A nucleoside-modified messenger RNA (modRNA) is a synthetic messenger RNA (mRNA) in which some nucleosides are replaced by other naturally modified nucleosides or by synthetic nucleoside analogues.[1] modRNA is used to induce the production of a desired protein in certain cells. An important application is the development of mRNA vaccines, of which the first authorized were COVID-19 vaccines (such as Comirnaty and Spikevax).