Transmission between Danes and migrants in DK
In 1992 through 2004, 61.6% of all TB cases notified were migrant cases. One fourth of the 403 clusters included both migrants and Danes. In accordance with previous findings [10–15], we found only limited transmission between migrants and Danes. The proportion of TB cases likely transmitted by other ethnic groups was higher in migrants (up to 7.9% (95% CI 7.0-8.9)) than in Danes (up to 5.8% (95% CI 4.8-7.0)), with odds ratios up to 2.5 for migrants (Table 3). Limited transmission between migrants and the resident population has previously been shown for Somalis and Danes [13, 14] in DK, and Mexicans and Americans in the US .
The limited transmission between migrants in DK and Danes is in part a result of poor integration , rather than an effective Danish migrant TB control strategy. Refugees in camps are only questioned about TB symptoms/screened at arrival, thus migrants only receive a very limited introduction to the Danish health care system. Also, active case finding within migrant TB high-risk groups has only been initiated on a minor basis the recent years.
Misclassification in registration of country of birth could confound the transmission proportion, as country of birth is a strong predictor for IR . It is most likely that children born in DK by migrant parents would be misclassified as migrants. However, if corrected, this would only strengthen the association. Also, the contribution to transmission from the youngest of children is believed to be limited, as the numbers of minor migrant children in mixed clusters in this study were limited (32 cases < 10 years) and as minor children are mostly considered non-infective .
Categorization of clusters in Danish, migrant or uncertain origin is a simplification with many limitations. However, the use of other criteria, ideally based on detailed epidemiological linkage information, was not available. Therefore, to meet the uncertainty regarding origin of clusters, we analyzed the origin of mixed clusters by 5 different criteria, with largely the same result, supporting the association.
It must be expected that any group of migrants from TB high incidence countries, living for decades in a TB low incidence country, will gradually transmit some MTB strains to the resident population. However, our data indicate, that the exchange occurs at a low rate. Fears that TB in migrants poses a threat for resident Danes seem exaggerated and unjustified.
Origin of migrant strains
In concordance with findings reported in a Dutch study among Somalis and Turks, where most recent transmission was attributable to transmission from cases with the same nationality , we also found limited transmission across ethnic subgroups. Even though some transmission has been confirmed by genotype and linkage information within/between ethnic subgroups in DK, in e.g. ethnic clubs, shelters, language schools, and the environment of homeless/socially marginalized, including khat-abusing Somalis in the capital region, clustering in migrants in DK more likely reflects reactivation disease . Other studies has documented that clustering in migrants may reflect recent transmission in the recipient country [11, 17], but that most cases are due to reactivation [14, 18–22].
Greenlanders had the highest proportion of clustered strains (Table 2). This is not surprising, as clustering, like in the Somalis, likely reflects imported reactivation disease, and the Greenlandic population in Greenland is relatively isolated, with only few introductions of new MTB genotypes into the population. The TB incidence  is high, with a notification rate (NR) at approximately 150. In DK, many micro epidemics involving Greenlanders are seen in larger cities and the capital area, often involving the two largest clusters in DK, C-1 and C-2 , mainly shared by Greenlanders and Danes. These clusters are associated with homelessness, alcohol abuse, and other social risk factors .
Migrants of "other origin" had the lowest proportion of clustered strains as expected, as the geographic origin in this pooled group of migrants is diverse and their time in DK short.
Compared to Danes, migrants in general presented with TB at an earlier age, reflecting early primary infection in their country of birth. IR of clustered TB distributed, as expected for high incidence countries, with an almost even overall sex-distribution, whereas Danes had around two to three times more males, higher at specific age groups, in accordance with NR for Danes . Also, compared to other Scandinavian countries, DK has a high TB NR in middle-aged Danish males, particular in the larger cities, associated with transmission of C-2 .
A recent study  has documented a high rate of transmission in Inuit children in Greenland. This has not yet reflected in DK, as Inuit children and adolescence are strongly underrepresented. So are elderly Somalis and migrants of other origin.
Micro epidemics and dominant clusters, like C-2, transmited among Danes, indicate that DK, being in a process aiming at TB elimination, still has unsolved TB control problems . In the migrant population, the Somalis carry a major disease burden, mainly through reactivation disease. However, decreasing IR of clustered TB in this group is presently seen, following increased focus on the problem and longer time interval since infection. This may resemble the development observed for Vietnamese boat refugees arriving in DK , a previous high-risk group.
Active case finding and effective treatment of symptomatic individuals, especially in high risk groups, mainly from larger cities and often with homelessness and abuse problems, must be intensified for IR to decrease. Educating staff in high risk settings like shelters, prisons, clubs/associations for migrants, and schools, to recognize and inform about TB symptoms should be strengthened to increase active case finding. Also, preventive therapy to certain high risk groups, like persons co-infected with HIV, with fibrotic lesions, or with recent infection with MTB , can reduce the pool of latently infected persons.
Coordination of activities on national and regional level by experienced parties is crucial. Mapping of TB in migrants, e.g. by genotyping, may identify specific risk patterns within ethnic subgroups. This information is valuable for further targeting of TB control.
It is important that the population has continues free access to screening, diagnostics and treatment for TB, and this must be combined with active case finding in certain high-risk groups. E.g. migrants should not only be offered screening at arrival, as they have a continuous increased risk of developing TB many years post-migration . However, the most important and cost-beneficial factor for TB control remains, especially for low incidence countries, investment in global TB control .