The VISION network includes Columbia University Irving Medical Center (CUIMC; New York), HealthPartners Institute (Minnesota and Wisconsin), Intermountain Healthcare (Utah), Kaiser Permanente Northern California (KPNC), Kaiser Permanente Northwest Center for Health Research (KPNW; Oregon and Washington), Regenstrief Institute (Indiana), and the University of Colorado. All seven network partners contributed data on hospitalizations and ICU visits from a total of 187 hospitals; three partners also contributed data on visits to a total of 167 emergency departments and a total of 54 urgent care clinics. The partners categorized their medical facilities into a total of 36 geographic subregions (Table 1, and Table S1 in the Supplementary Appendix, available with the full text of this article at NEJM.org).
A full description of the study methods is available in the Supplementary Appendix. In brief, in a study involving adults (≥50 years of age), we used a test-negative design to assess the effectiveness of Covid-19 vaccines with respect to hospitalization lasting more than 24 hours, ICU admission (as a subset of hospitalization), or an emergency department or urgent care clinic visit associated with laboratory-confirmed SARS-CoV-2 infection and a diagnosis consistent with Covid-19–like illness.15-17 (link)Covid-19–like illness was defined as a clinical diagnosis of acute respiratory illness (e.g., Covid-19, respiratory failure, or pneumonia) or signs or symptoms (e.g., cough, fever, dyspnea, vomiting, or diarrhea) that have been associated with Covid-19 in previous studies.12 (link),13 (link),18 (link) We identified Covid-19–like illness using discharge codes (Table S2) from the International Classification of Diseases, 9th and 10th Revisions (ICD) that are based on previous studies of other viral respiratory diseases.19 (link),20 (link) Data on hospital readmissions within 30 days after discharge, repeat emergency department visits within 24 hours, or repeat visits to urgent care clinics within 24 hours were combined and analyzed as single medical visits within each clinical setting.
The demographic characteristics of the patients and their underlying medical conditions (defined according to the ICD codes that were assigned at the visit) were extracted from medical records. The study protocol, available at NEJM.org, was reviewed and approved by the institutional review boards of the study partners. The study sponsor did not place limitations on publication or require confidentiality in the reporting of results.