Nosocomial SARS-CoV-2 Infections and Mortality During Unique COVID-19 Epidemic Waves

Abstract

Importance: Quantifying the burden of nosocomial SARS-CoV-2 infections and associated mortality is necessary to assess the need for infection prevention and control measures.

Objective: To investigate the occurrence of nosocomial SARS-CoV-2 infections and associated 30-day mortality among patients admitted to hospitals in Region Stockholm, Sweden.

Design, setting, and participants: A retrospective, matched cohort study divided the period from March 1, 2020, until September 15, 2022, into a prevaccination period, early vaccination and pre-Omicron (period 1), and late vaccination and Omicron (period 2). From among 303 898 patients 18 years or older living in Region Stockholm, 538 951 hospital admissions across all hospitals were included. Hospitalized admissions with nosocomial SARS-CoV-2 infections were matched to as many as 5 hospitalized admissions without nosocomial SARS-CoV-2 by age, sex, length of stay, admission time, and hospital unit.

Exposure: Nosocomial SARS-CoV-2 infection defined as the first positive polymerase chain reaction test result at least 8 days after hospital admission or within 2 days after discharge.

Main outcomes and measures: Primary outcome of 30-day mortality was analyzed using time-to-event analyses with a Cox proportional hazards regression model adjusted for age, sex, educational level, and comorbidities.

Results: Among 2193 patients with SARS-CoV-2 infections or reinfections (1107 women [50.5%]; median age, 80 [IQR, 71-87] years), 2203 nosocomial SARS-CoV-2 infections were identified. The incidence rate of nosocomial SARS-CoV-2 infections was 1.57 (95% CI, 1.51-1.64) per 1000 patient-days. In the matched cohort, 1487 hospital admissions with nosocomial SARS-CoV-2 infections were matched to 5044 hospital admissions without nosocomial SARS-CoV-2 infections. Thirty-day mortality was higher in the prevaccination period (adjusted hazard ratio [AHR], 2.97 [95% CI, 2.50-3.53]) compared with period 1 (AHR, 2.08 [95% CI, 1.50-2.88]) or period 2 (AHR, 1.22 [95% CI, 0.92-1.60]). Among patients with nosocomial SARS-CoV-2 infections, 30-day AHR comparing those with 2 or more doses of SARS-CoV-2 vaccination and those with less than 2 doses was 0.64 (95% CI, 0.46-0.88).

Conclusions and relevance: In this matched cohort study, nosocomial SARS-CoV-2 infections were associated with higher 30-day mortality during the early phases of the pandemic and lower mortality during the Omicron variant wave and after the introduction of vaccinations. Mitigation of excess mortality risk from nosocomial transmission should be a strong focus when population immunity is low through implementation of adequate infection prevention and control measures.

Figure 1.. Incidence Rate per 1000 Patient-Days and Daily Count of Nosocomial SARS-CoV-2 Infections

The left axis shows the daily count of nosocomial SARS-CoV-2 infections between March 2020 and September 2022. The right axis shows the incidence rate of nosocomial SARS-CoV-2 infections between March 2020 and September 2022 calculated using a 14-day moving mean. To calculate the patient-days at risk, only admissions with length of stay greater than 7 days were included.

Figure 2.. Unadjusted Kaplan-Meier Curves and Risk Tables for 30-Day All-Cause Mortality in the Matched Cohort

The P value represents the result of significance testing performed using log-rank tests between the COVID-19 and non–COVID-19 groups.

Figure 3.. Forest Plot of Crude and Adjusted Hazard Ratios for 30-Day Mortality Comparing the COVID-19 and Non–COVID-19 Groups

The Cox proportional hazards regression model was adjusted for age, sex, comorbidities, and educational level. Comorbidities included cancer, cardiovascular diseases, chronic kidney diseases, chronic lung diseases, diabetes, hypertension, and immunosuppression. Error bars indicate 95% CIs.