While the proportion of measles cases in vaccinees is expected to increase as vaccine coverage increases, such cases must be carefully investigated. The present study was conducted to examine possible contributions to vaccine failures (VFs) and to genetically characterize measles virus (MV) strains circulating in Novosibirsk, Russia during 20002005. Totally, 27 adult measles patients admitted to a regional hospital were prospectively enrolled in our study. Genetic characterization of the MV strains revealed circulation of genotypes A, D4 and D6 between 2000 and 2003 years; a genotype D6 MV was associated with the 2005 measles outbreak. Based on IgG avidity testing, half of the vaccinated patients demonstrated evidence of secondary vaccine failure (SVF). Patients, representing both levels of vaccine failure in our study were characterized by the lack of protective titers of neutralizing antibodies against circulating MVs, despite high IgG levels in many cases and high IgG avidity in SVF cases.