Measles outbreak prevention and control among adults: Lessons from an importation outbreak in Yunnan province, China, 2015

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.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.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.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.