GLOBAL TRAVEL PATTERNS AND RISK OF MEASLES IN ONTARIO AND QUEBEC, CANADA: 2007-2011

Sunday, 23rd of August 2015

GLOBAL TRAVEL PATTERNS AND RISK OF MEASLES IN ONTARIO AND QUEBEC, CANADA: 2007-2011

Excerpt below; full text, with figures, is at http://www.biomedcentral.com/1471-2334/15/341

Background

In 2011 the largest measles outbreak in North America in a decade occurred in Quebec, Canada with over 700 cases. In contrast, measles activity in neighbouring province Ontario remained low (8 cases). Our objective was to determine the extent to which the difference could be explained by differing travel patterns.

Methods

We explored the relationship between measles cases over 2007–2011, by importation classification, in Quebec and Ontario in relation to global travel patterns to each province using an ecological approach. Global measles exposure was estimated by multiplying the monthly traveler volume for each country of origin into Quebec or Ontario by the yearly measles incidence rate for the corresponding country. Visual inspection of temporal figures and calculation of Pearson correlation coefficients were performed.

Results

Global measles exposure was similar in Ontario and Quebec. In Quebec, there was a nearly perfectly linear relationship between annual measles cases and its global measles exposure index over 2007–2011 (r = 0.99, p = 0.001). In contrast, there was a non-significant association in Ontario. The 2011 rise in the Quebec index was largely driven by a dramatic increase in measles activity in France the same year.

Conclusions

Global measles activity was associated with measles epidemiology in Quebec. Global measles exposure risk is higher in Ontario than Quebec. Differences in measles epidemiology between Ontario and Quebec from 2007–2011 are not explained by greater exposure in Quebec. A combination of alternative factors may be responsible, including differences in population susceptibility.

Keywords:

Epidemiology; Measles; Outbreaks; Surveillance; Travel; Vaccine-preventable diseases

Background

Measles is the most communicable of vaccine-preventable diseases, requiring very high vaccine coverage for effective control. The Pan American Health Organization (PAHO) achieved its goal of endemic measles elimination by 2000 [1] and in Canada the last confirmed case of endemic measles virus was reported in 1997 [2]–[4]. In countries where elimination has been achieved, cases may still occur due to travel (importations) and some imported cases may result in limited local transmission (import-related cases). Measles epidemiology during elimination is therefore influenced by global measles activity and travel.

In Canada, measles vaccination policy and public health actions for measles control are provincial/territorial and regional responsibilities, respectively, and guidance on disease control and immunization is federal [5], [6]. Measles is reportable in all 13 of Canadas provinces and territories (P/Ts) [2] and all have a 2 dose measles vaccine policy. These measures successfully kept the annual incidence of measles from 0.02 to 0.31 per 100,000 from 2001 to 2010, with annual numbers of cases ranging from 6 (2005) to 102 (2007) [7]. In 2011, however, the province of Quebec experienced the largest measles outbreak in the region of the Americas in a decade with 725 confirmed cases reported between January 8 and December 22, 2011. In contrast, neighbouring Ontario confirmed a total of 8 measles cases in 2011 [8]. Ontario and Quebec are Canadas two most populous provinces with population sizes of 13.5 and 8.2 million, respectively.

Public health officials in Quebec attributed the 2011 measles outbreak to a combination of a chance superspreading event and sub-optimal measles immunity [9]. Our objective was to assess the contribution of travel patterns to the difference in measles epidemiology between the two provinces through a descriptive analysis and by exploring a new measure, the measles exposure index. We did this by comparing international travel and measles activity among the source countries of travelers arriving via commercial flights to Ontario and Quebec, to the measles epidemiology of each respective province over the period of 2007 to 2011. With the exception of the United States (US) (where measles has also been eliminated), international travel to and from Canada occurs almost exclusively by air rather than other modes of transport [10].