Epidemiological and molecular analysis of the first outbreak caused by genotype C mumps virus in China

Mumps remains a public health concern as an acute respiratory infectious disease, underscoring the necessity of conducting outbreak investigations and molecular surveillance. We investigated a mumps outbreak at a boarding school in Yunnan Province, China, with a focus on the mumps molecular epidemiology, and provide a basis for the formulation of mumps prevention and control policies. The epidemiological investigation of the outbreak was conducted. Clinical specimens (throat swabs and/or urine) were obtained to test for mumps virus (MuV) RNA, as well as for virus isolation, and genomic sequencing. Phylogenetic analysis was performed to identify circulating genotypes and trace transmission routes. In total, 157 cases of mumps (156 pupils and 1 faculty) were identified between March 4 and May 12, 2025. The field survey found that only 36.96% (601/1,626) of individuals had received at least one dose of mumps containing vaccine (MuCV). The five obtained MuV sequences shared 100% homology. In comparison to the 12 reference genotypes recognized by the World Health Organization (WHO), they showed 95.8% sequence homology and were all classified as genotype C. Phylogenetic analysis revealed that this was the first genotype C MuV-associated outbreak in China. This event also represents the initial detection of this genotype in Yunnan Province. Notably, it formed a distinct cluster from recent reported genotype C strains in India. This first-reported genotype C MuV outbreak in China underscores the critical importance of enhancing molecular surveillance, increasing MuCV coverage in target populations, and assessing the effectiveness of the current vaccine against this genotype.