In a household contact study of index cases with COVID-19 spanning pre-alpha, and alpha-virus (B.1.1.7) pandemic waves in the UK39, blood transcript levels of MX1 and IFI27 achieved equivalently good discrimination of contacts with and without prevalent SARS-CoV-2 infection at recruitment (day 0, AUROC 0.97, 0.92-1). The gene discussed is MX1; the disease is COVID-19.