An outbreak of mumps with genetic strain variation in a highly vaccinated student population in Scotland

Abstract

An outbreak of mumps within a student population in Scotland was investigated to assess the effect of previous vaccination on infection and clinical presentation, and any genotypic variation. Of the 341 cases, 79% were aged 18-24. Vaccination status was available for 278 cases of whom 84% had received at least one dose of mumps containing vaccine and 62% had received two. The complication rate was 5·3% (mainly orchitis), and 1·2% were admitted to hospital. Genetic sequencing of mumps virus isolated from cases across Scotland classified 97% of the samples as genotype G. Two distinct clusters of genotype G were identified, one circulating before the outbreak and the other thereafter, suggesting the virus that caused this outbreak was genetically different from the previously circulating virus. Whilst the poor vaccine effectiveness we found may be due to waning immunity over time, a contributing factor may be that the current mumps vaccine is less effective against some genotypes. Although the general benefits of the measles-mumps-rubella (MMR) vaccine should continue to be promoted, there may be value in reassessing the UK vaccination schedule and the current mumps component of the MMR vaccine.

Keywords: Community outbreaks; MMR vaccination; infectious disease epidemiology; mumps; viral genotyping.

Fig. 1.

Epidemic curve of mumps outbreak in Lothian, Scotland, October 2014–June 2015 (n = 341).

Fig. 2.

Phylogenetic analysis for the SH gene (316 nt) of mumps strains from patients in Scotland 2012–2015. Bootstrap values (%) are shown at each node. Scale bar indicates the number of substitutions per nucleotide position. Outbreak strain identified during the outbreak. Different strain detected during the outbreak. Outbreak strain but identified prior to the official start date of the outbreak. Pre-outbreak mumps virus circulating.