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There are two general ways in which immunization coverage is measured: (1) age appropriate and (2) up-to-date. Age appropriate vaccination measures the proportion of children who received the recommended vaccines at (or close to) the time when these vaccines are due. Up-to-date vaccination measures the proportion of children who have received all of the recommended vaccines by a certain age (i.e., age 24 months). Up-to-date vaccination can overlook periods of time when a child is left vulnerable to diseases. For example, if a child received the MMR vaccine at age 20 months, when measuring coverage at 24 months, the child is considered up-to-date. Yet, this child was vulnerable to these diseases for a significant time period. Additionally, up-to-date measures of coverage will typically include vaccines that are administered too early and not properly spaced between doses in the series, which may impact the protection the child has to these diseases.