Micro-projection array patches (MAPs) are an alternative approach to deliver vaccine without using needles and syringe. MAPs achieve improved vaccination efficiency by introducing a microtrauma in the skin, but a comprehensive understanding of all mechanisms behind this response remains elusive. Spatial frequency domain spectroscopy (SFDS) was used to quantify skin reactivity. This optical technique enables characterization of the skin response by identifying (1) functional tissue parameters, (2) changes in tissue structure and (3) measures of tissue damage. In a preliminary investigation, measurements were performed on 12 subjects in whom the MAP was applied on the lower forearm.