Background: Concerns have been raised about unauthorized immigrants importing measles to the United States (US). This potential risk has not been rigorously evaluated nor compared with the potential risk of measles importation by US residents traveling internationally or international travellers coming to the US. We compared the potential risk of measles importation from each of these populations. Methods: Using a cross-sectional, ecological design, we compared country-level measles vaccination and incidence data, for top (i) US resident international travel destinations, (ii) US-bound international travellers’ home countries and (iii) home countries of unauthorized immigrants to the US. Results: In 2014, US residents made 52.5 million trips to one of the top 10 international destinations. Five of these countries (10 958 000 US resident trips) had average first-dose measles vaccine coverage below 90%, and five (9 881 000 US resident trips) had average measles incidence over 1 case/100 000 population. Two of the 10 top US-bound international travellers’ home countries (5 597 259 international visitors) had average first-dose measles vaccine coverage below 90%, whereas five (13 333 545 international visitors) had average annual measles incidence over 1 case/100 000 population). In 2012, of 11.2 million unauthorized immigrants living in the US, 8.9 million (79.0%) were born in one of the top 10 unauthorized immigrant home countries. Four of those countries had average first-dose measles vaccine coverage below 90% (1.3 million unauthorized immigrants), whereas three of these countries had average measles incidence over 1 case/100 000 population (950 000 unauthorized immigrants). Overall, there are 10 times more annual US visitors to high measles incidence countries than there are unauthorized immigrants in the US from high measles incidence countries. Conclusions: Efforts to prevent reestablishment of indigenous measles transmission in the US should focus on evidence- based risk assessments, highlighting a greater potential measles importation risk of from US residents travelling internationally than unauthorized immigrants coming to the US. © International Society of Travel Medicine, 2016. All rights reserved.

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immigration human risk assessment travel statistics and numerical data Undocumented Immigrants Cross-Sectional Studies United States cross-sectional study undocumented immigrant Humans import disease measles vaccination Incidence Article measles vaccine Air Travel aviation virus transmission vaccination measles