Background
Between 1995 and 2014, an average of 23 cases of mumps were notified annually during non-outbreak years in Western Australia (WA). In 2007/08 there was an outbreak with 183 notifications due to genotype J mumps virus which primarily affected vaccinated young Aboriginal people from the Kimberley region. We describe an even larger outbreak that commenced in the Kimberley in April 2015 and spread to other remote areas of the state.
Methods
Mumps is notifiable by both laboratories and medical/nurse practitioners in WA. Cases were either laboratory confirmed or epidemiologically linked to a laboratory-confirmed case. Laboratory diagnosis was by PCR and/or serology with genotyping performed on a subset of cases. We extracted case information from the WA notifiable infectious diseases database.
Results
Between 1 April 2015 and 25 March 2016 more than 700 outbreak-related mumps cases were notified. Of these, 88% were Aboriginal, 55% male; with median age of 21 years (range: 8 months to 64 years). Thirty-four percent of cases were aged 10-19 years and 26% 20-29 years. Of cases aged <20 years 87% were fully vaccinated, i.e. had received 2 doses of MMR vaccine and another 6% had at least one MMR vaccination. Six percent of cases were hospitalised and 8% of males reported orchitis. Outbreak viruses were all genotype G. Compared with 2007, a greater proportion of mumps cases under 20 years were fully vaccinated, 87% vs. 78%.
Conclusions
The occurrence of two large mumps outbreaks affecting the same highly vaccinated cohorts within 8 years is unprecedented. While waning immunity, a vaccine-genotype mismatch, and social behavioural factors such as household density and high population mobility may partially explain these outbreaks, other factors that might potentially affect vaccine effectiveness warrant further investigation.