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This paper is a short extension of our previous paper [arXiv:2004.06033]about the use of the Test-Negative design to study risk factors for COVID-19[See: PubMed and ArXiv reference below] Reason for the extension is that theconditions under which people refer themselves for testing have greatlychanged: originally, in most countries priority was given to people withsymptoms, but nowadays people without symptoms are also tested for differentreasons, e.g., during contact tracing, or to be allowed on an (international)flight. Interestingly, this opens new possibilities to separately investigaterisk factors for infection and risk factors for becoming diseased. To use thisnew situation to best effect, one has to think carefully about how to elucidatethe different reasons for testing and what analyses one might do with thedifferent groups.

The evolving usefulness of the Test-Negative Design in studying risk factors for COVID-19 due to changes in testing policy