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Background. The Republic of the Marshall Islands (RMI) is a South Pacific nation freely associated with the United States. In 2003, the RMI experienced the largest measles outbreak within the United States or its associated areas for more than a decade, although the reported coverage of 1-dose measles-mumps-rubella (MMR) vaccine was 80%-93%. The outbreak ended only after vaccination of >35,000 persons among a population of 51,000. Of outbreak cases, 41% were reported to have been previously vaccinated. We studied measles attack rates in RMI households to assess vaccine effectiveness and patterns of disease transmission. Methods. For the household secondary attack rate study, households were selected by convenience sampling of outbreak measles cases. The primary case was defined as the first person with measles in a household. Secondary cases were household members with measles onset 7-18 days after the primary case's rash onset. Vaccine effectiveness analysis was limited to children aged 6 months to 14 years, with vaccination status verified against written records. Results. Seventy-two households were included in the study. The median household size was 11 persons, and the median number of persons per room was 5.5. Secondary cases were more likely than primary cases to be infants (46% vs. 13%; P = .03). MMR vaccine effectiveness was 92% (95% confidence interval [CI], 67%-98%) for 1 dose and 95% (95% CI, 82%-98%) for 2 doses. Conclusions. Measles vaccine effectiveness was high; thus, diminished effectiveness was not the main cause of the outbreak. In communities with high population density and household crowding, very high population immunity is needed to prevent measles outbreaks and to protect infants below the age of vaccination. This may require excellent implementation of a routine 2-dose measles vaccination strategy. [ABSTRACT FROM AUTHOR]