The COVID mRNA vaccines utilize the modified nucleobase N1-methylpseudouridine, in place of canonical uridine, to improve immunogenicity and protein yield. However, relatively few studies have investigated the effect of modified nucleobases on the fidelity of protein translation. Given the interest in the COVID mRNA vaccines, we sought to investigate how N1-methylpseudouridine (and the related modification pseudouridine) is read by ribosomes.