Occurrence of measles in a country with elimination status: Amplifying measles infection in hospitalized children due to imported virus

The Republic of Korea declared measles elimination in 2006. However, a measles outbreak occurred in 2013. This study aimed to identify the epidemiological characteristics of the sources of infection and the pattern of measles transmission in 2013 in South Korea. We utilized surveillance data, epidemiological data, immunization registry data, and genetic information. We describe the epidemiological characteristics of all measles case patients (sex, age distribution, vaccination status, sources of infection) as well as details of the outbreak (the pattern of transmission, duration, mean age of patients, and generation time). In 2013, a total of 107 measles cases were notified. Most patients were infants (43.0%) and unvaccinated individuals (60.7%). We identified 4 imported and 103 import-related cases. A total of 105 cases were related to four outbreaks that occurred in Gyeongnam, northern Gyeonggi, southern Gyeonggi, and Seoul. The predominant circulating genotype was B3 type, which was identified in the Gyeongnam, northern Gyeonggi, and southern Gyeonggi outbreaks. The B3 type had not been in circulation in South Korea in the previous 3 years; virologic evidence suggests that these outbreaks were import-related. Most measles cases in South Korea have been associated with imported measles virus. Although Korea has maintained a high level of herd immunity, clustering of susceptible people can cause such measles outbreaks.