Intended for healthcare professionals

CCBYNC Open access

Incidence, clinical outcomes, and transmission dynamics of severe coronavirus disease 2019 in California and Washington: prospective cohort study

BMJ 2020; 369 doi: (Published 22 May 2020) Cite this as: BMJ 2020;369:m1923

Read our latest coverage of the coronavirus pandemic

Linked Editorial

Covid-19 related hospital admissions in the United States

  1. Joseph A Lewnard, assistant professor123,
  2. Vincent X Liu, research scientist4,
  3. Michael L Jackson, associate investigator5,
  4. Mark A Schmidt, investigator6,
  5. Britta L Jewell, research fellow78,
  6. Jean P Flores, principal consultant9,
  7. Chris Jentz, principal consultant9,
  8. Graham R Northrup, graduate student3,
  9. Ayesha Mahmud, assistant professor10,
  10. Arthur L Reingold, professor1,
  11. Maya Petersen, associate professor1,
  12. Nicholas P Jewell, professor111,
  13. Scott Young, senior medical director912,
  14. Jim Bellows, managing director9
  1. 1Division of Epidemiology and Biostatistics, School of Public Health, University of California, Berkeley, Berkeley, CA, 94720, USA
  2. 2Division of Infectious Diseases and Vaccinology, School of Public Health, University of California, Berkeley, Berkeley, CA, USA
  3. 3Center for Computational Biology, College of Engineering, University of California, Berkeley, Berkeley, CA, USA
  4. 4Division of Research, Kaiser Permanente, Oakland, CA, USA
  5. 5Health Research Institute, Kaiser Permanente Washington, Seattle, WA, USA
  6. 6Center for Health Research, Kaiser Permanente Northwest, Portland, OR, USA
  7. 7MRC Centre for Global Infectious Disease Analysis, Abdul Latif Jameel Institute for Disease and Emergency Analytics, Imperial College London, London, UK
  8. 8Department of Infectious Disease Epidemiology, Imperial College London, London, UK
  9. 9The Care Management Institute, Kaiser Permanente, Oakland, California 94612
  10. 10Department of Demography, University of California, Berkeley, Berkeley, CA, USA
  11. 11Department of Medical Statistics, London School of Hygiene and Tropical Medicine, London, UK
  12. 12The Permanente Federation, Kaiser Permanente, Oakland, CA, USA
  1. Correspondence to: J A Lewnard jlewnard{at} (or @jLewnard on Twitter)
  • Accepted 12 May 2020


Objective To understand the epidemiology and burden of severe coronavirus disease 2019 (covid-19) during the first epidemic wave on the west coast of the United States.

Design Prospective cohort study.

Setting Kaiser Permanente integrated healthcare delivery systems serving populations in northern California, southern California, and Washington state.

Participants 1840 people with a first acute hospital admission for confirmed covid-19 by 22 April 2020, among 9 596 321 healthcare plan enrollees. Analyses of hospital length of stay and clinical outcomes included 1328 people admitted by 9 April 2020 (534 in northern California, 711 in southern California, and 83 in Washington).

Main outcome measures Cumulative incidence of first acute hospital admission for confirmed covid-19, and subsequent probabilities of admission to an intensive care unit (ICU) and mortality, as well as duration of hospital stay and ICU stay. The effective reproduction number (RE) describing transmission dynamics was estimated for each region.

Results As of 22 April 2020, cumulative incidences of a first acute hospital admission for covid-19 were 15.6 per 100 000 cohort members in northern California, 23.3 per 100 000 in southern California, and 14.7 per 100 000 in Washington. Accounting for censoring of incomplete hospital stays among those admitted by 9 April 2020, the estimated median duration of stay among survivors was 9.3 days (with 95% staying 0.8 to 32.9 days) and among non-survivors was 12.7 days (1.6 to 37.7 days). The censoring adjusted probability of ICU admission for male patients was 48.5% (95% confidence interval 41.8% to 56.3%) and for female patients was 32.0% (26.6% to 38.4%). For patients requiring critical care, the median duration of ICU stay was 10.6 days (with 95% staying 1.3 to 30.8 days). The censoring adjusted case fatality ratio was 23.5% (95% confidence interval 19.6% to 28.2%) among male inpatients and 14.9% (11.8% to 18.6%) among female inpatients; mortality risk increased with age for both male and female patients. Reductions in RE were identified over the study period within each region.

Conclusions Among residents of California and Washington state enrolled in Kaiser Permanente healthcare plans who were admitted to hospital with covid-19, the probabilities of ICU admission, of long hospital stay, and of mortality were identified to be high. Incidence rates of new hospital admissions have stabilized or declined in conjunction with implementation of social distancing interventions.


  • Contributors: JAL, VXL, MLJ, MAS, BLJ, JPF, NPJ, SY, and JB conceived the study. JPF, CJ, and JB collected data. JAL, VXL, MLJ, MAS, BLJ, JPF, GRN, AM, NPJ, and JB designed the analyses. JAL, JPF, CJ, and GRN conducted the analyses. JAL generated the figure and wrote the first draft of the manuscript. All authors critically revised the manuscript for intellectual content. JAL acts as guarantor for this paper and accepts full responsibility for the work, had access to the data, and controlled the final decision to publish. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.

  • Funding: This study was funded by Kaiser Permanente. VXL was supported by grant R35GM128672 from the US National Institutes of Health. VXL, MLJ, MAS, JPF, CJ, SY, and JB are employees of Kaiser Permanente; however, Kaiser Permanente officers and employees other than the authors did not play a role in study design; in the collection, analysis, and interpretation of data; in the writing of the report; or in the decision to submit the article for publication. All authors had full access to all of the data (including statistical reports and tables) in the study and can take responsibility for the integrity of the data and the accuracy of the data analysis.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at JAL, NPJ, and BLJ have received honorariums from Kaiser Permanente. VXL was supported by grant R35GM128672 from the US National Institutes of Health. All other authors declare: no support from any organization for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: Retrospective reviews of deidentified data for this study were considered exempt, non-human subjects research by the Kaiser Permanente institutional review boards for northern California, southern California, and Washington state.

  • Data sharing: The supplementary tables provide aggregate data to reproduce the analyses.

  • Dissemination to participants and related patient and public communities: Results of this study have been made available to the public through an open access preprint posted to MedRxiv (doi:10.1101/2020.04.12.20062943) and have been communicated directly to policy makers. A lay version of the results will be disseminated to Kaiser Permanente healthcare plan members and the public by the Kaiser Permanente press office.

  • The manuscript’s guarantor (JAL) affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as originally planned have been explained.

This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See:

View Full Text