An antiviral agent is provided, having a phosphorodiamidate morpholino oligomer with an antisense sequence to a portion of a genome of a strain of a coronavirus. The coronavirus may be SARS-CoV-2 or another βCoV. The antiviral agent finds many uses, such as in a pharmaceutical composition, a method of treating coronavirus-mediated disease, a method of preventing coronavirus-mediated disease, a method of reducing or preventing the replication of coronavirus in a host cell, a method of controlling the spread of coronavirus in donated tissue, a treated tissue sample, and in the manufacture of a medicament for the treatment or prevention or coronavirus-mediated disease.