As evidenced by an outbreak earlier in 2001 in Seattle/King County, Washington, susceptible young adults may continue to sustain disease transmission caused by measles virus. In this recent outbreak, 42% of the reported cases were 29-39 years of age. The age cohort currently between 27 and 44 years of age may be particularly vulnerable because: 1) they were enrolled in public school before the implementation of effective school immunization laws requiring proof of immunity; 2) they may have been vaccinated before 12 months of age, thus circulating maternal antibody could have blunted the immune response to vaccine; or 3) waning immunity from receiving only one measles vaccine [Ed. Note: This latter reason is unlikely; the 1989-1991 national outbreak showed that waning measles immunity from a single vaccination is a very uncommon cause of susceptibility. It is far more likely that primary vaccine failure (~ 5% following a single dose of vaccine) occurred].