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

  1. Kristoffer Strålin
  2. Erik Wahlström
  3. Sten Walther
  4. Anna M Bennet-Bark
  5. Mona Heurgren
  6. Thomas Lindén
  7. Johanna Holm
  8. Håkan Hanberger

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Abstract

Background

During 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.

Method

Retrospective nationwide observational study of all patients hospitalised in Sweden between March 1 st and December 31 st , 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.

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 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 context

Evidence before this study

During 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 value

This 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 evidence

While 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

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  1. ScreenIT

    SciScore for 10.1101/2021.03.29.21254557: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    Institutional Review Board Statementnot detected.
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.
    Sex as a biological variablenot detected.

    Table 2: Resources

    Software and Algorithms
    SentencesResources
    , SAS Institute Inc.
    SAS Institute
    suggested: (Statistical Analysis System, RRID:SCR_008567)

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:
    The study has limitations as mentioned before [1]. First, data on disease severity of non-ICU-treated patients and data on do-not-resuscitate orders were not available. Second, we did not have access to data on drug treatment of non-ICU-treated patients and could not assess its impact on outcome. Third, the study lacked information on hospital bed occupancy and staff-to-patient ratio, which is important when assessing association between patient load and outcome. In conclusion, the decrease in mortality that was noted during the summer of 2020 did not continue during the second pandemic wave. Instead the 60-day mortality increased. 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.

    Results from TrialIdentifier: No clinical trial numbers were referenced.

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

    About SciScore

    SciScore is an automated tool that is designed to assist expert reviewers by finding and presenting formulaic information scattered throughout a paper in a standard, easy to digest format. SciScore checks for the presence and correctness of RRIDs (research resource identifiers), and for rigor criteria such as sex and investigator blinding. For details on the theoretical underpinning of rigor criteria and the tools shown here, including references cited, please follow this link.

  2. Version published to 10.1101/2021.03.29.21254557 on medRxiv