International migrant populations are some of the most mobile populations in the world. This can make it difficult to determine how many migrants are living in host countries and how their characteristics differ from the locally-born population. This is because many host country systems are not equipped to capture the moves of migrants into and out of host countries. If migrants leave a host country and their exit is not recorded, they continue to exist in the host country’s databases even after they have migrated elsewhere. This issue is known as ‘over-coverage’. It gives the false impression there are more migrants in a host country than there truly are. For outcomes such as mortality, over-coverage is important. This is not only because of the possibility that they are officially counted in data sources when they have already left the country, but because they might also have died elsewhere. This issue of over-coverage could provide an explanation for the concept of the migrant mortality advantage, the term used to explain the finding of low mortality among international migrants compared to locally-born populations.
In their article, Matthew Wallace and Ben Wilson (Stockholm University) build upon previous research on age variation in migrant mortality and look to see if age differences in the migrant mortality advantage is genuine. They focus on the impact of over-coverage on the mortality of migrant populations in Sweden between 2010 and 2015 by asking the question: Are the higher levels of over-coverage at young adult ages driving the lower mortality of migrants at the same ages? They based their question on two facts: that the highest levels of over-coverage tend to be found at peak migration ages (15-39); and that the mortality of migrants tends to be lowest at these ages. To answer the question, they used a survival analysis of Sweden’s population registers.
Regarding the mortality of migrants, Wallace and Wilson found the same distinct age-specific U-shape of mortality advantage as documented in several other migrant-receiving countries. In short, young adult migrants had the largest mortality advantages over the locally-born population of Sweden, while the advantage among older migrants was weaker, absent, or even reversed. This pattern presented for some specific migrant origins, but not others. With respect to over-coverage, they found that levels of over-coverage were also highest at peak migration ages (i.e., the ages at which the mortality advantage was also largest). Crucially, however, over-coverage alone could not entirely explain the low mortality of international migrants at these ages.