Risk of childhood leukemia associated with vaccination, infection, and medication use in childhood: the Cross-Canada Childhood Leukemia Study.

Current hypotheses consonant with the peak in leukemia incidence in early childhood point to an infectious etiology. The authors examined the effect of postnatal exposures predicted to affect early immune functioning, including childhood vaccinations, illness, medication use, and breastfeeding patterns. Children 0-15 years of age diagnosed with leukemia from 1990 to 1994 and resident within principal cities across Canada were eligible for inclusion. Through pediatric oncology centers and population-based cancer registries, 399 cases were ascertained at the time of diagnosis. For each participating case, an age-, gender-, and area-matched control was randomly selected from government health insurance rolls. Risk factor information was obtained through personal interviews with each child's parents or guardians. Conditional logistic regression was used to calculate odds ratios, with adjustment for potential confounders. Use of immunosuppressant medication by the index child led to a deficit of risk (odds ratio = 0.37, 95% confidence interval: 0.16, 0.84), while vitamin intake was positively associated with leukemia (odds ratio = 1.66, 95% confidence interval: 1.18, 2.33). Breastfeeding for more than 6 months was also protective (p < 0.05). Results persisted for cases diagnosed with acute lymphoblastic leukemia and for children diagnosed at 1-5 years of age. These findings suggest a role for early immunologic challenge in the expression of childhood leukemia.