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

BackgroundDuring the first pandemic wave, a substantial decline in mortality was seen among hospitalised 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. MethodRetrospective nationwide observational study of all patients hospitalised 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, socio-economic data, comorbidity, care dependency, and country of birth. FindingsA 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{middle dot}7% (95% CI, 23{middle dot}0%-26{middle dot}5%) for March to 10{middle dot}4% (95% CI, 8{middle dot}9%-12{middle dot}1%) for July-September (as reported previously), later increased to 19{middle dot}9% (95% CI, 19{middle dot}1-20{middle dot}8) for December. RR for 60-day death for December (reference) was higher than those for June to November (RR ranging from 0{middle dot}74 to 0{middle dot}89; 95% CI <1 for all months). SARS-CoV-2 variants of concern were only sporadically found in Sweden before January 2021. InterpretationThe decreased mortality of hospitalised COVID-19 patients after the first wave turned and increased during the second wave. Focused research is urgent to describe if this increase was caused by a high load of patients, management and treatment, viral properties, or other factors. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSDuring the first pandemic wave, a substantial decline in mortality was seen among hospitalised COVID-19 patients in many countries. As the reason for this decline has not been clarified, no one could foresee how mortality would change during forthcoming waves. Added valueThis retrospective nationwide study of all patients hospitalised for COVID-19 in Sweden from March to December 2020 showed that the gradual decrease in mortality seen in the first pandemic wave was followed by an increased crude and adjusted 60-day all-cause mortality during the second wave. This increase in mortality occurred although the standard-of-care recommendations for hospitalised COVID-19 patients did not change in Sweden during the second half of 2020. Implications of all the available evidenceWhile improved standard-of-care was believed to be an important factor for the decrease in mortality during the first pandemic wave, the increasing mortality during the second wave has no apparent explanation. As the currently known virus variants of concern occurred only sporadically in Sweden before January 2021, they were most likely not involved. Focused research is urgent to describe if this increase in mortality was caused by a high load of patients, management and treatment factors, viral properties, or other circumstances