LARGEST MEASLES EPIDEMIC IN NORTH AMERICA IN A DECADE—QUEBEC, CANADA, 2011: CONTRIBUTION OF SUSCEPTIBILITY, SERENDIPITY, AND SUPER SPREADING EVENTS.

Sunday, 8th of June 2014

[source]Journal of Infectious Diseases[|source]

In 2011, Canada experienced several measles importations—mostly from Europe, where more than 30,000 cases were reported. Although these imported cases were generally followed by limited or no secondary transmission, one triggered the largest epidemic in North America. 

In this report, the authors describe key epidemiological features of the 2011 epidemic in Quebec and highlight observations that may be applicable in the broad context of measles elimination. More details are accessible at: http://jid.oxfordjournals.org/content/207/6/990.long

 

ABSTRACT

BACKGROUND. The largest measles epidemic in North America in the last decade, occurred in 2011 in Quebec, Canada, where rates of 1- and 2-dose vaccine coverage among children 3 years of age were 95%–97% and 90%, respectively, with 3%–5% unvaccinated.

METHODS. Case patients identified through passive surveillance and outbreak investigation were contacted to determine clinical course, vaccination status, and possible source of infection.

RESULTS. There were 21 measles importations and 725 cases. A super spreading event triggered by 1 importation resulted in sustained transmission and 678 cases. The overall incidence was 9.1 per 100 000; the highest incidence was in adolescents 12–17 years old (75.6 per 100 000), who comprised 56% of case patients. Among adolescents, 22% had received 2 vaccine doses. Outbreak investigation showed this proportion to have been an underestimate; active case finding identified 130% more cases among 2-dose recipients. Two-dose recipients had milder illness and a significantly lower risk of hospitalization than those who were unvaccinated or single-dose recipients.

CONCLUSIONS. A chance super spreading event revealed an overall level of immunity barely above the elimination threshold when unexpected vulnerability in 2-dose recipients was taken into account. Unvaccinated individuals remain the immunization priority, but a better understanding of susceptibility in 2-dose recipients is needed to define effective interventions if elimination is to be achieved.