The US Department of Veterans Affairs (VA) has established a definition for its health-care system users, specifically for studying COVID-19 cases. To be included in the study, individuals must have had at least two outpatient encounters, pharmacy encounters, and routine laboratory tests, each occurring at least 180 days apart within the two years prior to their test date.
There are two groups: those who tested positive for COVID-19 (positive group) and those who tested negative (control group). The ‘T0’ date is defined as the first positive test result in the positive group. For the control group, T0 corresponds to the date of the first negative test that aligns with the first positive test in the positive group.
The study excludes individuals from hospital admissions during the acute phase of infection and those diagnosed with respiratory viral illnesses during this period.
Source: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(24)00831-4/abstract