Converts 5-O-serinylthymidine (O-SerT) into 5-aminoethoxy-2'-deoxymethyluridine (5-NeOmdU) as a step in the pathway leading to thymidine hypermodifications in the viral genome (PubMed:34522950). As a final result of the pathway of hypermodification, 5-NeOmdU substitutes for about 40% of the thymidines in the viral DNA (PubMed:34522950, PubMed:29555775). These modifications probably prevent degradation of viral genome by the host restriction-modification antiviral defense system (PubMed:34522950). 5-O-(L-seryl)-dTMP in DNA + H(+) = 5-aminoethoxy-methyl-dUMP in DNA + CO2 Belongs to the pyridoxal-phosphate-dependent aminodecarboxylase family.