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Incidence, clinical outcomes, and transmission dynamics of hospitalized 2019 coronavirus disease among 9,596,321 individuals residing in California and Washington, United States: a prospective cohort study

Our take —

This study, available as a preprint and thus not yet peer reviewed, leveraged private healthcare insurance data from California and Washington state to describe clinical outcomes and transmission dynamics of COVID-19 prior to April 9, 2020. The cumulative incidence of hospitalization due to COVID-19 was 10.6-12.4 per 100,000; the probability of ICU admission and mortality among those hospitalized was 41.9% (34.1-51.4%) and 17.8% (14.3- 22.2%), respectively. The estimated effective reproductive number was lower, and likely <1, on March 20 compared to March 1, 2020. This study represents individuals with private health insurance and may be subject to selection bias, and assumes transmission exclusively within the health insurance cohorts which may not represent actual transmission patterns.

Study design

Prospective Cohort

Study population and setting

This study leveraged data of 9,596,321 individuals enrolled in comprehensive and integrated healthcare insurance plans from Kaiser Permanente in Northern California, Southern California, and Washington state. COVID-19 patients included 1277 hospitalized patients with laboratory- or clinically confirmed COVID-19 diagnosis by April 9, 2020.

Summary of Main Findings

Cumulative incidence of hospitalization due to COVID-19 among the cohort of individuals enrolled in health insurance with Kaiser Permanente was between 10.6-12.4 per 100,000. Among hospitalized confirmed cases, the probability of ICU admission was 41.9% (95% confidence interval: 34.1-51.4%) and of mortality was 17.8% (14.3- 22.2%); and the mean length of hospitalization was 11.3 days. The estimated effective reproductive number for infections acquired March 1, 2020 was 1.43 (1.17-1.73) in Northern California; 2.09 (1.63-2.69) in Southern California, and 1.47 (0.07-2.59) in Washington; and for those acquired March 20, 2020 was 0.98 (0.76-1.27), 0.89 (0.74-1.06), and 0.92 (0.05-1.55) respectively.

Study Strengths

This study leverages a large dataset of individuals enrolled in health insurance provided by a private company and is an example of utilizing such data to understand patient outcomes and transmission dynamics.

Limitations

The study population includes individuals covered by private health insurance, which may represent a population with higher employment or socioeconomic status than the general population, therefore there may be a selection bias and an underestimation of incidence and poor outcomes compared to data inclusive of individuals without access to health insurance. Lastly, the estimates for the reproductive number assumes transmission exclusively within the cohorts of individuals enrolled in Kaiser Permanente, which may not represent actual transmission patterns.

Value added

This study leverages a large longitudinal cohort and suggests a decrease in the effective reproductive number, signaling a slowing of transmission.

This review was posted on: 22 June 2020