Prevalence, risk factors, and outcomes of secondary infections among hospitalized patients with COVID-19 or post-COVID-19 conditions in Victoria, 2020-2023.

Objectives: Estimates of secondary infections are variedly reported, with few studies done in Australia. We investigated the occurrence and impact of secondary infections complicating COVID-19 and post-COVID-19 admissions in Victoria, Australia, 2020-2023.

Methods: We used linked population-wide data sets and specific International Classification of Disease, 10th Revision codes to identify and estimate the occurrence of secondary infections. Using hospital/intensive care unit length of stay in negative binomial regression and mortality, we examined the impact of secondary infections.

Results: Secondary infections were identified in 6.9% (13,467 of 194,660) of COVID-19 and post-COVID-19 admissions: 6.0% (11,651 of 194,660) bacterial, 0.9% (1691 of 194,660) viral, and 0.2% (385 of 194,660) fungal. Prevalence was highest during the pre-Delta (10.4%) and Omicron-BA2 (8.1%) periods. Sepsis and pneumonia were the most reported syndromes; the occurrence of sepsis declined gradually over time. The odds of secondary infections were higher among the ≥70-year-olds (adjusted odds ratio (aOR) 3.76, 95% confidence interval [CI] 3.43-4.14, vs 20-29-year-olds), individuals with chronic conditions (aOR 3.15, 95% CI 2.88-3.45, vs those without), the unvaccinated (aOR 1.59, 95% CI 1.45-1.75), and the lowest socioeconomic group (aOR 1.12, 95% CI 1.05-1.19). Patients with secondary infections had 2.43 times longer hospital length of stay and 9.60 times longer intensive care unit length of stay than those without secondary infections. The mortality risk was 2.17 times higher in those with secondary infections.

Conclusions: Secondary infections occurred in 69 per 1000 COVID-19-associated hospital admissions in Victoria, mostly in high-risk groups, and were associated with severe outcomes.