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Hum Vaccin Immunother. 2019 Jul 16:1-9. doi: 10.1080/21645515.2019.1619402. [Epub ahead of print]

Assessment of population immunity to measles in Ontario, Canada: a Canadian Immunization Research Network (CIRN) study.

Author information

1
a Public Health Ontario , Toronto , ON , Canada.
2
b Laboratory Medicine and Pathobiology , University of Toronto , Toronto , ON , Canada.
3
c Dalla Lana School of Public Health , University of Toronto , Toronto , ON , Canada.
4
d National Microbiology Laboratory , Public Health Agency of Canada , Winnipeg , MB , Canada.
5
e Department of Medical Microbiology and Infectious Diseases , University of Manitoba , Winnipeg , MB , Canada.
6
f Department of Pathology and Laboratory Medicine , Nova Scotia Health Authority , Halifax , NS , Canada.
7
g Department of Pathology , Dalhousie University , Halifax , NS , Canada.
8
h ICES , Toronto , ON , Canada.
9
i Département de médecine sociale et préventive , Université Laval , Québec City , QC , Canada.
10
j Canadian Center for Vaccinology , IWK Health Centre , Halifax , NS , Canada.
11
k Department of Microbiology , Mount Sinai Hospital , Toronto , ON , Canada.
12
l Laboratoire de santé publique du Québec/Institut national de santé publique du Québec , Sainte-Anne-de-Bellevue , QC , Canada.
13
m Department of Microbiology and Immunology , McGill University , Montreal , QC , Canada.
14
n Research Institute of the McGill University Health Centre , Montreal , QC , Canada.

Abstract

Canada eliminated measles in 1998. We conducted a sero-epidemiology study to estimate population immunity to measles in the province of Ontario, Canada and to identify groups at higher risk of outbreaks. We used a previously developed modified enzyme immunoassay to test 1,199 residual sera from patients aged 1-39 years. We re-tested negative and equivocal sera using a plaque reduction neutralization assay. We interpreted our results in the context of Ontario's immunization program and vaccine coverage data. Of 1,199 sera, 1035 (86.3%, 95% confidence interval (CI) 84.4, 88.2) were above the measles threshold for protection, 70 (5.8%, 95% CI 4.5, 7.2) were equivocal and 94 (7.8%, 95% CI 6.3, 9.4) were negative. The proportion of positive sera was highest for those 1-5 years, with 180/199 (90.5%, 95% CI 86.4, 94.5) positive sera, and lowest for those age 12-19 years, at 158/199 (79.4%, 95% CI 73.8, 85.0). Adjusted for age, females were more likely than males to have antibody titers above the threshold of protection (odds ratio = 1.60, 95% CI 1.14, 2.24). Most of the study cohort were eligible for two measles vaccine doses, and vaccine uptake in Ontario is >90% for school-aged cohorts. We observed a higher than expected proportion of sera with antibody levels below the threshold of protection, suggesting that immunity in some Ontario age-groups may be waning, despite high vaccine coverage. Alternatively, the traditional measles correlates of protection may not be an appropriate measure of population protection in measles-eliminated settings.

KEYWORDS:

Canada; Measles; Ontario; herd immunity; measles immunity; sero-epidemiology; threshold of protection

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