Background: The immune response of COVID-19-vaccinated women with vitamin D deficiency was investigated by giving vitamin D supplementation. Methods: The study consisted of young volunteer women who completed two doses of COVID-19 vaccines and spent an average of more than 65 days. Participants in the D group (n=14 Pfizer-BioNTech, 2 Sinovac) were given 150,000 IU vitamin D supplements, and no supplement was given to the C group (n=14 Pfizer-BioNTech, 3 Sinovac). Results: When the last measurements were examined (W3) after vitamin D supplementation, it was observed that serum 25(OH) D levels regularly increased (p=0.001). It was observed that the W1 measurement of the D group was lower than those of the C group (P<0.001), and there was a significantly higher increase in the W2 measure (P<0.001). In addition, although there was no correlation between serum IgG and 25(OH) D in the D group at the beginning, a significant positive correlation was found in the final measurement (r=0.558, p=0.031). Conclusion: It was observed that the use of vitamin D significantly increased serum IgG levels compared to those who did not use vitamin D, depending on the duration, and in the long term, it was observed that it remained above the baseline. It has been determined that there is a positive and significant correlation between the last measurement of serum IgG and 25(OH) vitamin D levels during the use of vitamin D.