We identified 767 references, of which 282 were duplicated and thus excluded. After reading the titles and abstracts of the 485 studies, 170 were selected for full text reading. Sixty-seven studies were included in the present review. Reasons for exclusion of the other 103 studies are displayed in Figure 1. Information on countries of origin and host continent was available in 16 studies, which were included in the meta-analyses.
Sixty-six studies were published in English and one in French(31), of which 15 reported active tuberculosis prevalence, 21 reported LTBI prevalence and 31 reported both. The total screened population was 599,072.
Active Tuberculosis Prevalence
Study Characteristics and Population
Among the 46 studies that reported active tuberculosis prevalence, 56.5% were cross-sectional; we did not identify any clinical trial (Table 1). Two thirds of these studies were published after 2009, among them half completed data collection before 2011. Sixty-three percent of studies included over 500 people.
The total screened population for active tuberculosis was 537,218, with one single study having a sample of 232,738 individuals. Eight one percent of studies were conducted among refugees (n = 437,264), 18% among asylum seekers (n = 95,283), and 0.9% among both (n = 4671).
The mean age, reported by 33% of studies(4,10,32–44), ranged from 18 to 40.7 years. Prevalence by sex was reported by only 17% of the studies.
The average time since arrival of refugees and asylum seekers to the host country to the time of evaluation for tuberculosis was 3.9 months (ranging from 0.7 to 12.8 months) among the 9% of studies(45–48) conveying this information.
Sixty-seven (31/46) percent of studies concomitantly evaluated the presence of LTBI in their populations. Among these, 57% used the LTBI diagnostic as a prerequisite to investigate the presence of active tuberculosis. In other words, they performed a diagnostic method for LTBI with TST or an IGRA to rule out active tuberculosis; if they were positive, a chest X-ray was performed; and if it was indicative of active tuberculosis, bacteriological tests were conducted. Otherwise, individuals were considered to have LTBI.
Main findings
Active tuberculosis prevalence rates varied from 0 to 35%, with 89% of studies reporting values under 5%. Considering studies that reported the prevalence by country of origin, the pooled measure was 1% (95% CI, 1–2), with high heterogeneity (I² = 98%) (Figure 2). The prevalence was higher among refugees from Syria (11%, 95% CI, 4–25), found in one single study with 44 hospitalized participants. Ethiopia, Ghana and Tunisia also had large confidence intervals, with populations smaller than 100 persons(10,48).
With reference to the host continent, refugees who emigrated to Europe, Asia and America presented a pooled prevalence of 1% (Figure 3). Europe was the continent that received refugees from most diverse nationalities, thus allowing an assessment of heterogeneity. In the other continents, this individual evaluation was not possible because of small numbers. Refugees from Eritrea, Ethiopia and Somalia immigrated to Europe(10,48) in the studies that presented this information, and with a slightly larger prevalence.
LTBI Prevalence
Study Characteristics and Population
Fifty four percent of the 52 studies that reported LTBI prevalence were cohort studies; no clinical trial was included (Table 2). Sixty percent of studies were published after 2009, among them half completed data collection before 2011; one study did not show this information. Thirty-one studies included over 500 participants.
A total of 271,544 individuals were screened for LTBI: 233,688 individuals were refugees (reported by 67% of studies) and 27,960 individuals were asylum seekers (reported by 21%). The remaining were studies including both types of situations.
The mean age, reported by 33% of studies(4,10,31–38,49–55), ranged from 3.5 to 39 years. Only 21% of studies reported the prevalence by gender.
The average time since arrival of refugees and asylum seekers to the host country at the time of evaluation for LTBI was 3.8 months (range: 0.7–12.8 months) among the 13% of studies that reported this information(45–48,56–58).
Eighty-nine percent of studies performed TST and 77% of these reported the TST cut-off point used to define LTBI: 10 mm was used in 78% of studies. Some studies considered different cut-off points to different populations (children, human immunodeficiency virus (HIV)-infected or BCG-vaccinated individuals) but did not report prevalence according to these cut-off points.
Main findings
Prevalence of LTBI ranged from 0.4% to 81.5%, with 61% of the studies reporting a prevalence rate higher than 30%.
In the meta-analysis by country of origin, prevalence rates were highly heterogeneous (I2 = 99.8%), with a pooled measure of 37% (95% CI, 23–52) (Figure 4). Refugees from Cuba and Iraq presented the lowest rates, 0 and 5% respectively, and from North Korea, the highest rate, 81%, systematically screened when arriving in South Korea(6). Targeted populations and sample sizes varied largely, with the Cuban study(55) evaluating 241 children under 7 years of age finding one LTBI case and large systematic screening for active tuberculosis of the Iraq refugees(58,59) applying for visa or recently arrived in the USA. Very small sample sizes resulted in some cases in very wide confidence intervals(10).
In the subgroup analysis by host continent, refugees who immigrated to Europe presented the highest prevalence (41%, 95% CI, 20–65), followed by those who went to the Americas (28%, 95% CI, 18–40) (Figure 5). However, one study in the U.S. A. excluded individuals with immunosuppressive conditions and thus had a high risk of false negative results(56). Somali refugees who went to America had a higher prevalence rate (54%) than the ones who went to Europe (38%), whilst Iraq refugees who went to the Americas had a higher prevalence rate (14%) than the ones who went to Asia (2%). Overall, there were very few studies with information by each country of origin and host continent.
Risk of bias
In 85% of studies (n = 569,880), routine screening of all the individuals who arrived in the host country was the reason for the enrolment and 9% of studies tested individuals who sought health service with symptoms (n = 11,234). Only one study was conducted in refugee camps(60) (n = 15,455). Among the 37 studies that informed the diagnostic method for active tuberculosis, 73% confirmed tuberculosis bacteriologically.
None of the 65 studies fulfilled all quality criteria. Among the 33 cross-sectional and the 32 cohort studies, only 13 and 11 respectively fulfilled 80% or more of the quality criteria (Figure S2 and S3). Two studies(58,61) were organization reports; it was not possible to perform the quality assessment.