Largest Measles Outbreak in the Americas since 2000: Quebec Ongoing Epidemic

Background: Despite a mass campaign that administered a second dose of measles vaccine to 89% of children aged 18 months to 16 years in 1996 and a routine 2 dose measles schedule (at 12 and 18 months) implemented the same year, Quebec (Canada) is affected since early 2011 by the largest measles outbreak reported in the Americas since 2000. We report  the epidemiological features of this epidemic.

Method: Measles reporting is mandatory in Canada. Cases analyzed had to meet the national case definition.

Results: Between January 1^st and August 3^rd, 727 cases have been notified: 682 meet the national case definition and 35% of them are laboratory confirmed. Before mid-March, the few sporadic cases were linked to importations from Europe. Since April, there has been continued transmission peaking in May, declining after the end of the school year in late June but still persisting as of August 3. While measles has been reported by 10 of the 18 regions of the province, two account for 71% and 20% of all cases. Only 2.2% of the cases have been acquired outside Canada, mostly from France (13/15). The mean age of cases was 15 years. The highest incidence was reported in 10-19 year olds (49 per 100 000) who comprise 65% of all cases. Incidence then decreased to 21 per 100 000 in preschoolers and to 14 per 100 000 in  5-9 year olds. In adults 20-39 year olds who were largely included in the 1996 mass campaign, the incidence was 4 per 100 000. Only 12 (1.8%) cases were older than 40 years. Almost 12% of cases have been hospitalized. This proportion was highest in children aged 12-17 months (29%) and adults 30 years and older (27%). Overall, 66% of cases were considered unvaccinated, 24% had written proof of vaccination and 10% reported being vaccinated without a written proof. In 5-19 year olds, 21% had written proof of having received measles vaccine (17% had two doses and 4% had one dose) and an additional 10% reported being vaccinated without a written proof.

Conclusion: While this still ongoing outbreak feeds largely on unvaccinated individuals, the high proportion of cases who received two doses raises concerns on vaccine effectiveness.