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

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 the conditions under which people refer themselves for testing have greatly changed: originally, in most countries priority was given to people with symptoms, but nowadays people without symptoms are also tested for different reasons, e.g., during contact tracing, or to be allowed on an (international) flight. Interestingly, this opens new possibilities to separately investigate risk factors for infection and risk factors for becoming diseased. To use this new situation to best effect, one has to think carefully about how to elucidate the different reasons for testing and what analyses one might do with the different groups.