Season of birth and risk of Hodgkin and non-Hodgkin lymphoma.

Infectious etiologies have been hypothesized for Hodgkin and non-Hodgkin lymphoma (HL and NHL) in early life, but findings to date for specific lymphomas and periods of susceptibility are conflicting. We conducted the first national cohort study to examine whether season of birth, a proxy for infectious exposures in the first few months of life, is associated with HL or NHL in childhood through young adulthood. A total of 3,571,574 persons born in Sweden in 1973-2008 were followed up through 2009 to examine the association between season of birth and incidence of HL (943 cases) or NHL (936 cases). We found a sinusoidal pattern in NHL risk by season of birth (p = 0.04), with peak risk occurring among birthdates in April. Relative to persons born in fall (September-November), odds ratios for NHL by season of birth were 1.25 [95% confidence interval (CI), 1.04-1.50; p = 0.02] for spring (March-May), 1.22 (95% CI, 1.01-1.48; p = 0.04) for summer (June-August) and 1.11 (95% CI, 0.91-1.35; p = 0.29) for winter (December-February). These findings did not vary by sex, age at diagnosis or major subtypes. In contrast, there was no seasonal association between birthdate and risk of HL (p = 0.78). In this large cohort study, birth in spring or summer was associated with increased risk of NHL (but not HL) in childhood through young adulthood, possibly related to immunologic effects of delayed infectious exposures compared with fall or winter birth. These findings suggest that immunologic responses in early infancy may play an important role in the development of NHL.