title: Day care in infancy and risk of childhood acute lymphoblastic leukaemia: findings from UK case-control study.
abstract: OBJECTIVE
  To test the hypothesis that reduced exposure to common infections in the first year of life increases the risk of developing acute lymphoblastic leukaemia. Design and setting The United Kingdom childhood cancer study (UKCCS) is a large population based case-control study of childhood cancer across 10 regions of the UK.
  
  
  METHODS
  6305 children (aged 2-14 years) without cancer; 3140 children with cancer (diagnosed 1991-6), of whom 1286 had acute lymphoblastic leukaemia (ALL).
  
  
  METHODS
  Day care and social activity during the first year of life were used as proxies for potential exposure to infection in infancy.
  
  
  RESULTS
  Increasing levels of social activity were associated with consistent reductions in risk of ALL; a dose-response trend was seen. When children whose mothers reported no regular activity outside the family were used as the reference group, odds ratios for increasing levels of activity were 0.73 (95% confidence interval 0.62 to 0.87) for any social activity, 0.62 (0.51 to 0.75) for regular day care outside the home, and 0.48 (0.37 to 0.62) for formal day care (attendance at facility with at least four children at least twice a week) (P value for trend < 0.001). Although not as striking, results for non-ALL malignancies showed a similar pattern (P value for trend < 0.001). When children with non-ALL malignancies were taken as the reference group, a significant protective effect for ALL was seen only for formal day care (odds ratio = 0.69, 0.51 to 0.93; P = 0.02). Similar results were obtained for B cell precursor common ALL and other subgroups, as well as for cases diagnosed above and below age 5 years.
  
  
  CONCLUSIONS
  These results support the hypothesis that reduced exposure to infection in the first few months of life increases the risk of developing acute lymphoblastic leukaemia.
authors.affiliation: Cancer Research UK Epidemiology and Genetics Unit, Institute of Cancer Research, Sutton SM2 5NG.
authors.forename: C
authors.initials: C
authors.lastname: Gilham
authors.forename: J
authors.initials: J
authors.lastname: Peto
authors.forename: J
authors.initials: J
authors.lastname: Simpson
authors.forename: E
authors.initials: E
authors.lastname: Roman
authors.forename: T O B
authors.initials: TO
authors.lastname: Eden
authors.forename: M F
authors.initials: MF
authors.lastname: Greaves
authors.forename: F E
authors.initials: FE
authors.lastname: Alexander
mesh_terms: D000293:Adolescent; D016022:Case-Control Studies; D002648:Child; D002656:Child Day Care Centers / Q000706:statistics & numerical data*; D002675:Child, Preschool; D003141:Communicable Diseases / Q000453:epidemiology*; D004781:Environmental Exposure / Q000009:adverse effects / Q000706:statistics & numerical data*; D006801:Humans; D007223:Infant; D016017:Odds Ratio; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000453:epidemiology* / Q000209:etiology; D012044:Regression Analysis; D012307:Risk Factors; D012919:Social Behavior; D006113:United Kingdom / Q000453:epidemiology