30-day mortality and morbidity in COVID-19 versus influenza: A populationbased study

Nersesjan, V.; Amiri, M.; Christensen, H. K.; Benros, M. E.; Kondziella, D.

Background As of July 2020, COVID-19 has caused 500,000 deaths worldwide. However, large-scale studies of COVID-19 mortality and new-onset comorbidity compared to influenza and individuals tested negative for COVID-19 are lacking. We aimed to investigate COVID-19 30-day mortality and newonset comorbidity compared to individuals with negative COVID-19 test results and individuals tested for influenza. Methods and findings This population-based cohort study utilized electronic health records covering roughly half (n=2,647,229) of Denmark's population, with nationwide linkage of microbiology test results and death records. All individuals [≥]18 years tested for COVID-19 and individuals tested for influenza were followed from November 1, 2017 to June 30, 2020. The main outcome was 30-day mortality after a test for either COVID-19 or influenza. Secondary outcomes were major comorbidity diagnoses 30-days after the test for either COVID-19 or influenza. In total, 224,639 individuals were tested for COVID-19. Among inpatients positive for COVID-19, 356 of 1657 (21%) died within 30 days, which was a 3.0 to 3.1-fold increased 30-day mortality rate, when compared to influenza and COVID-19-negative inpatients (all p<0.001). For outpatients, 128 of 6,263 (2%) COVID-19-positive patients died within 30 days, which was a 5.5 to 6.9-fold increased mortality rate compared to influenza and COVID-19-negative patients, respectively (all p<0.001). Compared to hospitalized patients with influenza, new-onset ischemic stroke, diabetes and nephropathy occurred more frequently in inpatients with COVID-19 (all p<0.05). Conclusions In this population-based study comparing COVID-19 with influenza, COVID-19 was associated with increased rates of major systemic and vascular comorbidity and substantially higher mortality, which is likely even higher than the stated 3.0 to 5.5-fold increase owing to more extensive testing for COVID-19.