Prevalence of risk factors for acquiring measles during the 2011 outbreak in Quebec and impact of the province-wide school-based vaccination campaign on population immunity.

Marie-Noëlle Billard,Gaston De Serres,Danuta M Skowronski,Nicole Boulianne,Monique Landry,Marie-Claude Gariépy,Eveline Toth,Ana Paula Arez

doi:10.1371/journal.pone.0186070

Abstract

BackgroundA large measles outbreak occurred in Quebec, Canada, in 2011. Although nearly two-thirds of the cases occurred in only two health districts, a mass vaccination campaign targeting all Quebec elementary and high school students without valid two-dose history was undertaken to prevent future outbreaks. We compared rates of non-vaccination and age at first measles vaccine dose among students in the two most-affected districts and the rest of the province and estimated the improvement in overall student measles immunity due to the mass school-based vaccination campaign.MethodsData were extracted from the provincial vaccination registry for students in kindergarten to grade 11 during the 2011/2012 school year. A telephone survey was conducted in three sub-groups: students whose first measles vaccine dose recorded in the vaccination registry was received during the 2011 school vaccination campaign; students with no dose recorded in the registry whose parents refused receipt during the school campaign; and students with no dose recorded in the registry and no information about parental consent/refusal during the school campaign.ResultsNeither the prevalence of being non-vaccinated nor a younger age at first pediatric dose were higher in the two most-affected districts versus the rest of the province. The school campaign vaccinated nearly 8% of all students including 7% who previously received at least one dose. Before the outbreak, 3% of students were not vaccinated and one-third of these (1%/3%) were vaccinated during the campaign. The campaign likely increased the absolute school population immunity by just 1.7%.ConclusionThe concentration of measles cases in the two most-affected health districts during the large Quebec outbreak is not explained by more students who were unvaccinated or who had received their first vaccine dose at a younger age. The vaccination campaign reached one-third of unvaccinated students and only marginally improved population immunity.

Highlights

In spring 2011, Quebec, Canada, was affected by the largest measles outbreak in North America in over a decade that included 678 confirmed cases [1]
Nearly two-thirds of the cases occurred in only two health districts, a mass vaccination campaign targeting all Quebec elementary and high school students without valid two-dose history was undertaken to prevent future outbreaks
A telephone survey was conducted in three sub-groups: students whose first measles vaccine dose recorded in the vaccination registry was received during the 2011 school vaccination campaign; students with no dose recorded in the registry whose parents refused receipt during the school campaign; and students with no dose recorded in the registry and no information about parental consent/refusal during the school campaign

Summary

Introduction

In spring 2011, Quebec, Canada, was affected by the largest measles outbreak in North America in over a decade that included 678 confirmed cases [1]. Despite the spread of measles to the community and into 8 of the 18 health regions, Montreal (the largest city of the province) was not affected and the outbreak remained concentrated in two of the 12 health districts of the most-affected region. These two districts accounted for 87% of all cases in that region and 65% of all provincial cases. Nearly two-thirds of the cases occurred in only two health districts, a mass vaccination campaign targeting all Quebec elementary and high school students without valid two-dose history was undertaken to prevent future outbreaks. We compared rates of non-vaccination and age at first measles vaccine dose among students in the two most-affected districts and the rest of the province and estimated the improvement in overall student measles immunity due to the mass school-based vaccination campaign.

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