Newcomer SR, Freeman RE, Wehner BK, et al. - In the present study, the researchers sought to quantify vaccine timeliness and the prevalence of undervaccination patterns in Montana and to measure the connections between timeliness and series completion by age 24 months. Days undervaccinated for the combined 7-vaccine series were estimated using data from Montana's centralized immunization information system from January 2015 to November 2019. Undervaccination patterns indicative of certain barriers to vaccination, including parental vaccine hesitancy, have been identified. Among 31,422 children, 38.0% got all vaccine doses on time; 24.3% received all doses, but some were received late; and 37.7% had not completed the combined 7-vaccine series. Fewer than 2 out of every 5 Montana children were fully vaccinated on time for the combined 7-vaccine series. Initiatives to boost vaccine confidence and remind parents to complete their vaccine series are needed to boost vaccination rates.