We investigated for the first time whether preexisting circulating CD4+FOXP3+CD127− Tregs influence antibody and cellular responses following vaccination with the live measles vaccine (MV) and killed diphtheria–tetanus–whole cell pertussis (DTP) vaccine in 9-month-old Gambian infants, and further to determine whether Treg functional capacity is altered by vaccination. Here, CD4 is linked to diphtheria.