Investigation and management of a large community mumps outbreak among young adults in Toronto, Canada, January 2017-February 2018.

Background: In 2017, a mumps outbreak was identified in a cohort of 18-34 year olds in Toronto, Canada.

Objective: To describe a large community mumps outbreak in an urban centre from January 2017 to February 2018 among young adults.

Methods: A broad range of interventions were implemented in an attempt to reach the target audience; including case and contact management, vaccination clinics at schools and clinicians' offices, school exclusions, bar inspections, traditional communication strategies (including health care provider updates and posters) and newer communication strategies (including three sequential social media campaigns).

Results: A total of 143 cases of mumps were identified. Although cases' ages ranged from three to 72 years, most (76%) were 18-34 year olds, many of whom had frequented bars and local food establishments in downtown Toronto. 84% (n=120) of the cases were community-acquired. Only 16% (n=23) of the cases reported exposures in schools and post-secondary school institutions. Of those, 39% (n=56) of cases had an unknown vaccination history; 34% (n=49) were either not vaccinated or partially vaccinated with one dose of measles-mumps-rubella vaccine; and 27% (n=38) had received the recommended two doses of mumps vaccine. Determining vaccination status was a challenge, in part due to the lack of a registry. Vaccination was recommended when subjects were known to have had fewer than two doses of vaccine or had an unknown vaccination status. A social media campaign, emphasizing the risk of social activities if not protected from the mumps, yielded over 500,000 impressions from Facebook and Twitter messages and ads and an impressive engagement rate of between 1% and 10x%.

Conclusion: This was the largest mumps outbreak in Toronto in over 20 years. Among young adults, ongoing social media and traditional communication campaigns can contribute to the control of community mumps outbreaks. Encouraging vaccine uptake is desirable, but without a vaccine registry it is difficult to assess vaccination coverage among adults. Susceptible cohorts of young adults who were not adequately vaccinated pose a risk for future outbreaks. Given that almost 30% of the mumps cases were fully vaccinated with two doses of mumps-containing vaccine, even two doses may not provide complete protection.