Relative Coronavirus Disease 2019 Mortality: A Swiss Population-based Study

Hothorn, T.; Bopp, M.; Guenthard, H. F.; Keiser, O.; Roelens, M.; Weibull, C. E.; Crowther, M. J.

Objective: Severity of the coronavirus disease 2019 (covid-19) has been assessed in terms of absolute mortality in SARS-CoV-2 positive cohorts. An assessment of mortality relative to mortality in the general population is presented. Design: Retrospective population-based study. Setting: Individual information on symptomatic confirmed SARS-CoV-2 patients and subsequent deaths from any cause were compared to the all-cause mortality in the Swiss population of 2018. Starting February 23, 2020, mortality in covid-19 patients was monitored for 80 days and compared to the population mortality observed in the same time-of-year starting February 23, 2018. Participants: 5 160 595 inhabitants of Switzerland aged 35 to 95 without covid-19 (general population in spring 2018) and 20 769 persons tested positively for covid-19 (spring 2020). Measurements: Sex- and age-specific mortality rates were estimated using Cox proportional hazards models. Absolute probabilities of death were predicted and risk was assessed in terms of relative mortality by taking the ratio between the sex- and age-specific absolute mortality in covid19 patients and the corresponding mortality in the 2018 general population. Results: A confirmed SARS-CoV-2 infection substantially increased the probability of death across all patient groups, ranging from nine (6 to 15) times the population mortality in 35-year old infected females to a 53-fold increase (46 to 59) for 95 year old infected males. The highest relative risks were observed among males and older patients. The magnitude of these effects was smaller compared to increases observed in absolute mortality risk. Male covid-19 patients exceeded the population hazard for males (hazard ratio 1.20, 1.00 to 1.44). Each additional year of age increased the population hazard in covid-19 patients (hazard ratio 1.04, 1.03 to 1.05). Limitations: Information about the distribution of relevant comorbidities was not available on population level and the associated risk was not quantified. Conclusions: Health care professionals, decision makers, and societies are provided with an additional population-adjusted assessment of covid-19 mortality risk. In combination with absolute measures of risk, the relative risks presented here help to develop a more comprehensive understanding of the actual impact of covid-19.