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

Metropolitan areas of the western United States were among the first to report COVID-19 importations and domestically-acquired cases. Washington state announced the first imported case of COVID-19 in the United States on January 21, 2020, while community transmission of COVID-19 has been identified since late February in Northern California. The vast majority of cases in California are concentrated in counties surrounding the San Francisco Bay in Northern California, and Los Angeles in Southern California. These regions were also among the first to implement intensive public health interventions aiming to curtail transmission. Social distancing recommendations for vulnerable populations were issued in San Francisco on March 6, 2020, and large gatherings were banned in Washington on March 11. Large-scale stay-at-home orders were implemented March 17 for the six counties of the San Francisco Bay area, and statewide in California and Washington on March 19 and March 24, respectively. Understanding the impact of these interventions is of crucial importance to inform their broader use, and may help to account for regional differences in the severity of COVID-19 epidemics across the country. 7 To inform the epidemiology of COVID-19 in these regions, we analyzed healthcare data covering all hospitalized COVID-19 cases within the cohort of 9,596,321 individuals receiving comprehensive, integrated care from Kaiser Permanente (KP) healthcare systems in Northern California (KPNC), Southern California (KPSC), and Washington state (KPWA).