As Yek and colleagues note, test-positive cases generally have more severe disease than untested infectees. This may lead to overestimation of absolute risks, but relative risks (RRs) are not necessarily biased unless the proportion of detected severe cases differs systematically between variants. Citing modelling results that indicated a declining infection detection rate in the USA during the delta-to-omicron transition, possibly driven by increasing proportions of undetected infectees with non-severe disease, Yek and colleagues hypothesise a mechanism for differential detection rates: the test-positive omicron cases may have included a relatively higher proportion of infectees prone to severe disease than test-positive delta cases, e.g. because a higher proportion among omicron infectees who sought testing had comorbidity.