SUMMARY:In this study, we have described the clinical characteristics of vaccine-modified measles to assess the performance of an expanded case definition in a school outbreak that occurred in 2010. The sensitivity, specificity, and the positive and negative predictive values were evaluated. Among 74 cases of vaccine-modified measles, 47 (64z) met the original case definition. Fever and rash were observed in 73z (54/74); fever was the most common (96z, 71/74) presenting symptom, and rash was noted in 77z (57/74) of the cases. The original case definition showed an overall sensitivity of 63.5z and a specificity of 100.0z. The expanded case definition combining fever and rash showed a higher sensitivity (72.9z) but a lower specificity (88.2z) than the original. The presence of fever and one or more of cough, coryza, or conjunctivitis scored the highest sensitivity among the combinations of signs and symptoms (77.0z), but scored the lowest specificity (52.9z). The expanded case definition was sensitive in identifying suspected cases of vaccine-modified measles. We suggest using this expanded definition for outbreak investigation in a closed community, and consider further discussions on expanding the case definition of measles for routine surveillance in South Korea.