BOSTON The measles outbreak in Canada continues
to occur in both vaccinated and unvaccinated individuals and is causing concern
over vaccine effectiveness, according to a presentation here during the IDSA
49th Annual Meeting.
While this still ongoing outbreak feeds largely on
unvaccinated individuals, the high proportion of cases who received two doses
raises concerns on vaccine effectiveness, Philippe Belanger, MSc,
Ministère de la santé et des services sociaux du Québec,
Montréal, told Infectious Diseases in Children. However,
the take-home message is still to get vaccinated.
Two mass campaigns were implemented in Quebec in 1996.
The goal of the first was to administer a second dose of measles vaccine to
children aged 18 months to 16 years, and the goal of the second was to
introduce a two-dose measles vaccine schedule in young children (at 12 and 18
months). The first initiative resulted in an 89% coverage rate for the second
dose. Despite these efforts, Belanger said that Quebec is experiencing the
largest measles outbreak reported in the Americas since 2000.
Between Jan. 1 and Aug. 3, 2011, 727 cases were
reported; 682 meet the national case definition and 35% of those were
laboratory confirmed. Prior to mid-March, the few sporadic cases were linked to
importations from Europe, but there was continued transmission that peaked in
May, and the cases declined after the end of the school year in late June.
However, cases continued to persist through Aug. 3.
Measles has been reported in 10 of the 18 regions of
Quebec province. Two of those regions account for 71% and 20% of all cases,
respectively. Just over 2% of the cases have been acquired outside Canada, and
those came mostly from France (13/15), according to Belanger.
The mean age of cases was 15 years, with the highest
incidence among 10- to 19-year-olds (65%). Incidence then decreased to 21 per
100,000 in preschoolers and to 14 per 100,000 in 5- to 9-year-olds.
The incidence among 20- to 39-year-olds was 4 per 100
000, which the investigators believe is because they were largely included in
the 1996 mass campaign. Only 12 (1.8%) cases were in individuals older than 40
Almost 12% of cases resulted in hospitalizations, with
the highest rates among children aged 12 to 17 months (29%) and adults aged 30
years and older (27%).
Overall, 66% of cases were considered to have occurred
in unvaccinated individuals: 24% had written proof of vaccination and 10%
reported being vaccinated without a written proof. In 5- to 19-year-olds, 21%
had written proof of measles vaccination (17% had two doses and 4% had one
dose), and an additional 10% reported being vaccinated without a written proof.
Disclosures: Dr. Belanger reports no relevant financial
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