Second wave mortality among patients hospitalised for COVID-19 in Sweden: a nationwide observational cohort study

Background During the first pandemic wave, a substantial decline in mortality was seen among hospitalized COVID-19 patients. We aimed to study if the decreased mortality continued during the second wave, using data compiled by the Swedish National Board of Health and Welfare. Method Retrospective nationwide observational study of all patients hospitalized in Sweden between March 1st and December 31st, 2020, with SARS-CoV-2 RNA positivity 14 days before to 5 days after admission and a discharge code for COVID-19. Outcome was 60-day all-cause mortality. Poisson regression was used to estimate the relative risk (RR) for death by month of admission, adjusting for age, sex, socioeconomic data, comorbidity, care dependency, and country of birth. Findings A total of 32 452 patients were included. December had the highest number of admissions/month (n=8253) followed by April (n=6430). The 60-day crude mortality decreased from 24.7% (95% CI, 23.0%-26.5%) for March to 10.4% (95% CI, 8.9%-12.1%) for July-September (as reported previously), later increased to 19.9% (95% CI, 19.1-20.8) for December. RR for 60-day death for December (reference) was higher than those for June to November (RR ranging from 0.74 to 0.89; 95% CI <1 for all months). SARS-CoV-2 variants of concern were only sporadically found in Sweden before January 2021. Interpretation The decreased mortality of hospitalized COVID-19 patients after the first wave turned and increased during the second wave.