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

Abstract

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 (R_E ) 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 R_E 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.

Fig 1

Cumulative incidence and daily incidence of first acute inpatient admissions in 2020 with confirmed coronavirus disease 2019 among participants with Kaiser Permanente health plans. Shaded regions (10 April onwards) indicate admission dates when participants were excluded from analyses of clinical outcomes (duration of hospital stay, admission to intensive care unit, and case fatality risk) owing to limited follow-up duration

Fig 2

Age specific cumulative incidence of first inpatient admissions in 2020 with confirmed coronavirus disease 2019 among participants with Kaiser Permanente health plans. Shaded regions (10 April onwards) indicate admission dates when participants were excluded from analyses of clinical outcomes (duration of hospital stay, admission to intensive care unit, and case fatality risk) owing to limited follow-up duration

Fig 3

Fitted durations of hospital admissions for first acute inpatient admissions with confirmed coronavirus disease 2019 by 9 April 2020 among all participants with Kaiser Permanente health plans admitted. Distributions of hospital length of stay for all acute inpatient admissions (top left); acute inpatient admissions, stratified by clinical outcome (top right); time to discharge or death among all acute inpatient admissions; and length of stay in an intensive care unit (ICU) for all inpatients admitted to ICU (bottom left). Box plots overlaid on density estimates indicate estimated medians (center line), interquartile ranges (boxed area), and 2.5% to 97.5% centile values (extended lines). Estimates were obtained by fitting Weibull distributed competing risk models to data on patient survival or non-survival as recorded by end of day 22 April 2020

Fig 4

Estimated probabilities of admission to intensive care unit (ICU) and mortality in enrollees of Kaiser Permanente health plans by age and sex for those with coronavirus disease 2019 (covid-19) admitted by 9 April 2020. Probabilities were stratified by age and sex for ICU admission (left) and mortality (right) for male and female patients within 10 year age stratums; children aged 0-9 years are excluded as none were admitted to hospital in this age group. Numerical estimates are indicated alongside plotted values. Sex specific estimates for all ages are plotted at the bottom. Estimates were obtained using parametric (Weibull) survival models to account for censoring of observations among incomplete hospital admissions