandemic through mitigating severe illness and death. However, immunocompromised, elderly and multimorbid patients remain at risk of poor outcomes and are overrepresented in hospital populations. The aim of this study was to describe the characteristics and outcomes of patients with nosocomial COVID-19 infection. This was a retrospective, observational study of patients who acquired COVID-19 after 7 days of hospital admission within the Southern Adelaide Local Health Network (SALHN) in South Australia between 1 June 2022 and 30 November 2022. Data were ascertained from the electronic medical record and the South Australian registry of births, deaths and marriages. Of 1084 COVID-19 inpatient cases managed in SALHN, 295 (27%) were nosocomial, with 215 included in the study. The median age of patients was 80 years (interquartile range [IQR], 68-88 years), the median Charlson Comorbidity Index score was 5 (IQR, 4-7) and 6% were immunocompromised. Most nosocomial COVID-19 infections were of mild severity (81%). The 30-day all-cause mortality rate following COVID-19 infection was 6%, and, in most cases, a cause of death other than COVID-19 was recorded on the death certificate. The majority of cases of nosocomial COVID-19 infection were mild, with a lower mortality rate than in earlier studies. This finding is likely attributable to immunity through vaccination and prior infection, early antiviral therapy and attenuated severity of the Omicron variant. The high proportion of nosocomial infections supports ongoing infection control measures.