Article

The largest measles epidemic in North America in a decade--Quebec, Canada, 2011: Contribution of susceptibility, serendipity and super-spreading events on elimination.

Institut national de santé publique du Québec, Quebec, Canada.
The Journal of Infectious Diseases (Impact Factor: 5.78). 12/2012; DOI: 10.1093/infdis/jis923
Source: PubMed

ABSTRACT Introduction. We describe the largest measles epidemic in North America in the last decade occurring in the population of Quebec, Canada in 2011 where one- and two-dose vaccine coverage among children 3years of age were 95-97% and 90%, respectively, with 3-5% unvaccinated.Methods. Cases identified through passive surveillance and outbreak investigation were contacted to describe clinical course, vaccination status and possible source of infection.Results. There were 21 measles importations and 725 cases. A super-spreading event triggered by one importation resulted in sustained transmission and 678 cases. The incidence per 100,000 was 9.1, highest in adolescents 12-17years old (75.6) comprising 56% of cases. Among adolescent cases, 22% had received two vaccine doses. Outbreak investigation showed this proportion to have been an under-estimate: active case-finding identified 130% more cases among two-dose recipients. Two-dose recipients had milder illness and significantly lower risk of hospitalization compared to unvaccinated or single-dose cases.Conclusion. A chance super-spreading event revealed an overall level of immunity barely above the elimination threshold when taking into account unexpected vulnerability in two-dose recipients. Unvaccinated individuals remain the immunization priority but a better understanding of susceptibility in two-dose recipients is needed to define effective interventions if elimination is to be achieved.

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