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

Joseph A Lewnard; Vincent X Liu; Michael L Jackson; Mark A Schmidt; Britta L Jewell; Jean P Flores; Chris Jentz; Graham R Northrup; Ayesha Mahmud; Arthur L Reingold; Maya Petersen; Nicholas P Jewell; Scott Young; Jim Bellows

The KPNC, KPSC, and KPWA systems deliver fully integrated healthcare to diverse membership cohorts generally resembling the commercially-insured populations of the surrounding geographic areas. [8] [9] [10] We analyzed clinical and administrative data captured from all KP members who had been hospitalized within these KP care delivery systems with COVID-19 laboratory or clinical diagnoses at any recorded healthcare encounter by April 9, 2020. We considered patients to be clinically confirmed if no positive test result was available and diagnosis codes included any of the six diagnoses listed in Table S1 . Clinical diagnoses were overruled by negative laboratory test results. Encounters included hospital, outpatient, and telehealth visits as well as uses of laboratory diagnostic services. We limited hospitalizations to acute inpatient admissions occurring between 14 days before to 28 days after the first encounter resulting in a COVID-19 diagnosis. Because only the most recent clinical encounter resulting in a COVID-19 diagnosis was available for KPWA patients, we considered COVID-19 hospitalizations to include those for which admission or discharge occurred within 30 days of the most recent COVID-19 encounter. Observational admissions were excluded. For the KPNC and KPSC cohorts, hospitalization events were sourced from electronic medical records and outside medical claims. Hospitalizations in the KPWA cohort were sourced from a centralized admissions database.