The focus of the present longitudinal study, to examine mother-infant interaction during the administration of immunizations at 2 and 6?months of age, used hidden Markov modelling, a time series approach that produces latent states to describe how mothers and infants work together to bring the infant to a soothed state. Results revealed a four-state model for the dyadic responses to a 2-month inoculation whereas a six-state model best described the dyadic process at 6?months. Two of the states at 2?months and three of the states at 6?months suggested a progression from high-intensity crying to no crying with parents using vestibular and auditory soothing methods. The use of feeding and/or pacifying to soothe the infant characterized one 2-month state and two 6-month states. These data indicate that with maturation and experience, the mother-infant dyad is becoming more organized around the soothing interaction. Using hidden Markov modelling to describe individual differences, as well as normative processes, is also presented and discussed.