Risk of leukaemia in children infected with enterovirus: a nationwide, retrospective, population-based, Taiwanese-registry, cohort study

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Risk of leukaemia in children infected with enterovirus: a nationwide, retrospective, population-based, Taiwanese-registry, cohort study Jiun-Nong Lin, Cheng-Li Lin, Ming-Chia Lin, Chung-Hsu Lai, Hsi-Hsun Lin, Chih-Hui Yang, Fung-Chang Sung, Chia-Hung Kao

Summary Background The association between enterovirus infections in children and risk of leukaemia is unclear. We aimed to assess the risk of leukaemia after enterovirus infection in children. Methods We did a nationwide retrospective cohort study by analysing data from the National Health Insurance Research Database (NHIRD) in Taiwan. Children with enterovirus infections aged younger than 18 years were identified. With use of computer-generated random numbers, children not infected with enterovirus were randomly selected and frequency matched (1:1) with children infected with enterovirus by sex, age, urbanisation level, parental occupation, and index year of enterovirus infection. We only included children with complete baseline data for age and sex and who had at least three clinic visits with the diagnosis of enterovirus infection. The diagnosis date of the first clinic visit for the enterovirus infection was defined as the index date for initiation of follow-up person-year measurement and participants. All study patients were followed up until they developed leukaemia, were lost to follow-up, withdrew from the NHI programme, or until the end of the study without leukaemia (censored). Our primary endpoint was a diagnosis of leukaemia during follow-up. Findings Insurance claims data for 3 054 336 children younger than 18 years were randomly selected from all insured children in the NHIRD. We identified 282 360 children infected with enterovirus and 282 355 children not infected with enterovirus between Jan 1, 2000, and Dec 31, 2007. The incidence density rates of leukaemia were 3·26 per 100 000 person-years for the enterovirus-infected and 5·84 per 100 000 person-years for the non-enterovirus-infected cohorts. The risk of leukaemia was significantly lower in the enterovirus-infected cohort than in the non-enterovirusinfected cohort (adjusted subhazard ratio [SHR] 0·44, 95% CI 0·31–0·60; p<0·0001). Children infected with enterovirus have a reduced risk of both lymphocytic leukaemia (adjusted SHR 0·44, 0·30–0·65; p<0·0001) and acute myeloid leukaemia (adjusted SHR 0·40, 0·17–0·97; p=0·04). Herpangina and hand-foot-and-mouth disease were the main diseases associated with the reduced risk of leukaemia. Interpretation The association between enterovirus infection and the reduced risk of developing leukaemia supports Greaves’ delayed infection hypothesis for the cause of childhood leukaemia. Funding Taiwan Ministry of Health and Welfare, Academia Sinica, NRPB Stroke Clinical Trial Consortium, TsengLien Lin Foundation, Taiwan Brain Disease Foundation, Katsuzo and Kiyo Aoshima Memorial Funds, China Medical University Hospital, and Taiwan Ministry of Education.

Introduction Leukaemia is the most common cancer in children, accounting for more than a third of childhood malignancies.1 Acute lymphoblastic leukaemia is the predominant leukaemia in children, accounting for 70–80% of cases, followed by acute myeloid leukaemia, which accounts for roughly 15–17%. Chronic myeloid leukaemia and other forms of myeloid leukaemia rarely exceed 4% of all types of leukaemia in children.1–3 However, the causes of leukaemia remain unclear. Several studies4,5 have proposed that genetic alterations in chromosomes and environmental factors are associated with leukaemia. Molecular genetic alterations might drive mutation, leading to childhood acute lymphoblastic leukaemia with racial and ethnic disparities. Genetic abnormalities have been associated with chromosomal translocations.4 Mutations in the IKZF1 gene and other genes encoding

kinase-activating proteins are important contributors to acute lymphoblastic leukaemia.4 Hispanic children are at a higher risk of developing acute lymphoblastic leukaemia but have poorer survival than children of non-Hispanic ethnic origin.5 The chromosomal translocations known to be associated with acute lymphoblastic leukaemia include t(12;21)(p13;q22) with ETV6-RUNX1 fusion, t(1;19) (q23;p13) with TCF3-PBX1 fusion, t(9;22)(q34;q11) with BCR-ABL1 fusion, t(4;11)(q21;q23) with MLL-AF4 fusion, hyperdiploidy, or hypodiploidy.4,5 Viral infections have been regarded as a crucial environmental risk factor for leukaemia.6–11 Greaves12,13 proposed the delayed infection hypothesis in childhood leukaemia. According to this hypothesis, children who have a delayed exposure to common infections have a more vigorous immune response. Proliferative immunological cells frequently undergo a second mutation leading to the

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Lancet Oncol 2015 Published Online August 28, 2015 http://dx.doi.org/10.1016/ S1470-2045(15)00060-1 See Online/Comment http://dx.doi.org/10.1016/ S1470-2045(15)00194-1 Department of Critical Care Medicine (J-N Lin MD), Division of Infectious Diseases, Department of Internal Medicine (J-N Lin, C-H Lai MD, H-H Lin MD), and Department of Nuclear Medicine (M-C Lin PhD), E-Da Hospital, and School of Medicine, College of Medicine (J-N Lin), I-Shou University, Kaohsiung, Taiwan; Management Office for Health Data, China Medical University Hospital, Taichung, Taiwan (C-L Lin MSc); College of Medicine, China Medical University, Taichung, Taiwan (C-L Lin); General Education Center, Meiho University, Pingtung, Taiwan (C-H Yang PhD); Graduate Institute of Clinical Medical Science and School of Medicine, College of Medicine, China Medical University, Taichung, Taiwan (Prof F-C Sung PhD, Prof C-H Kao MD); and Department of Nuclear Medicine and PET Center, China Medical University Hospital, Taichung, Taiwan (Prof C-H Kao) Correspondence to: Prof Chia-Hung Kao, Graduate Institute of Clinical Medical Science and School of Medicine, China Medical University, Taichung 404, Taiwan [email protected]

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Research in context Evidence before this study Before the start of this study, on March 12, 2015, we searched PubMed with combinations of the terms “leukaemia” and “enterovirus” (as well as variations thereof) without language restrictions. Although enteroviruses are common, we identified no research to evaluate the association between enterovirus infection and leukaemia. Added value of this study Our study showed a significantly reduced risk of leukaemia in children with enterovirus infection compared with those without infection. Herpangina and hand-foot-and-mouth

development of leukaemia in children. Conversely, exposure to common infections in early childhood is associated with a reduced risk of leukaemia.12,13 Kinlen14 proposed the population mixing hypothesis, suggesting that children might have an abnormal immune response to a common but unidentified infection; he suggested that when the infected population mixes with susceptible individuals, the risk of leukaemia increases. Enteroviruses belong to the Picornaviridae family and include more than 90 distinct viral serotypes, such as polioviruses, coxsackieviruses, echoviruses, and numerically named enteroviruses.15,16 Enterovirus infections are common in children and about 10–15 million children contract non-polio enterovirus infections in the USA every year.17 Enterovirus infections are prevalent in children in Taiwan, as they are worldwide. Multiple clinical manifestations of enterovirus infections have been recognised in human beings, including herpangina, handfoot-and-mouth disease, meningoencephalitis, acute flaccid paralysis, haemorrhagic conjunctivitis, respiratory tract infection, myocarditis, and pericarditis.16 Although enterovirus infections are common in children, the association between infection and leukaemia has not been assessed in a cohort study. We therefore aimed to establish whether the risk of developing leukaemia was greater in children infected with enteroviruses by analysing data from the National Health Insurance Research Database of Taiwan.

Methods Study population and design The National Health Insurance (NHI) programme was implemented in 1995 and has information about up to 99% of the 23·74 million people living in Taiwan.18 We compiled data files for children (aged <18 years) from the NHI programme, which were established and maintained by the National Health Research Institutes (NHRI). The dataset consisted of a randomly selected sample of half of all children in Taiwan who were insured from 1996 to 2008. Randomisation was done with use of computer-generated random numbers assigned to 2

disease were the major diseases associated with the reduced risk of leukaemia. To our knowledge, this study provides the first evidence to show a negative association between enterovirus infection and leukaemia risk. Implications of all the available evidence Findings from this study strongly support the epidemiological evidence for the role of viral infection in childhood leukaemia. Although this study does not show biological plausibility, experimental studies based on these findings to understand the pathogenesis of enteroviruses in leukaemia are suggested.

individual data files from the NHRI. To protect patient privacy, the NHRI had encrypted all personal identification numbers into unique numbers before releasing the data files to the public for research purposes. The disease criteria were defined and classified according to the diagnostic codes of the International Classifications of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM). To ensure the accuracy of disease diagnosis, the Bureau of NHI randomly reviewed the medical charts of one in 100 ambulatory and one in 20 inpatient claims. We identified children aged younger than 18 years with newly diagnosed enterovirus infections (ICD-9-CM codes 008.67, 047, 048, 074, 079.1, and 079.2) as the cohort infected with enteroviruses. To avoid coding errors in the claims data, we only included children who had at least three clinic visits with the diagnosis of enterovirus infection. The diagnosis date of the first clinic visit for the enterovirus infection was defined as the index date for initiation of follow-up person-year measurement. For each child with enterovirus infection, one child without enterovirus infection was randomly selected for the non-enterovirusinfected cohort with a frequency matching method to ensure both cohorts had the same distributions for strata of sex, age (every 1 year span), urbanisation level, parental occupation, and index year of enterovirus infection. Children with a history of cancer (ICD-9-CM codes 140–208) and with incomplete data for age or sex at baseline were excluded from both cohorts. This study was approved by the Institutional Review Board of China Medical University Hospital (CMUH-104-REC2-115 and CRREC-103-048). The sociodemographic variables in this study were age, sex, urbanisation level, and parental occupation (office jobs, manual labour jobs, or other). The NHRI stratified all city districts and townships in Taiwan (based on national administrative zones demarcation) into seven urbanisation levels based on population density (people per km²), proportion of residents with higher education, elderly and agricultural population, and the number of physicians per 100 000 people in each area.19 Level 1

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represented areas with the highest population density and socioeconomic status and level 7 represented areas with the lowest. Because few people lived in rural areas classified as levels 4–7, we grouped all these areas into the level 4 group. Office workers were employees characterised by indoor work, including public institutional workers, educators, and administrative personnel in business and industries. Manual labourers were characterised by increased hours of outdoor work, such as fishermen, farmers, and industrial labourers. Other occupations included mainly retired, unemployed, and low-income populations. Allergic diseases are comorbidities of interest in this study. We defined allergic diseases as a group encompassing atopic dermatitis (ICD-9-CM code 691.8), allergic rhinitis (ICD-9-CM code 477), and bronchial asthma (ICD-9-CM code 493). The study outcome was a diagnosis of leukaemia (ICD-9-CM codes 204–208) during the 9 year follow-up, according to the NHI catastrophic illness registry files. Leukaemia is categorised as a catastrophic illness in the NHI programme and patients newly diagnosed with leukaemia must apply for catastrophic illness certification. The certification is issued by the government after a stringent verification process including reviews of medical records, images, and pathology reports by a panel of specialists and experts on the disease. All study patients were followed up until they developed leukaemia, they were lost to follow-up, they withdrew from the NHI programme, or until the end of the study (Dec 31, 2008) without leukaemia (censored).

Statistical analysis We compared the distribution of sociodemographic status and allergic diseases between the enterovirusinfected and non-enterovirus-infected cohorts using the χ² test. Continuous data were expressed as mean (SD) and compared between the two cohorts with Student’s t test. The incidence density rates (per 100 000 personyears) were calculated for each cohort according to the leukaemia type. We used the Fine and Gray competing risks regression analysis20 to estimate the subhazard ratio (SHR) and 95% CI of leukaemia associated with enterovirus infection. The SHR was calculated due to the competing risk in the analyses of survival and cumulative incidence and can analyse the time to the first event, irrespective of which one this was. We calculated the standardised incidence ratio (SIR) of leukaemia using the indirect method of Breslow and Day.21 The multivariable models were simultaneously adjusted for age, sex, urbanisation-level, parental occupation, and allergic diseases. The age-specific, sex-specific, urbanisation level-specific, parental occupation-specific, and allergy-specific incidence densities of leukaemia were estimated for both cohorts. We did Kaplan-Meier analysis for the visual inspection of the cumulative incidence of leukaemia in the two cohorts to compute the Aalen-Johansen estimator adjusting for competing

risks.22 A p value lower than 0·05 was deemed statistically significant. We used SAS statistical package (version 9.3) to analyse the data. 3 054 336 children younger than 18 years randomly selected from all insured children in the NHIRD

285 597 newly diagnosed with an enterovirus infection in 2000–07

102 patients with cancers excluded before index date 3135 missing data for age or sex

282 360 enterovirus infection cases without cancers at baseline

282 355 matched participants without enterovirus infection assigned to control group

564 715 participants in enterovirus virus infection group and control group followed up and included in analysis

Figure 1: Flow diagram of the enrolment process NHIRD=National Health Insurance Research Database.

Non-enterovirus-infected group (n=282 355) Age (years)

2·39 (1·93)

Enterovirus-infected group (n=282 360) 2·39 (1·88)

Stratified age ≤2 years

154 922 (55%)

154 925 (55%)

3–5 years

102 801 (36%)

102 801 (36%)

>5 years

24 632 (9%)

24 634 (9%)

Sex Female

130 868 (46%)

130 870 (46%)

Male

151 487 (54%)

151 490 (54%)

Urbanisation level* 1

72 829 (26%)

72 829 (26%)

2

83 796 (30%)

83 796 (30%)

3

54 962 (19%)

54 962 (19%)

4

70 768 (25%)

70 773 (25%)

Parental occupation Office job

180 680 (64%)

180 685 (64%)

Manual labourer

68 372 (24%)

68 372 (24%)

Other†

33 303 (12%)

33 303 (12%)

Allergic disease No

251 315 (89%)

23 287 (82%)

Yes

31 040 (11%)

49 973 (18%)

Data are mean (SD) or n (%). *The urbanisation level was categorised by the population density of the residential area into four levels: level 1 as the most urbanised region and level 4 as the least urbanised region. †Other occupations included mainly retired, unemployed, and low-income populations.

Table 1: Demographic characteristics of participants

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Role of the funding source The funders of the study had no role in study design, data collection, data analysis, data interpretation, or writing of the report. C-LL had access to the raw data. The corresponding author had full access to all the data in the study and had final responsibility to submit for publication.

Results See Online for appendix

0·0010

Insurance claims data for 3 054 336 children younger than 18 years were randomly selected from the NHIRD

Without enterovirus infection With enterovirus infection

Cumulative incidence of leukaemia

SIR 0·57 (95% CI 0·43–0·75); p<0·0001 0·0008

0·0006

0·0004

0·0002

0 0

2

4

6

8

10

Follow-up (years)

Figure 2: Kaplan-Meier analysis of cumulative incidence of leukaemia for children with or without enterovirus infection SIR=adjusted subhazard ratio.

(figure 1). Of these children, 285 597 were newly diagnosed with an enterovirus infection and included in the enterovirus-infected cohort between Jan 1, 2000, and Dec 31, 2007. We excluded 102 children with an enterovirus infection and cancers before the index date and 3135 children with missing data for age or sex. 282 360 children were included in the enterovirusinfected cohort and 282 355 were included in the nonenterovirus-infected group (control group). Compared with the non-enterovirus-infected cohort, we noted that the enterovirus-infected cohort had a lower number of deaths (89 [0·03%] vs 218 [0·08%]) and loss to follow-up (1419 [0·50%] vs 4759 [1·69%]) than the control group. Baseline data for both groups were similar, with the exception of allergic diseases, which were more prevalent in the enterovirus-infected cohort than the control group (table 1). The median follow-up was 5·66 years (IQR 3·60–7·55) in the enterovirus-infected group and 5·63 years (3·71–7·55) in the control group. We compared the month of occurrence (first diagnosis) of acute lymphoblastic leukaemia in tertiles between the two cohorts. Acute lymphoblastic leukaemia was reported much earlier in the enterovirus-infected group (median 25·5 months, IQR 6·31–42·7) than in the non-enterovirus-infected children (median 37·9 months, IQR 28·8–60·8). The analysis of seasonality between enterovirus infection and acute lymphoblastic leukaemia showed a correlation coefficient of 0·45 (95% CI –0·17 to 0·81; p=0·15; appendix). Kaplan-Meier analysis showed that the cumulative incidence of leukaemia was significantly lower in the

Non-enterovirus-infected group

Enterovirus-infected group

Number Personyears of events

Number Personyears of events

Incidence density rate (per 100 000 person-years)

Adjusted SHR* (95% CI), p value

Incidence density rate (per 100 000 person-years)

All leukaemia (ICD-9-CM 204–208)

91

1 558 445

5·84

51

1 564 656

3·26

Lymphocytic leukaemia (ICD-9-CM 204)

65

··

4·17

38

··

2·43

0·44 (0·30–0·65), p<0·0001

Acute lymphoblastic leukaemia (ICD-9-CM 204.0, 204.00, 204.01)

39

··

2·5

23

··

1·47

0·43 (0·26–0·69), p=0·0006

Chronic lymphocytic leukaemia (ICD-9-CM 204.1, 204.10, 204.11)

0

··

0

0

··

0

··

Subacute type (ICD-9-CM 204.2, 204.20, 204.21)

0

··

0

0

··

0

··

Other (ICD-9-CM 204.8, 204.80, 204.81, 204.9, 204.90, 204.91)

0

··

0

1

··

0·06

21

··

1·35

12

··

0·77

0·41 (0·21–0·80), p=0·009

Myeloid leukaemia (ICD-9-CM 205)

0·44 (0·31–0·60), p<0·0001

··

Acute myeloid leukaemia (ICD-9-CM 205.0, 205.00, 205.01)

12

··

0·77

7

··

0·45

0·40 (0·17–0·97), p=0·04

Chronic myeloid leukaemia (ICD-9-CM 205.1, 205.10, 205.11)

2

··

0·13

1

··

0·06

0·30 (0·03–2·72), p=0·29

Subacute type (ICD-9-CM 205.2, 205.20, 205.21)

0

··

0

0

··

0

··

Other (ICD-9-CM 205.3, 205.30, 205.31, 205.8, 205.80, 2053.81, 205.9, 205.90, 205.91)

0

··

0

1

··

0·06

·· ··

Monocytic leukaemia (ICD-9-CM 206)

0

··

0

0

··

0

Other specified leukaemia (ICD-9-CM 207)

0

··

0

0

··

0

Leukaemia of unspecified cell type (ICD-9-CM 208)

5

··

0·32

1

··

0·06

·· 0·12 (0·02-0·91), p=0·04

SHR=subhazard ratio. ICD-9-CM=International Classification of Diseases, Ninth Revision, Clinical Modification. *Adjusted for age, sex, urbanisation level, parental occupation, and allergy.

Table 2: Type-specific incidence of childhood leukaemia in study cohorts and competing risk regression analysis

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Number of participants

Number of events

Person-years per 100 000 person-years

Incidence density rate (per 100 000 person-years)

Adjusted SHR (95% CI)*

282 355

91

1 558 445

5·84

1 (ref)

Enteritis caused by enterovirus (ICD-9-CM 008.67)

29

0

99·9

0

Meningitis caused by enterovirus (ICD-9-CM 047)

667

0

3843·0

0

··

Other enterovirus diseases of CNS (ICD-9-CM 048)

182

0

1229·7

0

··

281 167

51

1 557 654·3

3·27

0·44 (0·32–0·61), p<0·0001

225 751

45

1 224 922

3·67

0·54 (0·38–0·75), p=0·0003

Non-enterovirus-infected cohort Subtype of enterovirus infection

Specific diseases caused by coxsackievirus (ICD-9-CM 074) Herpangina (ICD-9-CM 074·0)

··

Epidemic pleurodynia ( ICD-9-CM 074.1)

30

0

174

0

Coxsackievirus carditis (ICD-9-CM 074.2)

31

0

195

0

52 108

6

309 665

3247

0

22 699

0

··

78

0

359·0

0

··

237

0

1470·0

0

··

Hand-foot-and-mouth disease (ICD-9-CM 074.3) Other specified diseases caused by coxsackievirus (ICD-9-CM 074.8) Echovirus infection in conditions classified elsewhere and of unspecified site (ICD-9-CM 079.1) Coxsackievirus infection in conditions classified elsewhere and of unspecified site (ICD-9-CM 079.2)

1·94

·· ·· 0·40 (0·18–0·90), p=0·03

SHR=subhazard ratio. ICD-9-CM=International Classification of Diseases, Ninth Revision, Clinical Modification.*Adjusted for age, sex, urbanisation level, parental occupation, and allergy.

Table 3: Incidence density rate and SHR of leukaemia in patients by type of enterovirus infection estimated with competing risk regression analysis

enterovirus-infected cohort than in the non-enterovirusinfected cohort after accounting for deaths and loss to follow-up as the competing risks (figure 2). The SIR for leukaemia calculated with the indirect method for enterovirus-infected children compared with the general population was 0·57 (95% CI 0·43–0·75; p<0·0001). The incidence density rates of leukaemia were 3·26 per 100 000 person-years for the enterovirus-infected group and 5·84 per 100 000 person-years for the nonenterovirus-infected cohorts (table 2). The risk of leukaemia was significantly lower in the enterovirusinfected cohort than in the non-enterovirus-infected cohort (adjusted SHR 0·44, 0·31–0·60; p<0·0001). Analyses of leukaemia by type showed that children with enterovirus infection had a significantly lower risk of both lymphocytic leukaemia and acute myeloid leukaemia than that of children without enterovirus infection. We also estimated the risk of leukaemia by subtypes of enterovirus infection (table 3). Lower risk of leukaemia was noted in children with herpangina and hand-footand-mouth disease; there were no other significant associations. The incidence rates of leukaemia for both cohorts were stratified by sex, the age at the diagnosis of enterovirus infection, urbanisation level, parental occupation, and allergic status (table 4). The gap in leukaemia incidence between the enterovirus-infected cohort and the non-enterovirus-infected cohort was greater for children aged 2 years or younger than for children of older age. The incidence differences of leukaemia between the two cohorts were greater in boys than in girls, in children living in less urbanised areas (level 3), and in children with parents with office jobs

(table 4). For children without allergic diseases, the incidence of leukaemia was significantly lower in the enterovirus-infected cohort than in the non-enterovirusinfected cohort. The analysis of interaction between the enterovirus exposure and each of the factors showed that age (p=0·27), sex (p=0·37), urbanisation level (p=0·80), and parental occupation (p=0·18) did not have significant interactions with the enterovirus infection. A statistically significant interaction only existed between enterovirus infection and allergic diseases (p=0·04; table 4).

Discussion In this study, we identified that the risk of leukaemia was significantly lower in the enterovirus-infected cohort than in the non-enterovirus-infected cohort. Herpangina and hand-foot-and-mouth disease were the main diseases associated with the reduced risk of leukaemia. The risk of leukaemia for children without allergic disease was significantly lower in the enterovirus-infected cohort than in the non-enterovirus-infected cohort. The incidence of leukaemia might differ between people from groups and countries of different ethnic origin.23–25 The incidence of childhood leukaemia ranges from 1·64 per 100 000 person-years in low-income countries to 4·09 per 100 000 person-years in highincome coutries.26 The Taiwan Cancer Registry, a government-supported and population-based cancer registry system, has reported that the average incidence of leukaemia was 4·3 per 100 000 person-years (95% CI 4·14–4·46) in children aged 0–14 years from 1996 to 2010.27 In our study, children aged 0–18 years with an enterovirus infection had lower incident leukaemia than the Taiwan Cancer Registry measured value. Moreover,

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Non-enterovirus-infected group

Enterovirus-infected group

Number Personof events years

Incidence density rate (per 100 000 person-years)

Number of events

Person-years

Adjusted SHR* (95% CI)

Incidence density rate (per 100 000 person-years)

Stratified age (years)†

0·27

≤2 years

71

857 148

8·28

35

861 971

4·06

0·39 (0·26–0·57)

>2 years

20

701 297

2·85

16

702 685

2·28

0·67 (0·36–1·25)

Female

34

721 179

4·71

23

724 246

3·18

0·42 (0·26–0·69)

Male

57

837 266

6·81

28

840 410

3·33

0·33 (0·22–0·50)

19

399 054

4·76

10

401 112

2·49

0·44 (0·21–0·92)

Sex‡

0·37

Urbanisation level§¶ 1 (highest)

0·80

2

31

462 538

6·70

22

464 477

4·74

0·53 (0·32–0·89)

3

21

305 058

6·88

7

306 374

2·28

0·28 (0·12–0·65)

4 (lowest)

20

391 794

5·10

12

392 693

3·06

0·44 (0·23–0·87)

Office job

66

993 991

6·64

32

997 670

3·21

0·34 (0·23–0·51)

Manual labourer

16

388 063

4·12

11

388 416

2·83

0·44 (0·22–0·89)

9

176 391

5·10

8

178 570

4·48

0·36 (0·16–0·81)

No

168

2 804 734

5·99

78

2 613 018

2·99

0·39 (0·27–0·56)

Yes

14

312 155

4·48

24

516 294

4·65

0·95 (0·39–2·29)

Parental occupation**

Others††

p value

0·18

Allergic diseases‡‡

0·04

SHR=subhazard ratio. *Adjusted for age, sex, urbanisation level, parental occupation, and allergy. †Adjusted SHR was calculated by Cox proportional hazard regression stratified by age and adjusted for sex, urbanisation level, parental occupation, and allergy. ‡Adjusted SHR was calculated by Cox proportional hazard regression stratified by sex, and adjusted for age, urbanisation level, parental occupation, and allergy. §Adjusted SHR was calculated by Cox proportional hazard regression stratified by urbanisation level and adjusted for age, sex, parental occupation, and allergy. ¶The urbanisation level was categorised by the population density of the residential area into four levels: level 1 as the most urbanised region and level four as the least urbanised region. **Adjusted HR was calculated by Cox proportional hazard regression stratified by parental occupation and adjusted for age, sex, urbanisation level, and allergy. ††Other occupations included primarily retired, unemployed, and low-income populations. ‡‡Adjusted SHR was calculated by Cox proportional hazard regression stratified by allergy and adjusted for age, sex, urbanisation level, and parental occupation.

Table 4: The risk of leukaemia in children and shared subhazard ratio for participants stratified by prespecified sociodemographic characteristics

the SIR of leukaemia for enterovirus-infected children compared with the general population aged 0–18 years also showed a reduced risk of leukaemia for children with enterovirus infection. Greaves12,13 proposed the delayed infection hypothesis for childhood leukaemia, and many epidemiological studies6–11,28–30 have supported this hypothesis. Common hygiene-related infections, such as infections of the respiratory system and gastrointestinal tract, occur frequently in the day-care centre;28 therefore, the attendance of young children in these settings is often regarded as an indicator of early exposure to infections. The UK Children’s Cancer Study, a large population based case-control study, used day-care and social activity in the children’s first year of life as proxies for potential exposure to infection to test the Greaves hypothesis. This UK study showed a dose–response association between increasing levels of social activity and a reduced risk of acute lymphoblastic leukaemia. The Northern California Childhood Leukemia Study,29 another large populationbased case-control study, also identified that extensive contact with other children in daycare settings resulted in a significantly reduced risk of acute lymphoid 6

lymphoma. A meta-analysis30 shows that day-care attendance, particularly in children aged 2 years or younger, is associated with a reduced risk of acute lymphoblastic leukaemia. The ESTELLE study9 done in France reported that early common infections through active day-care activities before the age of 1 year, breastfeeding, and regular contact with pets were also associated with a reduced risk of acute lymphoblastic leukaemia. Our study showed an inverse association between enterovirus infection and childhood leukaemia risk. The present study results have provided an adequate dataset with valid evidence to support the delayed infection hypothesis as a potential explanation for childhood leukaemia.13 In the population mixing hypothesis, Kinlen14 emphasises the association of leukaemia with exposure to previously unencountered infections. However, little is known about leukaemogenic infections. Marcotte and colleagues10 analysed the association of childhood acute lymphoblastic leukaemia from the California Cancer Registry with the respiratory syncytial virus season surveillance reports from the US Centers of Disease Control and Prevention and California Department of

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Public Health Influenza Surveillance Program. They identified that the risk of acute lymphoblastic leukaemia was higher in children with the infection of influenza or respiratory syncytial virus at 9–12 months of age than in children with the infections during the first 3 months of life.10 However, no direct records of influenza or respiratory syncytial virus infection were obtained from the database in their study. A study from New Zealand31 assessed the association between several infections and childhood acute lymphoblastic leukaemia using interviews or questionnaires. Their study identified that children infected with influenza during the first year of life were at an increased risk of childhood acute lymphoblastic leukaemia (odds ratio 6·8, 95% CI 1·8–25·7). However, no association was reported between childhood acute lymphoblastic leukaemia and measles, whooping cough, rubella, chickenpox, oral infection, eye infection, ear infection, persistent cough, and diarrhoea and vomiting. The information about serological tests were available in the study for measles IgG, Epstein-Barr virus capsid antigen IgG, cytomegalovirus IgG, and poliovirus type 1–3 antibody tests.31 An increased risk of childhood acute lymphoblastic leukaemia was associated with only poliovirus type 1 serology, after controlling for age, sex, and other confounding variables. But few cases were available for the analyses. Our study showed a reduced risk of leukaemia in children with enterovirus infection. However, we did not examine the association between other types of viral infections and leukaemia. To our knowledge, no study strongly supports the association between leukaemia risk and a unique microorganism so far. The incidence of childhood leukaemia is associated with socioeconomic status.32 Findings of the association were heterogeneous.33,34 An inverse association between childhood leukaemia and family income and parental education has been reported in studies in the USA that used data obtained from interviews or self-administered questionnaires for analysis. Conversely, a positive association between childhood leukaemia and a higher socioeconomic status was identified in record-based case-control studies from Europe.33,34 In our study, we examined the leukaemia risk in association with children’s parental occupation and urbanisation level where they lived. However, these statuses did not play an important part in the association between enterovirus infection and leukaemia in this study. Strachan35 proposed the hygiene hypothesis, and identified an inverse association between hay fever and eczema risk and the number of older children in the household. The hygiene hypothesis is also implicated with childhood leukaemia. The hygiene hypothesis suggests that a small family size and improved sanitation lower the tendency for cross infection in early childhood and thus results in a higher prevalence of allergies.35 This hypothesis has been supported by several epidemiological studies36–39 that reported a reduced risk of allergy in

children with a higher birth order or early day-care attendance. Moreover, childhood acute lymphoblastic leukaemia is inversely associated with a higher birth order and early day-care attendance, as in the case of the aforementioned delayed infection hypothesis of leukaemia.6–11 Therefore, a positive correlation between allergy and leukaemia can be expected. However, studies inspecting the association between allergy and leukaemia showed conflicting results. Most studies reported an inverse association between allergic disorders and the risk of childhood leukaemia.40–43 But Chang and colleagues analysed the data from the NHIRD of Taiwan and reported an increased risk of acute lymphoblastic leukaemia in children with allergic disorders occurring before 1 year of age, less than 1 year before acute lymphoblastic leukaemia diagnosis, and more than 1 year before acute lymphoblastic leukaemia diagnosis.44 Acute lymphoblastic leukaemia and allergic disorders possibly shared a mutual biological cause factor, the overactive and dysregulated immune reaction in response to infection.44 Tests for interaction between age, sex, urbanisation level, parental occupation, and allergic diseases on one hand, and enterovirus infection on the other hand, showed that the interactions were not statistically significant apart from the interaction of allergic diseases with the enterovirus infection (p=0·04). Our study showed that the risk of leukaemia for children without allergic disease was significantly lower in the enterovirus-infected cohort than in the non-enterovirusinfected cohort. This finding was not noted for those with allergic diseases, indicating that allergic diseases had no role in the causes of childhood leukaemia. The results in our study were not consistent with the findings from the study by Chang and colleagues.44 These differences might be explained by the dissimilar study designs. In our study, we focused on the relation of enterovirus infection to leukaemia and we did not examine the temporal sequence of the diagnoses of allergy and leukaemia. However, the exact reasons for the equivocal results between allergy and childhood leukaemia are unclear. The main strengths of this study include the large nationwide sample and comprehensive demographic characteristics. However, several limitations of this study exist. First, several risk factors for childhood leukaemia, such as air pollution, parental occupational chemical exposures, tobacco smoking, and pesticides in the household, have been reported.3 However, the NHIRD does not have this information. Second, because of the absence of information about the T-cell or B-cell subtypes of lymphocytic leukaemia, the association of enterovirus infections with these subtypes could not be analysed. Third, laboratory confirmation of enterovirus infection, such as serology or viral cultures, was unavailable in the NHIRD. However, the most common forms of enterovirus infections—namely, herpangina and handfoot-and-mouth disease—accounted for 277 859 (98%) of

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all 282 360 enterovirus infections in our study. Both these enterovirus infections are easily and reliably diagnosed solely on the basis of clinical manifestations. Fourth, children with mild symptoms of enterovirus infection might not seek health care, or might only have sought health care once or twice, and so were not included in the enterovirus cohort. Thus only those with more severe clinical manifestations were included. Whether children with benign or subclinical infections are associated with the risk of leukaemia development is unclear. Fifth, the precision of diagnoses based on the ICD-9-CM codes in the database might affect the study findings. However, the Bureau of NHI regularly reviews the charts and assesses the accuracy of claims files. Incorrect coding of diseases will result in no reimbursement, and the institutions are fined accordingly, and the high accuracy of data in the NHIRD has been proven by several studies.45,46 Finally, although the results of our study are consistent with the reports on the possible role of childhood infection in the reduction of leukaemia risk in children, biological plausibility and experimental explanation still does not exist. Further studies to understand the mechanism and pathogenesis between enterovirus infection and leukaemia are warranted. Overall, enterovirus infection is a crucial issue in children worldwide. Some highly virulent serotypes of enterovirus, such as EV71, have posed a serious challenge to public health because of their severe clinical manifestations and complications.15,16 To our knowledge, no similar investigation has examined the association of enterovirus infection with leukaemia in the scientific literature. This large nationwide retrospective cohort study showed a reduced risk of leukaemia in children infected with enteroviruses, particularly in those who had no allergic diseases. Additional investigations are needed to unravel the pathogenesis of enterovirus infections and childhood leukaemia. Contributors J-NL and C-HK designed the study. C-HK provided study materials. J-NL, C-LL, and C-HK collected and assembled data. J-NL, C-LL, M-CL, C-HL, H-HL, C-HY, F-CS, and C-HK analysed and interpreted the data. J-NL, C-LL, M-CL, C-HL, H-HL, C-HY, F-CS, and C-HK wrote the report. J-NL, C-LL, M-CL, C-HL, H-HL, C-HY, F-CS, and C-HK approved the final report. Declaration of interests We declare no competing interests. Acknowledgments This study is supported partly by Taiwan Ministry of Health and Welfare Clinical Trial and Research Center of Excellence (MOHW104-TDU-B-212-113002); China Medical University Hospital, Academia Sinica Taiwan Biobank, Stroke Biosignature Project (BM104010092); NRPB Stroke Clinical Trial Consortium (MOST 103-2325-B-039-006); Tseng-Lien Lin Foundation, Taichung, Taiwan; Taiwan Brain Disease Foundation, Taipei, Taiwan; Katsuzo and Kiyo Aoshima Memorial Funds, Japan; Health and Welfare Surcharge on tobacco products, China Medical University Hospital Cancer Research Center of Excellence (MOHW104-TDU-B-212-124-002, Taiwan); Bureau of Health Promotion, Taiwan Ministry of Health Welfare, Taiwan (DOH99-HP-1205); and China Medical University under the Aim for Top University Plan of the Ministry of Education, Taiwan.

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References 1 Spector LG, Ross JA, Olshan AF, COG Epidemiology Committee. Children’s Oncology Group’s 2013 blueprint for research: epidemiology. Pediatr Blood Cancer 2013; 60: 1059–62. 2 Feltbower RG, McNally RJQ, Kinsey SE, et al. Epidemiology of leukaemia and lymphoma in children and young adults from the north of England, 1990-2002. Eur J Cancer 2009; 45: 420–27. 3 Metayer C, Milne E, Clavel J, et al. The Childhood Leukemia International Consortium. Cancer Epidemiol 2013; 37: 336–47. 4 Mullighan CG. The molecular genetic makeup of acute lymphoblastic leukemia. Hematology Am Soc Hematol Educ Program 2012; 2012: 389–96. 5 Lim JY-S, Bhatia S, Robison LL, Yang JJ. Genomics of racial and ethnic disparities in childhood acute lymphoblastic leukemia. Cancer 2014; 120: 955–62. 6 Gilham C, Peto J, Simpson J, et al. Day care in infancy and risk of childhood acute lymphoblastic leukaemia: findings from UK case-control study. BMJ 2005; 330: 1294. 7 Ma X, Urayama K, Chang J, Wiemels JL, Buffler PA. Infection and pediatric acute lymphoblastic leukemia. Blood Cells Mol Dis 2009; 42: 117–20. 8 Urayama KY, Buffler PA, Gallagher ER, Ayoob JM, Ma X. A metaanalysis of the association between day-care attendance and childhood acute lymphoblastic leukaemia. Int J Epidemiol 2010; 39: 718–32. 9 Ajrouche R, Rudant J, Orsi L, et al. Childhood acute lymphoblastic leukaemia and indicators of early immune stimulation: the Estelle study (SFCE). Br J Cancer 2015; 112: 1017–26. 10 Marcotte EL, Ritz B, Cockburn M, Yu F, Heck JE. Exposure to infections and risk of leukemia in young children. Cancer Epidemiol Biomark Prev 2014; 23: 1195–203. 11 Westergaard T, Andersen PK, Pedersen JB, et al. Birth characteristics, sibling patterns, and acute leukemia risk in childhood: a populationbased cohort study. J Natl Cancer Inst 1997; 89: 939–47. 12 Greaves MF. Speculations on the cause of childhood acute lymphoblastic leukemia. Leukemia 1988; 2: 120–25. 13 Greaves M. Infection, immune responses and the aetiology of childhood leukaemia. Nat Rev Cancer 2006; 6: 193–203. 14 Kinlen L. Evidence for an infective cause of childhood leukaemia: comparison of a Scottish new town with nuclear reprocessing sites in Britain. Lancet 1988; 2: 1323–27. 15 Solomon T, Lewthwaite P, Perera D, Cardosa MJ, McMinn P, Ooi MH. Virology, epidemiology, pathogenesis, and control of enterovirus 71. Lancet Infect Dis 2010; 10: 778–90. 16 Abzug MJ. The enteroviruses: problems in need of treatments. J Infect 2014; 68 (suppl 1): S108–14. 17 Centers for Disease Control and Prevention. Non-polio enterovirus, overview non-polio enteroviruses, picornavirus. http://www.cdc.gov/ non-polio-enterovirus/about/overview.html (accessed April 10, 2015). 18 Cheng TM. Taiwan’s National Health Insurance system: high value for the dollar. In: Okma KGH, Crivelli L, eds. Six countries, six reform models: the health reform experience of Israel, the Netherlands, New Zealand, Singapore, Switzerland and Taiwan. New Jersey: World Scientific, 2009: 71–204. 19 Liu CY, Hung YT, Chuang YL, et al. Incorporating development stratification of Taiwan townships into sampling design of large scale health interview survey. J Health Manag 2006; 14: 1–22. 20 Fine JP, Gray RJ. A proportional hazards model for the subdistribution of a competing risk. J Am Stat Assoc 1999; 94: 496–509. 21 Breslow NE, Day NE. Statistical methods in cancer research. Volume II—The design and analysis of cohort studies. IARC Sci Publ 1987; 1–406. 22 Gooley TA, Leisenring W, Crowley J, Storer BE. Estimation of failure probabilities in the presence of competing risks: new representations of old estimators. Stat Med 1999; 18: 695–706. 23 Rodriguez-Galindo C, Friedrich P, Morrissey L, Frazier L. Global challenges in pediatric oncology. Curr Opin Pediatr 2013; 25: 3–15. 24 Belson M, Kingsley B, Holmes A. Risk factors for acute leukemia in children: a review. Environ Health Perspect 2007; 115: 138–45. 25 Oksuzyan S, Crespi CM, Cockburn M, Mezei G, Vergara X, Kheifets L. Race/ethnicity and the risk of childhood leukaemia: a case-control study in California. J Epidemiol Community Health 2015; 69: 795–802.

www.thelancet.com/oncology Published online August 28, 2015 http://dx.doi.org/10.1016/S1470-2045(15)00060-1

Articles

26 27

28 29

30

31

32

33

34

35 36

37

Howard SC, Metzger ML, Wilimas JA, et al. Childhood cancer epidemiology in low-income countries. Cancer 2008; 112: 461–72. Liu YL, Lo WC, Chiang CJ, et al. Incidence of cancer in children aged 0-14 years in Taiwan, 1996-2010. Cancer Epidemiol 2015; 39: 21–28. Osterholm MT. Infectious disease in child day care: an overview. Pediatrics 1994; 94: 987–90. Ma X, Buffler PA, Selvin S, et al. Daycare attendance and risk of childhood acute lymphoblastic leukaemia. Br J Cancer 2002; 86: 1419–24. Urayama KY, Ma X, Buffler PA. Exposure to infections through daycare attendance and risk of childhood leukaemia. Radiat Prot Dosimetry 2008; 132: 259–66. Dockerty JD. Epidemiology of childhood leukemia in New Zealand: studies of infectious hypotheses. Blood Cells Mol Dis 2009; 42: 113–16. McNally RJQ, Eden TOB. An infectious aetiology for childhood acute leukaemia: a review of the evidence. Br J Haematol 2004; 127: 243–63. Poole C, Greenland S, Luetters C, Kelsey JL, Mezei G. Socioeconomic status and childhood leukaemia: a review. Int J Epidemiol 2006; 35: 370–84. Adam M, Rebholz CE, Egger M, Zwahlen M, Kuehni CE. Childhood leukaemia and socioeconomic status: what is the evidence? Radiat Prot Dosimetry 2008; 132: 246–54. Strachan DP. Hay fever, hygiene, and household size. BMJ 1989; 299: 1259–60. Kinra S, Davey Smith G, Jeffreys M, Gunnell D, Galobardes B, McCarron P. Association between sibship size and allergic diseases in the Glasgow Alumni Study. Thorax 2006; 61: 48–53. Bernsen RMD, de Jongste JC, van der Wouden JC. Birth order and sibship size as independent risk factors for asthma, allergy, and eczema. Pediatr Allergy Immunol 2003; 14: 464–69.

38

39

40

41

42

43

44

45

46

Westergaard T, Rostgaard K, Wohlfahrt J, Andersen PK, Aaby P, Melbye M. Sibship characteristics and risk of allergic rhinitis and asthma. Am J Epidemiol 2005; 162: 125–32. Haby MM, Marks GB, Peat JK, Leeder SR. Daycare attendance before the age of two protects against atopy in preschool age children. Pediatr Pulmonol 2000; 30: 377–84. Wen W, Shu XO, Linet MS, et al. Allergic disorders and the risk of childhood acute lymphoblastic leukemia (United States). Cancer Causes Control 2000; 11: 303–07. Lariou M-S, Dikalioti SK, Dessypris N, et al. Allergy and risk of acute lymphoblastic leukemia among children: a nationwide case control study in Greece. Cancer Epidemiol 2013; 37: 146–51. Dahl S, Schmidt LS, Vestergaard T, Schüz J, Schmiegelow K. Allergy and the risk of childhood leukemia: a meta-analysis. Leukemia 2009; 23: 2300–04. Schüz J, Morgan G, Böhler E, Kaatsch P, Michaelis J. Atopic disease and childhood acute lymphoblastic leukemia. Int J Cancer 2003; 105: 255–60. Chang JS, Tsai YW, Tsai CR, Wiemels JL. Allergy and risk of childhood acute lymphoblastic leukemia: a population-based and record-based study. Am J Epidemiol 2012; 176: 970–78. Cheng CL, Kao YH, Lin SJ, Lee CH, Lai ML. Validation of the National Health Insurance Research Database with ischemic stroke cases in Taiwan. Pharmacoepidemiol Drug Saf 2011; 20: 236–42. Yu YB, Gau JP, Liu CY, et al. A nation-wide analysis of venous thromboembolism in 497,180 cancer patients with the development and validation of a risk-stratification scoring system. Thromb Haemost 2012; 108: 225–35.

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