Great disparity in tuberculosis (TB) rates has been reported in different parts of the world.1 Patients with TB from 22 high-burden areas accounted for over 80% of all notified TB cases in the world.1 Immigrants from these high-burden areas have often been blamed for their impact on the TB situation in many developed areas.2 3 4 5 6 7 8 9 10 11 12 A rapid increase in population was observed in Hong Kong in the last century, largely due to a heavy influx of immigrants from Mainland China.13 14 15 Despite remarkable socio-economic improvement over the past four decades, TB remains a common disease in Hong Kong. In 2006, the TB notification rate remained as high as 84.1/100 000.16 With continuing population movement between the Mainland and Hong Kong, there has been major concern about cross-border transmission of infections including TB.

A large-scale population census has been conducted in Hong Kong every 10 years since 1961, with a smaller by-census in-between. Tuberculosis is a statutorily notifiable disease, and basic demographic, clinical, and bacteriological data of notified cases are regularly captured by the TB notification registry. All residents are issued an identity card, and the identity card is used by both the TB notification registry and death registry as a unique personal identifier. Eighteen government chest clinics offer free programmatic case-finding and treatment services for TB patients under a centralised Tuberculosis and Chest Service of the Department of Health, with estimated programme coverage of over 80% of the population. Sputum culture and drug susceptibility testing are regularly performed by a centralised laboratory that is a Supranational Reference Laboratory within the World Health Organization/International Union Against Tuberculosis and Lung Diseases (WHO/IUATLD) network. Standard short-course regimens are used in line with the WHO recommendations. Patients are regularly followed up for 2 years after initiation of TB treatment to facilitate cohort analysis of treatment outcome. Using regularly captured data within the statutory registries and government TB programme, a longitudinal cohort study was performed to examine the impact of immigrant populations on the epidemiology of TB in Hong Kong.

Data on sex, age, place of birth, residency status, case category (new or retreatment), disease form (pulmonary with or without extrapulmonary involvement, or extrapulmonary only), sputum smear and culture results of consecutive patients notified within 2006 were obtained from the territory-wide TB notification registry of Hong Kong. An active case of TB was defined as positive isolation of Mycobacterium tuberculosis complex or, in the case of absent bacteriological confirmation, disease diagnosed on clinical, radiological, and/or histological grounds together with an appropriate response to anti-TB treatment. As part of the public health surveillance, bacteriological results of notified cases were verified with the reports from the central TB reference laboratory, with drug susceptibility test results for streptomycin, isoniazid, rifampicin, and ethambutol retrieved for culture-positive cases.

The sex- and age-stratified population data were obtained from the 2006 By-census for the following places of birth: Hong Kong (Group I); Mainland China (Group II); India, Pakistan, and Bangladesh (Indian subcontinent: Group III); Philippines, Thailand, Indonesia, and Nepal (other key Asian minority groups, Group IV); Vietnam (Group V); and other miscellaneous places of birth (Group VI).17 The crude incidence of TB among each of the above population groups was calculated with adjustment made by a multiplication factor (total notified cases / [total notified cases – cases with missing place of birth]) for cases with missing data on place of birth. The sex- and age-specific (by 5-year age-group) TB rates were derived from the overall population data and applied to the corresponding sex-age groups of each of the above six population groups to obtain the expected number of cases. The observed number of TB cases for each population group was compared with the respective number of expected cases to obtain the indirectly standardised TB incidence ratio. The 95% confidence intervals (CIs) were calculated by assuming a Poisson distribution in the occurrence of events. For those born in Mainland China, further stratification was made by the duration of residence in Hong Kong.

The treatment outcome 12 months after initiation of treatment was ascertained for those patients being managed by the government chest clinics under the Tuberculosis and Chest Service from the programme record form of the Tuberculosis and Chest Service. Treatment success was defined as cure or treatment completion (successfully completed treatment of ≥6 months for new cases and ≥8 months for retreatment cases), irrespective of subsequent relapse or death or loss to follow-up. All other treatment outcomes (including death before treatment completion, default, transferring out, still on treatment at 12 months after treatment initiation) were regarded as unsuccessful. Permanent residents who successfully completed treatment under the government TB programme were subsequently tracked up to 30 June 2013 through the territory-wide TB notification registry and death registry for relapse of TB or death.

Chi squared and Fisher’s exact tests were used as appropriate for categorical variables and analysis of variance was used for continuous variables. Logistic regression modelling was used for multivariate analysis of 12-month outcome. For censored data of TB relapse during follow-up, Kaplan-Meier analysis was used in univariate analysis and Cox proportional hazards modelling was used in multivariate analysis to adjust for potential confounders. A two-tailed P value of 0.05 was considered statistically significant.

The study was part of a public health surveillance exercise in tracking the profile and outcome of statutory TB notifications. It did not involve intervention on human subjects.

A total of 6246 TB notifications were received in 2006, 480 of which were excluded because of duplicate notification or revised diagnosis, leaving 5766 cases in the 2006 TB notification registry. Of these, 18 cases involving tourists, 17 cases involving illegal immigrants, and 329 cases without information on place of birth were also excluded, leaving 5402 cases for analysis. Table 1 shows their basic characteristics as stratified by place of birth. The vast majority (92.3%) of TB cases involved residents born in either Hong Kong or Mainland China. Significant differences were observed between the six population groups in terms of sex and age distribution as well as proportions of new and pulmonary cases, but not proportions of either smear-positive or culture-confirmed cases.

Table 2 summarises the incidence of TB and indirectly sex- and age-standardised incidence ratio (SIR) of TB for the six population groups. The TB SIR was significantly above 1 for those born in Mainland China (males and combined), Group III (females and combined), Group IV (females and combined), and Group V (males and combined), but significantly below 1 for those born in Hong Kong (males, females, and combined) and other miscellaneous places of birth (males, females, and combined). Mainland-born permanent residents (staying in Hong Kong for ≥7 years) maintained a higher TB risk than the population average for both sexes and combined. Nonetheless, recent Mainland immigrants with duration of stay of less than 7 years actually had a lower TB risk than the general population, despite sex and age standardisation.

Table 3 shows the resistance profile of 3474 (98.1%) culture-confirmed cases (with available drug susceptibility testing results) by place of birth. Table 4 summarises the results of univariate and multiple logistic analyses with respect to isoniazid, rifampicin, and multidrug resistance (resistance to both isoniazid and rifampicin) of 3434 culture-confirmed cases after combining all patients born in Asian countries listed under Groups III, IV and V, and excluding 40 patients with miscellaneous places of birth in Group VI that included very few drug-resistant cases. In the multiple logistic regression models using a backward stepwise elimination approach, only age <65 years, place of birth, and history of previous treatment (ie retreatment cases) remained important independent predictors of isoniazid, rifampicin, and multidrug resistance.

Of the 5402 subjects included in this study, 4319 (80.0%) were managed, at least at some stage of the disease, within the government TB programme. A total of 3304 (76.5%) patients successfully completed treatment within 12 months after initiation of treatment. Table 5 summarises the factors associated with 12-month treatment outcome in both univariate analysis and multivariate logistic regression analysis. Of those patients who successfully completed treatment, 3176 (96.1%) permanent residents in Hong Kong were followed up by cross-linking with the TB notification registry and death registry until relapse of TB, death or 30 June 2013, whichever was the earliest. After a mean (± standard deviation) duration of 5.28 ± 1.64 years of follow-up, 80 (2.5%) relapses were detected at a median (range) time interval of 1004 (225-2640) days after initiation of treatment, 37 (46.3%) of which were bacteriologically confirmed. In Kaplan-Meier analysis, the relapse risk was higher among permanent residents born outside Hong Kong than among those born in Hong Kong (3.0% vs 1.9%; log rank test, P=0.019). A consistently higher relapse risk was present among those born outside Hong Kong (adjusted hazard ratio=1.76; 95% CI, 1.07-2.89; P=0.025) after adjustment for gender, age, case category (new or retreatment), type of TB (pulmonary or extrapulmonary only), sputum smear, culture, and drug resistance to isoniazid and/or rifampicin at the baseline. The Figure shows the cumulative hazard curves by place of birth in and outside Hong Kong in Cox proportional hazards modelling.

In this study, persons born in Hong Kong had a SIR of 0.90 (95% CI, 0.87-0.94), while those born in Mainland China (Group II), Indian subcontinent (Group III), Philippines, Indonesia, Thailand, Nepal (Group IV), and Vietnam (Group V) had significantly higher SIRs of 1.06, 2.02, 1.59, and 3.11 respectively (Table 2). Recent Mainland migrants (with length of stay <7 years), however, had a significantly lower SIR (0.51 vs 1.09) than other Mainland-born residents. Age <65 years, birth in Mainland or Groups III-V Asian countries, and history of previous treatment were independently associated with resistance to isoniazid and/or rifampicin (Table 4). Older age, birth in Groups III-V Asian countries, non-permanent residents, retreatment case, and resistance to isoniazid and/or rifampicin were independently associated with lower treatment success (cure/treatment completion) rate at 1 year (Table 5). Birth outside Hong Kong (Groups II-V combined) was an independent predictor of TB relapse among permanent residents after successful treatment completion under the government TB programme (Fig).

With the successful control of recent transmission of TB in Hong Kong, the majority of TB cases arose from endogenous reactivation of past infection.18 The higher SIR and drug resistance prevalence among Mainland immigrants and immigrants from Groups III-V Asia countries corroborate reports of higher TB incidence2 3 4 5 6 8 and drug resistance7 12 among immigrants in low-TB-burden countries. These higher risks among immigrants may have resulted at least in part from reactivation of latent TB infection9 10 11 acquired during their previous residence in, and/or travel to, their places of birth with higher burdens of TB and/or drug resistance.1 19 20 21 22 Apart from possible selection factors in migration, the progressive fall in TB prevalence in the Mainland over the recent decades22 could also have contributed to a lower burden of latent TB infection, and hence SIR, among recent Mainland immigrants compared with those who immigrated longer ago. Taking into consideration the independent effects of birth outside Hong Kong on both treatment outcome and relapse (Table 5 and Fig), population mobility may have adversely affected treatment adherence, and thus impacted on treatment outcome and/or relapse with possible acquisition of drug resistance. In line with these observations, a previous case-control study also identified younger age, non-permanent residents, and frequent travel as independent predictors of multidrug-resistant TB among previously treated patients in Hong Kong.23

Although this study showed an increased risk of TB among immigrants in Hong Kong, which is a metropolitan city with intermediate TB burden, the vast majority of TB cases still occurred among the majority population groups of local-born persons or permanent residents born in Mainland China (both of which were largely of Chinese ethnicity). This is contrary to the situation in most low-burden areas, where the majority of TB cases often came from foreign-born minority groups.2 3 12 The relatively high crude TB incidence rate of 55/100 000, even among the local-born (both genders combined) in this intermediate burden area (Table 2), could have reduced the risk differential between the immigrant groups and the local-born, thus reducing the influence of immigrants on the overall TB incidence. Nonetheless, the higher drug resistance rate, poorer treatment outcome, and higher relapse rate among immigrants in this study remain critical areas of concern. In a recent study on the transmission of drug-resistant TB in Hong Kong,24 46% of all multidrug-resistant TB cases were new cases with no previous history of treatment. This suggests ongoing transmission of these difficult-to-treat TB cases within our community. The degree of molecular clustering was as high as 65% among extensively drug-resistant TB cases, the majority of which did not have obvious epidemiological linkage, suggesting active transmission outside households or other conventional close contact settings.24

This study was based on territory-wide data from by-census, statutory registries, a centralised government TB programme, and centralised TB laboratory. The well-developed health care infrastructure in Hong Kong with easy access to free TB care services allowed capture of relevant information from TB patients notified in a by-census year and successful tracking of the majority of them for treatment outcome and relapse. Some degree of incomplete case ascertainment was still likely as in all other public health surveillance systems. Even though around 20% of the notified patients were managed outside the government TB programme, this might not have substantially confounded the internal comparisons among different population groups if access to care could be assumed to be roughly parallel. If the usual inverse care law25 did apply, the direction of bias would likely be underestimation of the risks among the immigrants as an underprivileged group. With the limited amount of socio-demographic and clinical information contained in the various statutory registries and programme forms, this study may not be in a strong position to analyse the complex mechanisms that underlie the observed associations between immigrants and treatment outcome or relapse. Further studies are therefore warranted to identify potential areas of intervention for specific minority groups.

No external grant or support has been received for this study.

1. Global tuberculosis control report 2013. WHO/HTM/TB/2013.11. Available from: http://apps.who.int/iris/bitstream/10665/91355/1/9789241564656_eng.pdf. Accessed May 2015.

2. Gilbert RL, Antoine D, French CE, Abubakar I, Watson JM, Jones JA. The impact of immigration on tuberculosis rates in the United Kingdom compared with other European countries. Int J Tuberc Lung Dis 2009;13:645-51.

3. Das D, Baker M, Venugopal K, McAllister S. Why the tuberculosis incidence rate is not falling in New Zealand. N Z Med J 2006;119:U2248.

4. Cain KP, Haley CA, Armstrong LR, et al. Tuberculosis among foreign-born persons in the United States: achieving tuberculosis elimination. Am J Respir Crit Care Med 2007;175:75-9. Crossref

5. Dye C, Lönnroth K, Jaramillo E, Williams BG, Raviglione M. Trends in tuberculosis incidence and their determinants in 134 countries. Bull World Health Organ 2009;87:683-91. Crossref

6. Svensson E, Millet J, Lindqvist A, et al. Impact of immigration on tuberculosis epidemiology in a low-incidence country. Clin Microbiol Infect 2011;17:881-7. Crossref

7. Baussano I, Mercadante S, Pareek M, Lalvani A, Bugiani M. High rates of Mycobacterium tuberculosis among socially marginalized immigrants in low-incidence area, 1991-2010, Italy. Emerg Infect Dis 2013;19:1437-45. Crossref

8. Manangan L, Elmore K, Lewis B, et al. Disparities in tuberculosis between Asian/Pacific Islanders and non-Hispanic Whites, United States, 1993-2006. Int J Tuberc Lung Dis 2009;13:1077-85.

9. Farah MG, Meyer HE, Selmer R, Heldal E, Bjune G. Long-term risk of tuberculosis among immigrants in Norway. Int J Epidemiol 2005;34:1005-11. Crossref

10. Patel S, Parsyan AE, Gunn J, et al. Risk of progression to active tuberculosis among foreign-born persons with latent tuberculosis. Chest 2007;131:1811-6. Crossref

11. McPherson ME, Kelly H, Patel MS, Leslie D. Persistent risk of tuberculosis in migrants a decade after arrival in Australia. Med J Aust 2008;188:528-31.

12. Long R, Langlois-Klassen D. Increase in multidrug-resistant tuberculosis (MDR-TB) in Alberta among foreign-born persons: implications for tuberculosis management. Can J Public Health 2013;104:e22-7.

13. Tuberculosis and Chest Service. Annual Report of Tuberculosis and Chest Service 1949. Hong Kong: Department of Health; 1950: 3.

14. Tuberculosis and Chest Service. Annual Report of Tuberculosis and Chest Service 1961. Hong Kong: Department of Health; 1962: 3.

15. Hong Kong Monthly Digest of Statistics April 2012, p.9. Available from: http://www.census2011.gov.hk/pdf/Feature_articles/Trends_Pop_DH.pdf. Accessed 3 Jul 2013.

16. Tuberculosis and Chest Service. Annual Report of Tuberculosis and Chest Service 2006. Hong Kong: Department of Health; 2008.

17. Census and Social Statistics Department of Hong Kong, 2006 Population By-census. Available from: http://www.bycensus2006.gov.hk/en/data/data2/index.htm. Accessed 3 Jul 2014.

18. Chan-Yeung M, Kam KM, Leung CC, et al. Population-based prospective molecular and conventional epidemiological study of tuberculosis in Hong Kong. Respirology 2006;11:442-8. Crossref

19. Zignol M, van Gemert W, Falzon D, et al. Surveillance of anti-tuberculosis drug resistance in the world: an updated analysis, 2007-2010. Bull World Health Organ 2012;90:111-119D. Crossref

20. Zhao Y, Xu S, Wang L, et al. National survey of drug-resistant tuberculosis in China. N Engl J Med 2012;366:2161-70. Crossref

21. Udwadia ZF, Amale RA, Ajbani KK, Rodrigues C. Totally drug-resistant tuberculosis in India. Clin Infect Dis 2012;54:579-81. Crossref

22. Wang L, Zhang H, Ruan Y, et al. Tuberculosis prevalence in China, 1990-2010; a longitudinal analysis of national survey data. Lancet 2014;383:2057-64. Crossref

23. Law WS, Yew WW, Leung CC, et al. Risk factors for multidrug-resistant tuberculosis in Hong Kong. Int J Tuberc Lung Dis 2008;12:1065-70.

24. Leung EC, Leung CC, Kam KM, et al. Transmission of multidrug-resistant and extensively drug-resistant tuberculosis in a metropolitan city. Eur Respir J 2013;41:901-8. Crossref

25. Hart JT. The inverse care law. Lancet 1971;1:405-12. Crossref