  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.
