Measles Outbreak Prevention and Control among Adults: Lessons from an Importation Outbreak in Yunnan Province, China, 2015

Background: The incidence of measles in Yunnan Province among vaccine target-age children has decreased markedly after attaining and sustaining high 2-dose coverage of measles containing vaccine (MCV) through routine immunization services and supplementary immunization activities (SIAs). Most cases of measles now occur among adults. In 2015, we investigated a measles outbreak among adults to determine transmission patterns and the potential role of nosocomial transmission. Methods: We enhanced measles surveillance using retrospective active case search. We conducted case investigations to determine sources of infection and routes of transmission; laboratory testing included serologic and molecular diagnostic methods. Results: Twenty-two outbreak-associated cases of measles were identified; most (86.36%) were among individuals 20 to 39 years of age (range, 7 months to 43 years). We interviewed 19 individuals who had acquired measles. The first 3 cases were infected in Tibet; 12 (63%) were hospital-acquired infections; 2 (11%) were community-acquired; and 2 (11%) were family-contact cases. We conducted outbreak response immunization (ORI) that provided MCV without regard to vaccination status; 10,596 residents between 20 and 40 years of age were vaccinated. A serological survey conducted during the ORI showed that 84% of the 20–40 year-olds were immune to measles prior to vaccination. Post-vaccination serological testing showed 100% were immune. Conclusions: Despite high population immunity among children and adolescents, imported measles virus transmission occurred among adults in a provincial cross-border area. Nosocomial transmission and measles immunity gaps among adults poses a threat to measles elimination and highlights the strategy of targeting adults during ORI to outbreaks with adult-to-adult transmission.