Genomic characterization of mumps viruses from a large-scale mumps outbreak in Arkansas, 2016

In 2016, a year-long large-scale mumps outbreak occurred in Arkansas among a highly-vaccinated population. A total of 2954 mumps cases were identified during this outbreak. The majority of cases (1676 (57%)) were school-aged children (5–17 years), 1536 (92%) of these children had completed the mumps vaccination schedule. To weigh the possibility that the mumps virus evaded vaccine-induced immunity in the affected Arkansas population, we established a pipeline for genomic characterization of the outbreak strains. Our pipeline produces whole-genome sequences along with phylogenetic analysis of the outbreak mumps virus strains. We collected buccal swab samples of patients who tested positive for the mumps virus during the 2016 Arkansas outbreak, and used the portable Oxford Nanopore Technology to sequence the extracted strains. Our pipeline identified the genotype of the Arkansas mumps strains as genotype G and presented a genome-based phylogenetic tree with superior resolution to a standard small hydrophobic (SH) gene-based tree. We phylogenetically compared the Arkansas whole-genome sequences to all publicly available mumps strains. While these analyses show that the Arkansas mumps strains are evolutionarily distinct from the vaccine strains, we observed no correlation between vaccination history and phylogenetic grouping. Furthermore, we predicted potential B-cell epitopes encoded by the Arkansas mumps strains using a random forest prediction model trained on antibody-antigen protein structures. Over half of the predicted epitopes of the Jeryl-Lynn vaccine strains in the Hemagglutinin-Neuraminidase (HN) surface glycoprotein (a major target of neutralizing antibodies) region are missing in the Arkansas mumps strains. In silico analyses of potential epitopes may indicate that the Arkansas mumps strains display antigens with reduced immunogenicity, which may contribute to reduced vaccine effectiveness. However, our in silico findings should be assessed by robust experiments such as cross neutralization assays. Metadata analysis showed that vaccination history had no effect on the evolution of the Arkansas mumps strains during this outbreak. We conclude that the driving force behind the spread of the mumps virus in the 2016 Arkansas outbreak remains undetermined.