Vitamin D is a lipid-soluble vitamin that acts as a hormone as well as a micronutrient. Through vitamin D receptors (VDR) expressed on immune cells, vitamin D polarizes Th1/Th2 cytokine balance in pregnancy regulating humoral immune responses triggered by B cell antigens. How plasma concentrations of 25-hydroxyvitamin D affect the quantity of anti-measles virus and anti-diphtheria toxoid vaccine-antigen antibodies during pregnancy in early life years remain unclear. This study assumed that vitamin D affects antibody responses to infections and therefore investigated its association with IgG levels following measles virus and diphtheria toxoid vaccination during childhood. Specifically, the study quantified plasma concentrations of 25 (OH) D, anti-measles virus, and anti diphtheria toxoid IgG, and computed the association between 25(OH) D and the vaccine-induced IgG responses in mother-child pairs. This was a hospital-based prospective cohort study of pregnant mothers from their first trimester to delivery, and mother-infant pairs from delivery/birth up to 18 months old.